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<p><b>If you are upgrading from a version prior to 2.1.x, please see the note below about the change to the schema used to store consumer offsets.
    Once you have changed the inter.broker.protocol.version to the latest version, it will not be possible to downgrade to a version prior to 2.1.</b></p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g., <code>2.6</code>, <code>2.5</code>, etc.)</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from version 0.11.0.x or above, and you have not overridden the message format, then you only need to override
        the inter-broker protocol version.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g., <code>2.6</code>, <code>2.5</code>, etc.)</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. Once you have done so, the
        brokers will be running the latest version and you can verify that the cluster's behavior and performance meets expectations.
        It is still possible to downgrade at this point if there are any problems.
    </li>
    <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
        <code>inter.broker.protocol.version</code> and setting it to <code>2.7</code>.
    </li>
    <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
        protocol version, it will no longer be possible to downgrade the cluster to an older version.
    </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.7 on each broker and restart them one by one. Note that the older Scala clients,
        which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs
        (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java clients must be used.
    </li>
</ol>

<h5><a id="upgrade_270_notable" href="#upgrade_270_notable">Notable changes in 2.7.0</a></h5>
<ul>
    <li>
        The 2.7.0 release includes the core Raft implementation specified in
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-595%3A+A+Raft+Protocol+for+the+Metadata+Quorum">KIP-595</a>.
        There is a separate "raft" module containing most of the logic. Until integration with the
        controller is complete, there is a standalone server that users can use for testing the p
        erformance of the Raft implementation.  See the README.md in the raft module for details
    </li>
    <li>
        KIP-651 <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-651+-+Support+PEM+format+for+SSL+certificates+and+private+key">adds support</a>
        for using PEM files for key and trust stores.
    </li>
    <li>
        KIP-612 <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-612%3A+Ability+to+Limit+Connection+Creation+Rate+on+Brokers">adds support</a>
        for enforcing broker-wide and per-listener connection create rates.  The 2.7.0 release contains
        the first part of KIP-612 with dynamic configuration coming in the 2.8.0 release.
    </li>
    <li>
        The ability to throttle topic and partition creations or
        topics deletions to prevent a cluster from being harmed via
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-599%3A+Throttle+Create+Topic%2C+Create+Partition+and+Delete+Topic+Operations">KIP-599</a>
    </li>
    <li>
        When new features become available in Kafka there are two main issues:
        <ol>
            <li>How do Kafka clients become aware of broker capabilities?</li>
            <li>How does the broker decide which features to enable?</li>
        </ol>
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-584%3A+Versioning+scheme+for+features">KIP-584</a>
        provides a flexible and operationally easy solution for client discovery, feature gating and rolling upgrades using a single restart.
    </li>
    <li>
        The ability to print record offsets and headers with the <code>ConsoleConsumer</code> is now possible
        via <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-431%3A+Support+of+printing+additional+ConsumerRecord+fields+in+DefaultMessageFormatter">KIP-431</a>
    </li>
    <li>
      The addition of <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-554%3A+Add+Broker-side+SCRAM+Config+API">KIP-554</a>
        continues progress towards the goal of Zookeeper removal from Kafka. The addition of KIP-554
        means you don't have to connect directly to ZooKeeper anymore for managing SCRAM credentials.
    </li>
    <li>Altering non-reconfigurable configs of existent listeners causes <code>InvalidRequestException</code>.
        By contrast, the previous (unintended) behavior would have caused the updated configuration to be persisted,
        but it wouldn't
        take effect until the broker was restarted. See <a href="https://github.com/apache/kafka/pull/9284">KAFKA-10479</a> for more discussion.
        See <code>DynamicBrokerConfig.DynamicSecurityConfigs</code> and <code>SocketServer.ListenerReconfigurableConfigs</code>
        for the supported reconfigurable configs of existent listeners.
    </li>

    <li>
        Kafka Streams adds support for
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-450%3A+Sliding+Window+Aggregations+in+the+DSL">Sliding Windows Aggregations</a>
        in the KStreams DSL.
    </li>
    <li>
        Reverse iteration over state stores enabling more efficient most recent update searches with
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-617%3A+Allow+Kafka+Streams+State+Stores+to+be+iterated+backwards">KIP-617</a>
    </li>
    <li>
        End-to-End latency metrics in Kafka Steams see
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-613%3A+Add+end-to-end+latency+metrics+to+Streams">KIP-613</a>
        for more details
    </li>
    <li>
        Kafka Streams added metrics reporting default RocksDB properties with
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-607%3A+Add+Metrics+to+Kafka+Streams+to+Report+Properties+of+RocksDB">KIP-607</a>
    </li>
    <li>
        Better Scala implicit Serdes support from
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-616%3A+Rename+implicit+Serdes+instances+in+kafka-streams-scala">KIP-616</a>
    </li>
</ul>
<h4><a id="upgrade_2_6_0" href="#upgrade_2_6_0">Upgrading to 2.6.0 from any version 0.8.x through 2.5.x</a></h4>

<p><b>If you are upgrading from a version prior to 2.1.x, please see the note below about the change to the schema used to store consumer offsets.
    Once you have changed the inter.broker.protocol.version to the latest version, it will not be possible to downgrade to a version prior to 2.1.</b></p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g., <code>2.5</code>, <code>2.4</code>, etc.)</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from version 0.11.0.x or above, and you have not overridden the message format, then you only need to override
        the inter-broker protocol version.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g., <code>2.5</code>, <code>2.4</code>, etc.)</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. Once you have done so, the
        brokers will be running the latest version and you can verify that the cluster's behavior and performance meets expectations.
        It is still possible to downgrade at this point if there are any problems.
    </li>
    <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
        <code>inter.broker.protocol.version</code> and setting it to <code>2.6</code>.
    </li>
    <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
        protocol version, it will no longer be possible to downgrade the cluster to an older version.
    </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.6 on each broker and restart them one by one. Note that the older Scala clients,
        which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs
        (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java clients must be used.
    </li>
</ol>

<h5 class="anchor-heading"><a id="upgrade_260_notable" class="anchor-link"></a><a href="#upgrade_260_notable">Notable changes in 2.6.0</a></h5>
<ul>
    <li>Kafka Streams adds a new processing mode (requires broker 2.5 or newer) that improves application
        scalability using exactly-once guarantees
        (cf. <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-447%3A+Producer+scalability+for+exactly+once+semantics">KIP-447</a>)
    </li>
    <li>TLSv1.3 has been enabled by default for Java 11 or newer. The client and server will negotiate TLSv1.3 if
        both support it and fallback to TLSv1.2 otherwise. See
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-573%3A+Enable+TLSv1.3+by+default">KIP-573</a> for more details.
    </li>
    <li>The default value for the <code>client.dns.lookup</code> configuration has been changed from <code>default</code>
        to <code>use_all_dns_ips</code>. If a hostname resolves to multiple IP addresses, clients and brokers will now
        attempt to connect to each IP in sequence until the connection is successfully established. See
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-602%3A+Change+default+value+for+client.dns.lookup">KIP-602</a>
        for more details.
    </li>
    <li><code>NotLeaderForPartitionException</code> has been deprecated and replaced with <code>NotLeaderOrFollowerException</code>.
        Fetch requests and other requests intended only for the leader or follower return NOT_LEADER_OR_FOLLOWER(6) instead of REPLICA_NOT_AVAILABLE(9)
        if the broker is not a replica, ensuring that this transient error during reassignments is handled by all clients as a retriable exception.
    </li>
</ul>

<h4><a id="upgrade_2_5_0" href="#upgrade_2_5_0">Upgrading to 2.5.0 from any version 0.8.x through 2.4.x</a></h4>

<p><b>If you are upgrading from a version prior to 2.1.x, please see the note below about the change to the schema used to store consumer offsets.
    Once you have changed the inter.broker.protocol.version to the latest version, it will not be possible to downgrade to a version prior to 2.1.</b></p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g., <code>2.4</code>, <code>2.3</code>, etc.)</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from version 0.11.0.x or above, and you have not overridden the message format, then you only need to override
        the inter-broker protocol version.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g., <code>2.4</code>, <code>2.3</code>, etc.)</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. Once you have done so, the
        brokers will be running the latest version and you can verify that the cluster's behavior and performance meets expectations.
        It is still possible to downgrade at this point if there are any problems.
    </li>
    <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
        <code>inter.broker.protocol.version</code> and setting it to <code>2.5</code>.
    </li>
    <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
        protocol version, it will no longer be possible to downgrade the cluster to an older version.
    </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.5 on each broker and restart them one by one. Note that the older Scala clients,
        which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs
        (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java clients must be used.
    </li>

    <li>There are several notable changes to the reassignment tool <code>kafka-reassign-partitions.sh</code>
        following the completion of
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-455%3A+Create+an+Administrative+API+for+Replica+Reassignment">KIP-455</a>.
        This tool now requires the <code>--additional</code> flag to be provided when changing the throttle of an
        active reassignment. Reassignment cancellation is now possible using the
        <code>--cancel</code> command. Finally, reassignment with <code>--zookeeper</code>
        has been deprecated in favor of <code>--bootstrap-server</code>. See the KIP for more detail.
    </li>
</ol>

<h5 class="anchor-heading"><a id="upgrade_250_notable" class="anchor-link"></a><a href="#upgrade_250_notable">Notable changes in 2.5.0</a></h5>
<ul>
    <li>When <code>RebalanceProtocol#COOPERATIVE</code> is used, <code>Consumer#poll</code> can still return data
        while it is in the middle of a rebalance for those partitions still owned by the consumer; in addition
        <code>Consumer#commitSync</code> now may throw a non-fatal <code>RebalanceInProgressException</code> to notify
        users of such an event, in order to distinguish from the fatal <code>CommitFailedException</code> and allow
        users to complete the ongoing rebalance and then reattempt committing offsets for those still-owned partitions.</li>
    <li>For improved resiliency in typical network environments, the default value of
        <code>zookeeper.session.timeout.ms</code> has been increased from 6s to 18s and
        <code>replica.lag.time.max.ms</code> from 10s to 30s.</li>
    <li>New DSL operator <code>cogroup()</code> has been added for aggregating multiple streams together at once.</li>
    <li>Added a new <code>KStream.toTable()</code> API to translate an input event stream into a KTable.</li>
    <li>Added a new Serde type <code>Void</code> to represent null keys or null values from input topic.</li>
    <li>Deprecated <code>UsePreviousTimeOnInvalidTimestamp</code> and replaced it with <code>UsePartitionTimeOnInvalidTimeStamp</code>.</li>
    <li>Improved exactly-once semantics by adding a pending offset fencing mechanism and stronger transactional commit
        consistency check, which greatly simplifies the implementation of a scalable exactly-once application.
        We also added a new exactly-once semantics code example under
        <a href="https://github.com/apache/kafka/tree/2.5/examples">examples</a> folder. Check out
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-447%3A+Producer+scalability+for+exactly+once+semantics">KIP-447</a>
        for the full details.</li>
    <li>Added a new public api <code>KafkaStreams.queryMetadataForKey(String, K, Serializer) to get detailed information on the key being queried.
        It provides information about the partition number where the key resides in addition to hosts containing the active and standby partitions for the key.</code></li>
    <li>Provided support to query stale stores (for high availability) and the stores belonging to a specific partition by deprecating <code>KafkaStreams.store(String, QueryableStoreType)</code> and replacing it with <code>KafkaStreams.store(StoreQueryParameters)</code>.</li>
    <li>Added a new public api to access lag information for stores local to an instance with <code>KafkaStreams.allLocalStorePartitionLags()</code>.</li>
    <li>Scala 2.11 is no longer supported. See
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-531%3A+Drop+support+for+Scala+2.11+in+Kafka+2.5">KIP-531</a>
        for details.</li>
    <li>All Scala classes from the package <code>kafka.security.auth</code> have been deprecated. See
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-504+-+Add+new+Java+Authorizer+Interface">KIP-504</a>
        for details of the new Java authorizer API added in 2.4.0.  Note that <code>kafka.security.auth.Authorizer</code>
        and <code>kafka.security.auth.SimpleAclAuthorizer</code> were deprecated in 2.4.0.
    </li>
    <li>TLSv1 and TLSv1.1 have been disabled by default since these have known security vulnerabilities. Only TLSv1.2 is now
        enabled by default. You can continue to use TLSv1 and TLSv1.1 by explicitly enabling these in the configuration options
        <code>ssl.protocol</code> and <code>ssl.enabled.protocols</code>.
    </li>
    <li>ZooKeeper has been upgraded to 3.5.7, and a ZooKeeper upgrade from 3.4.X to 3.5.7 can fail if there are no snapshot files in the 3.4 data directory.
        This usually happens in test upgrades where ZooKeeper 3.5.7 is trying to load an existing 3.4 data dir in which no snapshot file has been created.
        For more details about the issue please refer to <a href="https://issues.apache.org/jira/browse/ZOOKEEPER-3056">ZOOKEEPER-3056</a>.
        A fix is given in <a href="https://issues.apache.org/jira/browse/ZOOKEEPER-3056">ZOOKEEPER-3056</a>, which is to set <code>snapshot.trust.empty=true</code>
        config in <code>zookeeper.properties</code> before the upgrade.
    </li>
    <li>ZooKeeper version 3.5.7 supports TLS-encrypted connectivity to ZooKeeper both with or without client certificates,
        and additional Kafka configurations are available to take advantage of this.
        See <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-515%3A+Enable+ZK+client+to+use+the+new+TLS+supported+authentication">KIP-515</a> for details.
    </li>
</ul>

<h4><a id="upgrade_2_4_0" href="#upgrade_2_4_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x, 0.11.0.x, 1.0.x, 1.1.x, 2.0.x or 2.1.x or 2.2.x or 2.3.x to 2.4.0</a></h4>

<p><b>If you are upgrading from a version prior to 2.1.x, please see the note below about the change to the schema used to store consumer offsets.
    Once you have changed the inter.broker.protocol.version to the latest version, it will not be possible to downgrade to a version prior to 2.1.</b></p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.10.0, 0.11.0, 1.0, 2.0, 2.2).</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from version 0.11.0.x or above, and you have not overridden the message format, then you only need to override
        the inter-broker protocol version.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (0.11.0, 1.0, 1.1, 2.0, 2.1, 2.2, 2.3).</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. Once you have done so, the
        brokers will be running the latest version and you can verify that the cluster's behavior and performance meets expectations.
        It is still possible to downgrade at this point if there are any problems.
    </li>
    <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
        <code>inter.broker.protocol.version</code> and setting it to 2.4.
    </li>
    <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
        protocol version, it will no longer be possible to downgrade the cluster to an older version.
    </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.4 on each broker and restart them one by one. Note that the older Scala clients,
        which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs
        (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java clients must be used.
    </li>
</ol>

<p><b>Additional Upgrade Notes:</b></p>

<ol>
    <li>ZooKeeper has been upgraded to 3.5.6. ZooKeeper upgrade from 3.4.X to 3.5.6 can fail if there are no snapshot files in 3.4 data directory.
        This usually happens in test upgrades where ZooKeeper 3.5.6 is trying to load an existing 3.4 data dir in which no snapshot file has been created.
        For more details about the issue please refer to <a href="https://issues.apache.org/jira/browse/ZOOKEEPER-3056">ZOOKEEPER-3056</a>.
        A fix is given in <a href="https://issues.apache.org/jira/browse/ZOOKEEPER-3056">ZOOKEEPER-3056</a>, which is to set <code>snapshot.trust.empty=true</code>
        config in <code>zookeeper.properties</code> before the upgrade. But we have observed data loss in standalone cluster upgrades when using
        <code>snapshot.trust.empty=true</code> config. For more details about the issue please refer to <a href="https://issues.apache.org/jira/browse/ZOOKEEPER-3644">ZOOKEEPER-3644</a>.
        So we recommend the safe workaround of copying empty <a href="https://issues.apache.org/jira/secure/attachment/12928686/snapshot.0">snapshot</a> file to the 3.4 data directory,
        if there are no snapshot files in 3.4 data directory. For more details about the workaround please refer to <a href="https://cwiki.apache.org/confluence/display/ZOOKEEPER/Upgrade+FAQ">ZooKeeper Upgrade FAQ</a>.
    </li>
    <li>
        An embedded Jetty based <a href="http://zookeeper.apache.org/doc/r3.5.6/zookeeperAdmin.html#sc_adminserver">AdminServer</a> added in ZooKeeper 3.5.
        AdminServer is enabled by default in ZooKeeper and is started on port 8080.
        AdminServer is disabled by default in the ZooKeeper config (<code>zookeeper.properties</code>) provided by the Apache Kafka distribution.
        Make sure to update your local <code>zookeeper.properties</code> file with <code>admin.enableServer=false</code> if you wish to disable the AdminServer.
        Please refer <a href="http://zookeeper.apache.org/doc/r3.5.6/zookeeperAdmin.html#sc_adminserver">AdminServer config</a> to configure the AdminServer.
    </li>
</ol>

<h5><a id="upgrade_240_notable" href="#upgrade_240_notable">Notable changes in 2.4.0</a></h5>
<ul>
    <li>A new Admin API has been added for partition reassignments. Due to changing the way Kafka propagates reassignment information,
        it is possible to lose reassignment state in failure edge cases while upgrading to the new version. It is not recommended to start reassignments while upgrading.</li>
    <li>ZooKeeper has been upgraded from 3.4.14 to 3.5.6. TLS and dynamic reconfiguration are supported by the new version.</li>
    <li>The <code>bin/kafka-preferred-replica-election.sh</code> command line tool has been deprecated. It has been replaced by <code>bin/kafka-leader-election.sh</code>.</li>
    <li>The methods <code>electPreferredLeaders</code> in the Java <code>AdminClient</code> class have been deprecated in favor of the methods <code>electLeaders</code>.</li>
    <li>Scala code leveraging the <code>NewTopic(String, int, short)</code> constructor with literal values will need to explicitly call <code>toShort</code> on the second literal.</li>
    <li>The argument in the constructor <code>GroupAuthorizationException(String)</code> is now used to specify an exception message.
        Previously it referred to the group that failed authorization. This was done for consistency with other exception types and to
        avoid potential misuse. The constructor <code>TopicAuthorizationException(String)</code> which was previously used for a single
        unauthorized topic was changed similarly.
    </li>
    <li>The internal <code>PartitionAssignor</code> interface has been deprecated and replaced with a new <code>ConsumerPartitionAssignor</code> in the public API. Some
        methods/signatures are slightly different between the two interfaces. Users implementing a custom PartitionAssignor should migrate to the new interface as soon as possible.
    </li>
    <li>The <code>DefaultPartitioner</code> now uses a sticky partitioning strategy. This means that records for specific topic with null keys and no assigned partition 
        will be sent to the same partition until the batch is ready to be sent. When a new batch is created, a new partition is chosen. This decreases latency to produce, but
        it may result in uneven distribution of records across partitions in edge cases. Generally users will not be impacted, but this difference may be noticeable in tests and
        other situations producing records for a very short amount of time.
    </li>
    <li>The blocking <code>KafkaConsumer#committed</code> methods have been extended to allow a list of partitions as input parameters rather than a single partition.
        It enables fewer request/response iterations between clients and brokers fetching for the committed offsets for the consumer group.
        The old overloaded functions are deprecated and we would recommend users to make their code changes to leverage the new methods (details
        can be found in <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-520%3A+Add+overloaded+Consumer%23committed+for+batching+partitions">KIP-520</a>).
    </li>
    <li>We've introduced a new <code>INVALID_RECORD</code> error in the produce response to distinguish from the <code>CORRUPT_MESSAGE</code> error.
        To be more concrete, previously when a batch of records were sent as part of a single request to the broker and one or more of the records failed
        the validation due to various causes (mismatch magic bytes, crc checksum errors, null key for log compacted topics, etc), the whole batch would be rejected
        with the same and misleading <code>CORRUPT_MESSAGE</code>, and the caller of the producer client would see the corresponding exception from either
        the future object of <code>RecordMetadata</code> returned from the <code>send</code> call as well as in the <code>Callback#onCompletion(RecordMetadata metadata, Exception exception)</code>
        Now with the new error code and improved error messages of the exception, producer callers would be better informed about the root cause why their sent records were failed.
    </li>
    <li>We are introducing incremental cooperative rebalancing to the clients' group protocol, which allows consumers to keep all of their assigned partitions during a rebalance
        and at the end revoke only those which must be migrated to another consumer for overall cluster balance. The <code>ConsumerCoordinator</code> will choose the latest <code>RebalanceProtocol</code>
        that is commonly supported by all of the consumer's supported assignors. You can use the new built-in <code>CooperativeStickyAssignor</code> or plug in your own custom cooperative assignor. To do
        so you must implement the <code>ConsumerPartitionAssignor</code> interface and include <code>RebalanceProtocol.COOPERATIVE</code> in the list returned by <code>ConsumerPartitionAssignor#supportedProtocols</code>.
        Your custom assignor can then leverage the <code>ownedPartitions</code> field in each consumer's <code>Subscription</code> to give partitions back to their previous owners whenever possible. Note that when
        a partition is to be reassigned to another consumer, it <em>must</em> be removed from the new assignment until it has been revoked from its original owner. Any consumer that has to revoke a partition will trigger
        a followup rebalance to allow the revoked partition to safely be assigned to its new owner. See the
        <a href="https://kafka.apache.org/24/javadoc/index.html?org/apache/kafka/clients/consumer/ConsumerPartitionAssignor.RebalanceProtocol.html">ConsumerPartitionAssignor RebalanceProtocol javadocs</a> for more information.
        <br>
        To upgrade from the old (eager) protocol, which always revokes all partitions before rebalancing, to cooperative rebalancing, you must follow a specific upgrade path to get all clients on the same <code>ConsumerPartitionAssignor</code>
        that supports the cooperative protocol. This can be done with two rolling bounces, using the <code>CooperativeStickyAssignor</code> for the example: during the first one, add "cooperative-sticky" to the list of supported assignors
        for each member (without removing the previous assignor -- note that if previously using the default, you must include that explicitly as well). You then bounce and/or upgrade it.
        Once the entire group is on 2.4+ and all members have the "cooperative-sticky" among their supported assignors, remove the other assignor(s) and perform a second rolling bounce so that by the end all members support only the
        cooperative protocol. For further details on the cooperative rebalancing protocol and upgrade path, see <a href="https://cwiki.apache.org/confluence/x/vAclBg">KIP-429</a>.
    </li>
    <li>There are some behavioral changes to the <code>ConsumerRebalanceListener</code>, as well as a new API. Exceptions thrown during any of the listener's three callbacks will no longer be swallowed, and will instead be re-thrown
        all the way up to the <code>Consumer.poll()</code> call. The <code>onPartitionsLost</code> method has been added to allow users to react to abnormal circumstances where a consumer may have lost ownership of its partitions
        (such as a missed rebalance) and cannot commit offsets. By default, this will simply call the existing <code>onPartitionsRevoked</code> API to align with previous behavior. Note however that <code>onPartitionsLost</code> will not
        be called when the set of lost partitions is empty. This means that no callback will be invoked at the beginning of the first rebalance of a new consumer joining the group.
        <br>
        The semantics of the <code>ConsumerRebalanceListener's</code> callbacks are further changed when following the cooperative rebalancing protocol described above. In addition to <code>onPartitionsLost</code>, <code>onPartitionsRevoked</code>
        will also never be called when the set of revoked partitions is empty. The callback will generally be invoked only at the end of a rebalance, and only on the set of partitions that are being moved to another consumer. The
        <code>onPartitionsAssigned</code> callback will however always be called, even with an empty set of partitions, as a way to notify users of a rebalance event (this is true for both cooperative and eager). For details on
        the new callback semantics, see the <a href="https://kafka.apache.org/24/javadoc/index.html?org/apache/kafka/clients/consumer/ConsumerRebalanceListener.html">ConsumerRebalanceListener javadocs</a>.
    </li>
    <li>The Scala trait <code>kafka.security.auth.Authorizer</code> has been deprecated and replaced with a new Java API
        <code>org.apache.kafka.server.authorizer.Authorizer</code>. The authorizer implementation class
        <code>kafka.security.auth.SimpleAclAuthorizer</code> has also been deprecated and replaced with a new
        implementation <code>kafka.security.authorizer.AclAuthorizer</code>. <code>AclAuthorizer</code> uses features
        supported by the new API to improve authorization logging and is compatible with <code>SimpleAclAuthorizer</code>.
        For more details, see <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-504+-+Add+new+Java+Authorizer+Interface">KIP-504</a>.
    </li>
</ul>

<h4><a id="upgrade_2_3_0" href="#upgrade_2_3_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x, 0.11.0.x, 1.0.x, 1.1.x, 2.0.x or 2.1.x or 2.2.x to 2.3.0</a></h4>

<p><b>If you are upgrading from a version prior to 2.1.x, please see the note below about the change to the schema used to store consumer offsets.
    Once you have changed the inter.broker.protocol.version to the latest version, it will not be possible to downgrade to a version prior to 2.1.</b></p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.8.2, 0.9.0, 0.10.0, 0.10.1, 0.10.2, 0.11.0, 1.0, 1.1).</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from 0.11.0.x, 1.0.x, 1.1.x, 2.0.x, or 2.1.x, and you have not overridden the message format, then you only need to override
        the inter-broker protocol version.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (0.11.0, 1.0, 1.1, 2.0, 2.1, 2.2).</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. Once you have done so, the
        brokers will be running the latest version and you can verify that the cluster's behavior and performance meets expectations.
        It is still possible to downgrade at this point if there are any problems.
    </li>
    <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
        <code>inter.broker.protocol.version</code> and setting it to 2.3.
    </li>
    <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
        protocol version, it will no longer be possible to downgrade the cluster to an older version.
    </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.3 on each broker and restart them one by one. Note that the older Scala clients,
        which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs
        (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java clients must be used.
    </li>
</ol>

<h5><a id="upgrade_230_notable" href="#upgrade_230_notable">Notable changes in 2.3.0</a></h5>
<ul>
    <li> We are introducing a new rebalancing protocol for Kafka Connect based on
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-415%3A+Incremental+Cooperative+Rebalancing+in+Kafka+Connect">incremental cooperative rebalancing</a>.
        The new protocol does not require stopping all the tasks during a rebalancing phase between Connect workers. Instead, only the tasks that need to be exchanged
        between workers are stopped and they are started in a follow up rebalance. The new Connect protocol is enabled by default beginning with 2.3.0.
        For more details on how it works and how to enable the old behavior of eager rebalancing, checkout
        <a href="/{{version}}/documentation/#connect_administration">incremental cooperative rebalancing design</a>.
    </li>
    <li> We are introducing static membership towards consumer user. This feature reduces unnecessary rebalances during normal application upgrades or rolling bounces.
        For more details on how to use it, checkout <a href="/{{version}}/documentation/#static_membership">static membership design</a>.
    </li>
    <li> Kafka Streams DSL switches its used store types. While this change is mainly transparent to users, there are some corner cases that may require code changes.
        See the <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_230">Kafka Streams upgrade section</a> for more details.
    </li>
    <li>Kafka Streams 2.3.0 requires 0.11 message format or higher and does not work with older message format.</li>
</ul>

<h4><a id="upgrade_2_2_0" href="#upgrade_2_2_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x, 0.11.0.x, 1.0.x, 1.1.x, 2.0.x or 2.1.x to 2.2.0</a></h4>

<p><b>If you are upgrading from a version prior to 2.1.x, please see the note below about the change to the schema used to store consumer offsets.
    Once you have changed the inter.broker.protocol.version to the latest version, it will not be possible to downgrade to a version prior to 2.1.</b></p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.8.2, 0.9.0, 0.10.0, 0.10.1, 0.10.2, 0.11.0, 1.0, 1.1).</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from 0.11.0.x, 1.0.x, 1.1.x, or 2.0.x and you have not overridden the message format, then you only need to override
        the inter-broker protocol version.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (0.11.0, 1.0, 1.1, 2.0).</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. Once you have done so, the
        brokers will be running the latest version and you can verify that the cluster's behavior and performance meets expectations.
        It is still possible to downgrade at this point if there are any problems.
    </li>
    <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
        <code>inter.broker.protocol.version</code> and setting it to 2.2.
    </li>
    <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
        protocol version, it will no longer be possible to downgrade the cluster to an older version.
    </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.2 on each broker and restart them one by one. Note that the older Scala clients,
        which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs
        (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java clients must be used.
    </li>
</ol>

<h5><a id="upgrade_221_notable" href="#upgrade_221_notable">Notable changes in 2.2.1</a></h5>
<ul>
    <li>Kafka Streams 2.2.1 requires 0.11 message format or higher and does not work with older message format.</li>
</ul>

<h5><a id="upgrade_220_notable" href="#upgrade_220_notable">Notable changes in 2.2.0</a></h5>
<ul>
    <li>The default consumer group id has been changed from the empty string (<code>""</code>) to <code>null</code>. Consumers who use the new default group id will not be able to subscribe to topics,
        and fetch or commit offsets. The empty string as consumer group id is deprecated but will be supported until a future major release. Old clients that rely on the empty string group id will now
        have to explicitly provide it as part of their consumer config. For more information see
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-289%3A+Improve+the+default+group+id+behavior+in+KafkaConsumer">KIP-289</a>.</li>
    <li>The <code>bin/kafka-topics.sh</code> command line tool is now able to connect directly to brokers with <code>--bootstrap-server</code> instead of zookeeper. The old <code>--zookeeper</code>
        option is still available for now. Please read <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-377%3A+TopicCommand+to+use+AdminClient">KIP-377</a> for more information.</li>
    <li>Kafka Streams depends on a newer version of RocksDBs that requires MacOS 10.13 or higher.</li>
</ul>

<h4><a id="upgrade_2_1_0" href="#upgrade_2_1_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x, 0.11.0.x, 1.0.x, 1.1.x, or 2.0.0 to 2.1.0</a></h4>

<p><b>Note that 2.1.x contains a change to the internal schema used to store consumer offsets. Once the upgrade is
  complete, it will not be possible to downgrade to previous versions. See the rolling upgrade notes below for more detail.</b></p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.8.2, 0.9.0, 0.10.0, 0.10.1, 0.10.2, 0.11.0, 1.0, 1.1).</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from 0.11.0.x, 1.0.x, 1.1.x, or 2.0.x and you have not overridden the message format, then you only need to override
        the inter-broker protocol version.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (0.11.0, 1.0, 1.1, 2.0).</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. Once you have done so, the
        brokers will be running the latest version and you can verify that the cluster's behavior and performance meets expectations.
        It is still possible to downgrade at this point if there are any problems. 
    </li>
    <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
        <code>inter.broker.protocol.version</code> and setting it to 2.1.
    </li>
    <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
        protocol version, it will no longer be possible to downgrade the cluster to an older version.
    </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.1 on each broker and restart them one by one. Note that the older Scala clients,
        which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs 
        (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java clients must be used.
    </li>
</ol>

<p><b>Additional Upgrade Notes:</b></p>

<ol>
    <li>Offset expiration semantics has slightly changed in this version. According to the new semantics, offsets of partitions in a group will
        not be removed while the group is subscribed to the corresponding topic and is still active (has active consumers). If group becomes
        empty all its offsets will be removed after default offset retention period (or the one set by broker) has passed (unless the group becomes
        active again). Offsets associated with standalone (simple) consumers, that do not use Kafka group management, will be removed after default
        offset retention period (or the one set by broker) has passed since their last commit.</li>
    <li>The default for console consumer's <code>enable.auto.commit</code> property when no <code>group.id</code> is provided is now set to <code>false</code>.
        This is to avoid polluting the consumer coordinator cache as the auto-generated group is not likely to be used by other consumers.</li>
    <li>The default value for the producer's <code>retries</code> config was changed to <code>Integer.MAX_VALUE</code>, as we introduced <code>delivery.timeout.ms</code>
        in <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-91+Provide+Intuitive+User+Timeouts+in+The+Producer">KIP-91</a>,
        which sets an upper bound on the total time between sending a record and receiving acknowledgement from the broker. By default,
        the delivery timeout is set to 2 minutes.</li>
    <li>By default, MirrorMaker now overrides <code>delivery.timeout.ms</code> to <code>Integer.MAX_VALUE</code> when
        configuring the producer. If you have overridden the value of <code>retries</code> in order to fail faster,
        you will instead need to override <code>delivery.timeout.ms</code>.</li>
    <li>The <code>ListGroup</code> API now expects, as a recommended alternative, <code>Describe Group</code> access to the groups a user should be able to list.
        Even though the old <code>Describe Cluster</code> access is still supported for backward compatibility, using it for this API is not advised.</li>
    <li><a href="https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=87298242">KIP-336</a> deprecates the ExtendedSerializer and ExtendedDeserializer interfaces and
        propagates the usage of Serializer and Deserializer. ExtendedSerializer and ExtendedDeserializer were introduced with
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-82+-+Add+Record+Headers">KIP-82</a> to provide record headers for serializers and deserializers
        in a Java 7 compatible fashion. Now we consolidated these interfaces as Java 7 support has been dropped since.</li>
</ol>

<h5><a id="upgrade_210_notable" href="#upgrade_210_notable">Notable changes in 2.1.0</a></h5>
<ul>
    <li>Jetty has been upgraded to 9.4.12, which excludes TLS_RSA_* ciphers by default because they do not support forward
        secrecy, see https://github.com/eclipse/jetty.project/issues/2807 for more information.</li>
    <li>Unclean leader election is automatically enabled by the controller when <code>unclean.leader.election.enable</code> config is dynamically updated by using per-topic config override.</li>
    <li>The <code>AdminClient</code> has added a method <code>AdminClient#metrics()</code>. Now any application using the <code>AdminClient</code> can gain more information
        and insight by viewing the metrics captured from the <code>AdminClient</code>. For more information
        see <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-324%3A+Add+method+to+get+metrics%28%29+in+AdminClient">KIP-324</a>
    </li>
    <li>Kafka now supports Zstandard compression from <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-110%3A+Add+Codec+for+ZStandard+Compression">KIP-110</a>.
        You must upgrade the broker as well as clients to make use of it. Consumers prior to 2.1.0 will not be able to read from topics which use
        Zstandard compression, so you should not enable it for a topic until all downstream consumers are upgraded. See the KIP for more detail.
    </li>
</ul>

<h4><a id="upgrade_2_0_0" href="#upgrade_2_0_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x, 0.11.0.x, 1.0.x, or 1.1.x to 2.0.0</a></h4>
<p>Kafka 2.0.0 introduces wire protocol changes. By following the recommended rolling upgrade plan below,
    you guarantee no downtime during the upgrade. However, please review the <a href="#upgrade_200_notable">notable changes in 2.0.0</a> before upgrading.
</p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.8.2, 0.9.0, 0.10.0, 0.10.1, 0.10.2, 0.11.0, 1.0, 1.1).</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from 0.11.0.x, 1.0.x, or 1.1.x and you have not overridden the message format, then you only need to override
        the inter-broker protocol format.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (0.11.0, 1.0, 1.1).</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. </li>
    <li> Once the entire cluster is upgraded, bump the protocol version by editing <code>inter.broker.protocol.version</code> and setting it to 2.0.
    <li> Restart the brokers one by one for the new protocol version to take effect.</li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.0 on each broker and restart them one by one. Note that the older Scala consumer
        does not support the new message format introduced in 0.11, so to avoid the performance cost of down-conversion (or to
        take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>), the newer Java consumer must be used.</li>
</ol>

<p><b>Additional Upgrade Notes:</b></p>

<ol>
    <li>If you are willing to accept downtime, you can simply take all the brokers down, update the code and start them back up. They will start
        with the new protocol by default.</li>
    <li>Bumping the protocol version and restarting can be done any time after the brokers are upgraded. It does not have to be immediately after.
        Similarly for the message format version.</li>
    <li>If you are using Java8 method references in your Kafka Streams code you might need to update your code to resolve method ambiguities.
        Hot-swapping the jar-file only might not work.</li>
    <li>ACLs should not be added to prefixed resources,
        (added in <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-290%3A+Support+for+Prefixed+ACLs">KIP-290</a>),
        until all brokers in the cluster have been updated.
        <p><b>NOTE:</b> any prefixed ACLs added to a cluster, even after the cluster is fully upgraded, will be ignored should the cluster be downgraded again.
    </li>
</ol>

<h5><a id="upgrade_200_notable" href="#upgrade_200_notable">Notable changes in 2.0.0</a></h5>
<ul>
    <li><a href="https://cwiki.apache.org/confluence/x/oYtjB">KIP-186</a> increases the default offset retention time from 1 day to 7 days. This makes it less likely to "lose" offsets in an application that commits infrequently. It also increases the active set of offsets and therefore can increase memory usage on the broker. Note that the console consumer currently enables offset commit by default and can be the source of a large number of offsets which this change will now preserve for 7 days instead of 1. You can preserve the existing behavior by setting the broker config <code>offsets.retention.minutes</code> to 1440.</li>
    <li>Support for Java 7 has been dropped, Java 8 is now the minimum version required.</li>
    <li> The default value for <code>ssl.endpoint.identification.algorithm</code> was changed to <code>https</code>, which performs hostname verification (man-in-the-middle attacks are possible otherwise). Set <code>ssl.endpoint.identification.algorithm</code> to an empty string to restore the previous behaviour. </li>
    <li><a href="https://issues.apache.org/jira/browse/KAFKA-5674">KAFKA-5674</a> extends the lower interval of <code>max.connections.per.ip</code> minimum to zero and therefore allows IP-based filtering of inbound connections.</li>
    <li><a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-272%3A+Add+API+version+tag+to+broker%27s+RequestsPerSec+metric">KIP-272</a>
        added API version tag to the metric <code>kafka.network:type=RequestMetrics,name=RequestsPerSec,request={Produce|FetchConsumer|FetchFollower|...}</code>.
        This metric now becomes <code>kafka.network:type=RequestMetrics,name=RequestsPerSec,request={Produce|FetchConsumer|FetchFollower|...},version={0|1|2|3|...}</code>. This will impact
        JMX monitoring tools that do not automatically aggregate. To get the total count for a specific request type, the tool needs to be
        updated to aggregate across different versions.
    </li>
    <li><a href="https://cwiki.apache.org/confluence/x/uaBzB">KIP-225</a> changed the metric "records.lag" to use tags for topic and partition. The original version with the name format "{topic}-{partition}.records-lag" has been removed.</li>
    <li>The Scala consumers, which have been deprecated since 0.11.0.0, have been removed. The Java consumer has been the recommended option
        since 0.10.0.0. Note that the Scala consumers in 1.1.0 (and older) will continue to work even if the brokers are upgraded to 2.0.0.</li>
    <li>The Scala producers, which have been deprecated since 0.10.0.0, have been removed. The Java producer has been the recommended option
        since 0.9.0.0. Note that the behaviour of the default partitioner in the Java producer differs from the default partitioner
        in the Scala producers. Users migrating should consider configuring a custom partitioner that retains the previous behaviour.
        Note that the Scala producers in 1.1.0 (and older) will continue to work even if the brokers are upgraded to 2.0.0.</li>
    <li>MirrorMaker and ConsoleConsumer no longer support the Scala consumer, they always use the Java consumer.</li>
    <li>The ConsoleProducer no longer supports the Scala producer, it always uses the Java producer.</li>
    <li>A number of deprecated tools that rely on the Scala clients have been removed: ReplayLogProducer, SimpleConsumerPerformance, SimpleConsumerShell, ExportZkOffsets, ImportZkOffsets, UpdateOffsetsInZK, VerifyConsumerRebalance.</li>
    <li>The deprecated kafka.tools.ProducerPerformance has been removed, please use org.apache.kafka.tools.ProducerPerformance.</li>
    <li>New Kafka Streams configuration parameter <code>upgrade.from</code> added that allows rolling bounce upgrade from older version. </li>
    <li><a href="https://cwiki.apache.org/confluence/x/DVyHB">KIP-284</a> changed the retention time for Kafka Streams repartition topics by setting its default value to <code>Long.MAX_VALUE</code>.</li>
    <li>Updated <code>ProcessorStateManager</code> APIs in Kafka Streams for registering state stores to the processor topology. For more details please read the Streams <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_200">Upgrade Guide</a>.</li>
    <li>
        In earlier releases, Connect's worker configuration required the <code>internal.key.converter</code> and <code>internal.value.converter</code> properties.
        In 2.0, these are <a href="https://cwiki.apache.org/confluence/x/AZQ7B">no longer required</a> and default to the JSON converter.
        You may safely remove these properties from your Connect standalone and distributed worker configurations:<br />
        <code>internal.key.converter=org.apache.kafka.connect.json.JsonConverter</code>
        <code>internal.key.converter.schemas.enable=false</code>
        <code>internal.value.converter=org.apache.kafka.connect.json.JsonConverter</code>
        <code>internal.value.converter.schemas.enable=false</code>
    </li>
    <li><a href="https://cwiki.apache.org/confluence/x/5kiHB">KIP-266</a> adds a new consumer configuration <code>default.api.timeout.ms</code>
        to specify the default timeout to use for <code>KafkaConsumer</code> APIs that could block. The KIP also adds overloads for such blocking
        APIs to support specifying a specific timeout to use for each of them instead of using the default timeout set by <code>default.api.timeout.ms</code>.
        In particular, a new <code>poll(Duration)</code> API has been added which does not block for dynamic partition assignment.
        The old <code>poll(long)</code> API has been deprecated and will be removed in a future version. Overloads have also been added
        for other <code>KafkaConsumer</code> methods like <code>partitionsFor</code>, <code>listTopics</code>, <code>offsetsForTimes</code>,
        <code>beginningOffsets</code>, <code>endOffsets</code> and <code>close</code> that take in a <code>Duration</code>.</li>
    <li>Also as part of KIP-266, the default value of <code>request.timeout.ms</code> has been changed to 30 seconds.
        The previous value was a little higher than 5 minutes to account for maximum time that a rebalance would take.
        Now we treat the JoinGroup request in the rebalance as a special case and use a value derived from
        <code>max.poll.interval.ms</code> for the request timeout. All other request types use the timeout defined
        by <code>request.timeout.ms</code></li>
    <li>The internal method <code>kafka.admin.AdminClient.deleteRecordsBefore</code> has been removed. Users are encouraged to migrate to <code>org.apache.kafka.clients.admin.AdminClient.deleteRecords</code>.</li>
    <li>The AclCommand tool <code>--producer</code> convenience option uses the <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-277+-+Fine+Grained+ACL+for+CreateTopics+API">KIP-277</a> finer grained ACL on the given topic. </li>
    <li><a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-176%3A+Remove+deprecated+new-consumer+option+for+tools">KIP-176</a> removes
        the <code>--new-consumer</code> option for all consumer based tools. This option is redundant since the new consumer is automatically
        used if --bootstrap-server is defined.
    </li>
    <li><a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-290%3A+Support+for+Prefixed+ACLs">KIP-290</a> adds the ability
        to define ACLs on prefixed resources, e.g. any topic starting with 'foo'.</li>
    <li><a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-283%3A+Efficient+Memory+Usage+for+Down-Conversion">KIP-283</a> improves message down-conversion
        handling on Kafka broker, which has typically been a memory-intensive operation. The KIP adds a mechanism by which the operation becomes less memory intensive
        by down-converting chunks of partition data at a time which helps put an upper bound on memory consumption. With this improvement, there is a change in
        <code>FetchResponse</code> protocol behavior where the broker could send an oversized message batch towards the end of the response with an invalid offset.
        Such oversized messages must be ignored by consumer clients, as is done by <code>KafkaConsumer</code>.
    <p>KIP-283 also adds new topic and broker configurations <code>message.downconversion.enable</code> and <code>log.message.downconversion.enable</code> respectively
       to control whether down-conversion is enabled. When disabled, broker does not perform any down-conversion and instead sends an <code>UNSUPPORTED_VERSION</code>
       error to the client.</p></li>
    <li>Dynamic broker configuration options can be stored in ZooKeeper using kafka-configs.sh before brokers are started.
        This option can be used to avoid storing clear passwords in server.properties as all password configs may be stored encrypted in ZooKeeper.</li>
    <li>ZooKeeper hosts are now re-resolved if connection attempt fails. But if your ZooKeeper host names resolve
    to multiple addresses and some of them are not reachable, then you may need to increase the connection timeout
    <code>zookeeper.connection.timeout.ms</code>.</li>
</ul>

<h5><a id="upgrade_200_new_protocols" href="#upgrade_200_new_protocols">New Protocol Versions</a></h5>
<ul>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-279%3A+Fix+log+divergence+between+leader+and+follower+after+fast+leader+fail+over">KIP-279</a>: OffsetsForLeaderEpochResponse v1 introduces a partition-level <code>leader_epoch</code> field. </li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-219+-+Improve+quota+communication">KIP-219</a>: Bump up the protocol versions of non-cluster action requests and responses that are throttled on quota violation.</li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-290%3A+Support+for+Prefixed+ACLs">KIP-290</a>: Bump up the protocol versions ACL create, describe and delete requests and responses.</li>
</ul>


<h5><a id="upgrade_200_streams_from_11" href="#upgrade_200_streams_from_11">Upgrading a 1.1 Kafka Streams Application</a></h5>
<ul>
    <li> Upgrading your Streams application from 1.1 to 2.0 does not require a broker upgrade.
         A Kafka Streams 2.0 application can connect to 2.0, 1.1, 1.0, 0.11.0, 0.10.2 and 0.10.1 brokers (it is not possible to connect to 0.10.0 brokers though). </li>
    <li> Note that in 2.0 we have removed the public APIs that are deprecated prior to 1.0; users leveraging on those deprecated APIs need to make code changes accordingly.
         See <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_200">Streams API changes in 2.0.0</a> for more details. </li>
</ul>

<h4><a id="upgrade_1_1_0" href="#upgrade_1_1_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x, 0.11.0.x, or 1.0.x to 1.1.x</a></h4>
<p>Kafka 1.1.0 introduces wire protocol changes. By following the recommended rolling upgrade plan below,
    you guarantee no downtime during the upgrade. However, please review the <a href="#upgrade_110_notable">notable changes in 1.1.0</a> before upgrading.
</p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.8.2, 0.9.0, 0.10.0, 0.10.1, 0.10.2, 0.11.0, 1.0).</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from 0.11.0.x or 1.0.x and you have not overridden the message format, then you only need to override
        the inter-broker protocol format.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (0.11.0 or 1.0).</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. </li>
    <li> Once the entire cluster is upgraded, bump the protocol version by editing <code>inter.broker.protocol.version</code> and setting it to 1.1.
    <li> Restart the brokers one by one for the new protocol version to take effect. </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 1.1 on each broker and restart them one by one. Note that the older Scala consumer
        does not support the new message format introduced in 0.11, so to avoid the performance cost of down-conversion (or to
        take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>), the newer Java consumer must be used.</li>
</ol>

<p><b>Additional Upgrade Notes:</b></p>

<ol>
    <li>If you are willing to accept downtime, you can simply take all the brokers down, update the code and start them back up. They will start
        with the new protocol by default.</li>
    <li>Bumping the protocol version and restarting can be done any time after the brokers are upgraded. It does not have to be immediately after.
        Similarly for the message format version.</li>
    <li>If you are using Java8 method references in your Kafka Streams code you might need to update your code to resolve method ambiguties.
        Hot-swapping the jar-file only might not work.</li>
</ol>

<h5><a id="upgrade_111_notable" href="#upgrade_111_notable">Notable changes in 1.1.1</a></h5>
<ul>
    <li> New Kafka Streams configuration parameter <code>upgrade.from</code> added that allows rolling bounce upgrade from version 0.10.0.x </li>
    <li> See the <a href="/{{version}}/documentation/streams/upgrade-guide.html"><b>Kafka Streams upgrade guide</b></a> for details about this new config.
</ul>

<h5><a id="upgrade_110_notable" href="#upgrade_110_notable">Notable changes in 1.1.0</a></h5>
<ul>
    <li>The kafka artifact in Maven no longer depends on log4j or slf4j-log4j12. Similarly to the kafka-clients artifact, users
        can now choose the logging back-end by including the appropriate slf4j module (slf4j-log4j12, logback, etc.). The release
        tarball still includes log4j and slf4j-log4j12.</li>
    <li><a href="https://cwiki.apache.org/confluence/x/uaBzB">KIP-225</a> changed the metric "records.lag" to use tags for topic and partition. The original version with the name format "{topic}-{partition}.records-lag" is deprecated and will be removed in 2.0.0.</li>
    <li>Kafka Streams is more robust against broker communication errors. Instead of stopping the Kafka Streams client with a fatal exception,
	Kafka Streams tries to self-heal and reconnect to the cluster. Using the new <code>AdminClient</code> you have better control of how often
	Kafka Streams retries and can <a href="/{{version}}/documentation/streams/developer-guide/config-streams">configure</a>
	fine-grained timeouts (instead of hard coded retries as in older version).</li>
    <li>Kafka Streams rebalance time was reduced further making Kafka Streams more responsive.</li>
    <li>Kafka Connect now supports message headers in both sink and source connectors, and to manipulate them via simple message transforms. Connectors must be changed to explicitly use them. A new <code>HeaderConverter</code> is introduced to control how headers are (de)serialized, and the new "SimpleHeaderConverter" is used by default to use string representations of values.</li>
    <li>kafka.tools.DumpLogSegments now automatically sets deep-iteration option if print-data-log is enabled
        explicitly or implicitly due to any of the other options like decoder.</li>
</ul>

<h5><a id="upgrade_110_new_protocols" href="#upgrade_110_new_protocols">New Protocol Versions</a></h5>
<ul>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-226+-+Dynamic+Broker+Configuration">KIP-226</a> introduced DescribeConfigs Request/Response v1.</li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-227%3A+Introduce+Incremental+FetchRequests+to+Increase+Partition+Scalability">KIP-227</a> introduced Fetch Request/Response v7.</li>
</ul>

<h5><a id="upgrade_110_streams_from_10" href="#upgrade_110_streams_from_10">Upgrading a 1.0 Kafka Streams Application</a></h5>
<ul>
    <li> Upgrading your Streams application from 1.0 to 1.1 does not require a broker upgrade.
        A Kafka Streams 1.1 application can connect to 1.0, 0.11.0, 0.10.2 and 0.10.1 brokers (it is not possible to connect to 0.10.0 brokers though). </li>
    <li> See <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_110">Streams API changes in 1.1.0</a> for more details. </li>
</ul>

<h4><a id="upgrade_1_0_0" href="#upgrade_1_0_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x or 0.11.0.x to 1.0.0</a></h4>
<p>Kafka 1.0.0 introduces wire protocol changes. By following the recommended rolling upgrade plan below,
    you guarantee no downtime during the upgrade. However, please review the <a href="#upgrade_100_notable">notable changes in 1.0.0</a> before upgrading.
</p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.8.2, 0.9.0, 0.10.0, 0.10.1, 0.10.2, 0.11.0).</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
		following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
	If you are upgrading from 0.11.0.x and you have not overridden the message format, you must set
	both the message format version and the inter-broker protocol version to 0.11.0.
        <ul>
            <li>inter.broker.protocol.version=0.11.0</li>
            <li>log.message.format.version=0.11.0</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. </li>
    <li> Once the entire cluster is upgraded, bump the protocol version by editing <code>inter.broker.protocol.version</code> and setting it to 1.0.
    <li> Restart the brokers one by one for the new protocol version to take effect. </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 1.0 on each broker and restart them one by one. If you are upgrading from
        0.11.0 and log.message.format.version is set to 0.11.0, you can update the config and skip the rolling restart.
        Note that the older Scala consumer does not support the new message format introduced in 0.11, so to avoid the
        performance cost of down-conversion (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java consumer must be used.</li>
</ol>

<p><b>Additional Upgrade Notes:</b></p>

<ol>
    <li>If you are willing to accept downtime, you can simply take all the brokers down, update the code and start them back up. They will start
        with the new protocol by default.</li>
    <li>Bumping the protocol version and restarting can be done any time after the brokers are upgraded. It does not have to be immediately after.
        Similarly for the message format version.</li>
</ol>

<h5><a id="upgrade_102_notable" href="#upgrade_102_notable">Notable changes in 1.0.2</a></h5>
<ul>
    <li> New Kafka Streams configuration parameter <code>upgrade.from</code> added that allows rolling bounce upgrade from version 0.10.0.x </li>
    <li> See the <a href="/{{version}}/documentation/streams/upgrade-guide.html"><b>Kafka Streams upgrade guide</b></a> for details about this new config.
</ul>

<h5><a id="upgrade_101_notable" href="#upgrade_101_notable">Notable changes in 1.0.1</a></h5>
<ul>
    <li>Restored binary compatibility of AdminClient's Options classes (e.g. CreateTopicsOptions, DeleteTopicsOptions, etc.) with
        0.11.0.x. Binary (but not source) compatibility had been broken inadvertently in 1.0.0.</li>
</ul>

<h5><a id="upgrade_100_notable" href="#upgrade_100_notable">Notable changes in 1.0.0</a></h5>
<ul>
    <li>Topic deletion is now enabled by default, since the functionality is now stable. Users who wish to
        to retain the previous behavior should set the broker config <code>delete.topic.enable</code> to <code>false</code>. Keep in mind that topic deletion removes data and the operation is not reversible (i.e. there is no "undelete" operation)</li>
    <li>For topics that support timestamp search if no offset can be found for a partition, that partition is now included in the search result with a null offset value. Previously, the partition was not included in the map.
        This change was made to make the search behavior consistent with the case of topics not supporting timestamp search.
    <li>If the <code>inter.broker.protocol.version</code> is 1.0 or later, a broker will now stay online to serve replicas
        on live log directories even if there are offline log directories. A log directory may become offline due to IOException
        caused by hardware failure. Users need to monitor the per-broker metric <code>offlineLogDirectoryCount</code> to check
        whether there is offline log directory. </li>
    <li>Added KafkaStorageException which is a retriable exception. KafkaStorageException will be converted to NotLeaderForPartitionException in the response
        if the version of client's FetchRequest or ProducerRequest does not support KafkaStorageException. </li>
    <li>-XX:+DisableExplicitGC was replaced by -XX:+ExplicitGCInvokesConcurrent in the default JVM settings. This helps
        avoid out of memory exceptions during allocation of native memory by direct buffers in some cases.</li>
    <li>The overridden <code>handleError</code> method implementations have been removed from the following deprecated classes in
        the <code>kafka.api</code> package: <code>FetchRequest</code>, <code>GroupCoordinatorRequest</code>, <code>OffsetCommitRequest</code>,
        <code>OffsetFetchRequest</code>, <code>OffsetRequest</code>, <code>ProducerRequest</code>, and <code>TopicMetadataRequest</code>.
        This was only intended for use on the broker, but it is no longer in use and the implementations have not been maintained.
        A stub implementation has been retained for binary compatibility.</li>
    <li>The Java clients and tools now accept any string as a client-id.</li>
    <li>The deprecated tool <code>kafka-consumer-offset-checker.sh</code> has been removed. Use <code>kafka-consumer-groups.sh</code> to get consumer group details.</li>
    <li>SimpleAclAuthorizer now logs access denials to the authorizer log by default.</li>
    <li>Authentication failures are now reported to clients as one of the subclasses of <code>AuthenticationException</code>.
        No retries will be performed if a client connection fails authentication.</li>
    <li>Custom <code>SaslServer</code> implementations may throw <code>SaslAuthenticationException</code> to provide an error
        message to return to clients indicating the reason for authentication failure. Implementors should take care not to include
        any security-critical information in the exception message that should not be leaked to unauthenticated clients.</li>
    <li>The <code>app-info</code> mbean registered with JMX to provide version and commit id will be deprecated and replaced with
        metrics providing these attributes.</li>
    <li>Kafka metrics may now contain non-numeric values. <code>org.apache.kafka.common.Metric#value()</code> has been deprecated and
        will return <code>0.0</code> in such cases to minimise the probability of breaking users who read the value of every client
        metric (via a <code>MetricsReporter</code> implementation or by calling the <code>metrics()</code> method).
        <code>org.apache.kafka.common.Metric#metricValue()</code> can be used to retrieve numeric and non-numeric metric values.</li>
    <li>Every Kafka rate metric now has a corresponding cumulative count metric with the suffix <code>-total</code>
        to simplify downstream processing. For example, <code>records-consumed-rate</code> has a corresponding
        metric named <code>records-consumed-total</code>.</li>
    <li>Mx4j will only be enabled if the system property <code>kafka_mx4jenable</code> is set to <code>true</code>. Due to a logic
        inversion bug, it was previously enabled by default and disabled if <code>kafka_mx4jenable</code> was set to <code>true</code>.</li>
    <li>The package <code>org.apache.kafka.common.security.auth</code> in the clients jar has been made public and added to the javadocs.
        Internal classes which had previously been located in this package have been moved elsewhere.</li>
    <li>When using an Authorizer and a user doesn't have required permissions on a topic, the broker
        will return TOPIC_AUTHORIZATION_FAILED errors to requests irrespective of topic existence on broker.
        If the user have required permissions and the topic doesn't exists, then the UNKNOWN_TOPIC_OR_PARTITION
        error code will be returned. </li>
    <li>config/consumer.properties file updated to use new consumer config properties.</li>
</ul>

<h5><a id="upgrade_100_new_protocols" href="#upgrade_100_new_protocols">New Protocol Versions</a></h5>
<ul>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-112%3A+Handle+disk+failure+for+JBOD">KIP-112</a>: LeaderAndIsrRequest v1 introduces a partition-level <code>is_new</code> field. </li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-112%3A+Handle+disk+failure+for+JBOD">KIP-112</a>: UpdateMetadataRequest v4 introduces a partition-level <code>offline_replicas</code> field. </li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-112%3A+Handle+disk+failure+for+JBOD">KIP-112</a>: MetadataResponse v5 introduces a partition-level <code>offline_replicas</code> field. </li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-112%3A+Handle+disk+failure+for+JBOD">KIP-112</a>: ProduceResponse v4 introduces error code for KafkaStorageException. </li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-112%3A+Handle+disk+failure+for+JBOD">KIP-112</a>: FetchResponse v6 introduces error code for KafkaStorageException. </li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-152+-+Improve+diagnostics+for+SASL+authentication+failures">KIP-152</a>:
         SaslAuthenticate request has been added to enable reporting of authentication failures. This request will
         be used if the SaslHandshake request version is greater than 0. </li>
</ul>

<h5><a id="upgrade_100_streams_from_0110" href="#upgrade_100_streams_from_0110">Upgrading a 0.11.0 Kafka Streams Application</a></h5>
<ul>
    <li> Upgrading your Streams application from 0.11.0 to 1.0 does not require a broker upgrade.
         A Kafka Streams 1.0 application can connect to 0.11.0, 0.10.2 and 0.10.1 brokers (it is not possible to connect to 0.10.0 brokers though).
         However, Kafka Streams 1.0 requires 0.10 message format or newer and does not work with older message formats. </li>
    <li> If you are monitoring on streams metrics, you will need make some changes to the metrics names in your reporting and monitoring code, because the metrics sensor hierarchy was changed. </li>
    <li> There are a few public APIs including <code>ProcessorContext#schedule()</code>, <code>Processor#punctuate()</code> and <code>KStreamBuilder</code>, <code>TopologyBuilder</code> are being deprecated by new APIs.
         We recommend making corresponding code changes, which should be very minor since the new APIs look quite similar, when you upgrade.
    <li> See <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_100">Streams API changes in 1.0.0</a> for more details. </li>
</ul>

<h5><a id="upgrade_100_streams_from_0102" href="#upgrade_100_streams_from_0102">Upgrading a 0.10.2 Kafka Streams Application</a></h5>
<ul>
    <li> Upgrading your Streams application from 0.10.2 to 1.0 does not require a broker upgrade.
         A Kafka Streams 1.0 application can connect to 1.0, 0.11.0, 0.10.2 and 0.10.1 brokers (it is not possible to connect to 0.10.0 brokers though). </li>
    <li> If you are monitoring on streams metrics, you will need make some changes to the metrics names in your reporting and monitoring code, because the metrics sensor hierarchy was changed. </li>
    <li> There are a few public APIs including <code>ProcessorContext#schedule()</code>, <code>Processor#punctuate()</code> and <code>KStreamBuilder</code>, <code>TopologyBuilder</code> are being deprecated by new APIs.
         We recommend making corresponding code changes, which should be very minor since the new APIs look quite similar, when you upgrade.
    <li> If you specify customized <code>key.serde</code>, <code>value.serde</code> and <code>timestamp.extractor</code> in configs, it is recommended to use their replaced configure parameter as these configs are deprecated. </li>
    <li> See <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_0110">Streams API changes in 0.11.0</a> for more details. </li>
</ul>

<h5><a id="upgrade_100_streams_from_0101" href="#upgrade_1100_streams_from_0101">Upgrading a 0.10.1 Kafka Streams Application</a></h5>
<ul>
    <li> Upgrading your Streams application from 0.10.1 to 1.0 does not require a broker upgrade.
         A Kafka Streams 1.0 application can connect to 1.0, 0.11.0, 0.10.2 and 0.10.1 brokers (it is not possible to connect to 0.10.0 brokers though). </li>
    <li> You need to recompile your code. Just swapping the Kafka Streams library jar file will not work and will break your application. </li>
    <li> If you are monitoring on streams metrics, you will need make some changes to the metrics names in your reporting and monitoring code, because the metrics sensor hierarchy was changed. </li>
    <li> There are a few public APIs including <code>ProcessorContext#schedule()</code>, <code>Processor#punctuate()</code> and <code>KStreamBuilder</code>, <code>TopologyBuilder</code> are being deprecated by new APIs.
         We recommend making corresponding code changes, which should be very minor since the new APIs look quite similar, when you upgrade.
    <li> If you specify customized <code>key.serde</code>, <code>value.serde</code> and <code>timestamp.extractor</code> in configs, it is recommended to use their replaced configure parameter as these configs are deprecated. </li>
    <li> If you use a custom (i.e., user implemented) timestamp extractor, you will need to update this code, because the <code>TimestampExtractor</code> interface was changed. </li>
    <li> If you register custom metrics, you will need to update this code, because the <code>StreamsMetric</code> interface was changed. </li>
    <li> See <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_100">Streams API changes in 1.0.0</a>,
         <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_0110">Streams API changes in 0.11.0</a> and
         <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_0102">Streams API changes in 0.10.2</a> for more details. </li>
</ul>

<h5><a id="upgrade_100_streams_from_0100" href="#upgrade_100_streams_from_0100">Upgrading a 0.10.0 Kafka Streams Application</a></h5>
<ul>
    <li> Upgrading your Streams application from 0.10.0 to 1.0 does require a <a href="#upgrade_10_1">broker upgrade</a> because a Kafka Streams 1.0 application can only connect to 0.1, 0.11.0, 0.10.2, or 0.10.1 brokers. </li>
    <li> There are couple of API changes, that are not backward compatible (cf. <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_100">Streams API changes in 1.0.0</a>,
         <a href="/{{version}}/documentation/streams#streams_api_changes_0110">Streams API changes in 0.11.0</a>,
         <a href="/{{version}}/documentation/streams#streams_api_changes_0102">Streams API changes in 0.10.2</a>, and
         <a href="/{{version}}/documentation/streams#streams_api_changes_0101">Streams API changes in 0.10.1</a> for more details).
         Thus, you need to update and recompile your code. Just swapping the Kafka Streams library jar file will not work and will break your application. </li>
    <li> Upgrading from 0.10.0.x to 1.0.2 requires two rolling bounces with config <code>upgrade.from="0.10.0"</code> set for first upgrade phase
        (cf. <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-268%3A+Simplify+Kafka+Streams+Rebalance+Metadata+Upgrade">KIP-268</a>).
        As an alternative, an offline upgrade is also possible.
        <ul>
            <li> prepare your application instances for a rolling bounce and make sure that config <code>upgrade.from</code> is set to <code>"0.10.0"</code> for new version 0.11.0.3 </li>
            <li> bounce each instance of your application once </li>
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<p><b>If you are upgrading from a version prior to 2.1.x, please see the note below about the change to the schema used to store consumer offsets.
    Once you have changed the inter.broker.protocol.version to the latest version, it will not be possible to downgrade to a version prior to 2.1.</b></p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g., <code>2.6</code>, <code>2.5</code>, etc.)</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from version 0.11.0.x or above, and you have not overridden the message format, then you only need to override
        the inter-broker protocol version.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g., <code>2.6</code>, <code>2.5</code>, etc.)</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. Once you have done so, the
        brokers will be running the latest version and you can verify that the cluster's behavior and performance meets expectations.
        It is still possible to downgrade at this point if there are any problems.
    </li>
    <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
        <code>inter.broker.protocol.version</code> and setting it to <code>2.7</code>.
    </li>
    <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
        protocol version, it will no longer be possible to downgrade the cluster to an older version.
    </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.7 on each broker and restart them one by one. Note that the older Scala clients,
        which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs
        (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java clients must be used.
    </li>
</ol>

<h5><a id="upgrade_270_notable" href="#upgrade_270_notable">Notable changes in 2.7.0</a></h5>
<ul>
    <li>
        The 2.7.0 release includes the core Raft implementation specified in
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-595%3A+A+Raft+Protocol+for+the+Metadata+Quorum">KIP-595</a>.
        There is a separate "raft" module containing most of the logic. Until integration with the
        controller is complete, there is a standalone server that users can use for testing the p
        erformance of the Raft implementation.  See the README.md in the raft module for details
    </li>
    <li>
        KIP-651 <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-651+-+Support+PEM+format+for+SSL+certificates+and+private+key">adds support</a>
        for using PEM files for key and trust stores.
    </li>
    <li>
        KIP-612 <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-612%3A+Ability+to+Limit+Connection+Creation+Rate+on+Brokers">adds support</a>
        for enforcing broker-wide and per-listener connection create rates.  The 2.7.0 release contains
        the first part of KIP-612 with dynamic configuration coming in the 2.8.0 release.
    </li>
    <li>
        The ability to throttle topic and partition creations or
        topics deletions to prevent a cluster from being harmed via
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-599%3A+Throttle+Create+Topic%2C+Create+Partition+and+Delete+Topic+Operations">KIP-599</a>
    </li>
    <li>
        When new features become available in Kafka there are two main issues:
        <ol>
            <li>How do Kafka clients become aware of broker capabilities?</li>
            <li>How does the broker decide which features to enable?</li>
        </ol>
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-584%3A+Versioning+scheme+for+features">KIP-584</a>
        provides a flexible and operationally easy solution for client discovery, feature gating and rolling upgrades using a single restart.
    </li>
    <li>
        The ability to print record offsets and headers with the <code>ConsoleConsumer</code> is now possible
        via <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-431%3A+Support+of+printing+additional+ConsumerRecord+fields+in+DefaultMessageFormatter">KIP-431</a>
    </li>
    <li>
      The addition of <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-554%3A+Add+Broker-side+SCRAM+Config+API">KIP-554</a>
        continues progress towards the goal of Zookeeper removal from Kafka. The addition of KIP-554
        means you don't have to connect directly to ZooKeeper anymore for managing SCRAM credentials.
    </li>
    <li>Altering non-reconfigurable configs of existent listeners causes <code>InvalidRequestException</code>.
        By contrast, the previous (unintended) behavior would have caused the updated configuration to be persisted,
        but it wouldn't
        take effect until the broker was restarted. See <a href="https://github.com/apache/kafka/pull/9284">KAFKA-10479</a> for more discussion.
        See <code>DynamicBrokerConfig.DynamicSecurityConfigs</code> and <code>SocketServer.ListenerReconfigurableConfigs</code>
        for the supported reconfigurable configs of existent listeners.
    </li>

    <li>
        Kafka Streams adds support for
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-450%3A+Sliding+Window+Aggregations+in+the+DSL">Sliding Windows Aggregations</a>
        in the KStreams DSL.
    </li>
    <li>
        Reverse iteration over state stores enabling more efficient most recent update searches with
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-617%3A+Allow+Kafka+Streams+State+Stores+to+be+iterated+backwards">KIP-617</a>
    </li>
    <li>
        End-to-End latency metrics in Kafka Steams see
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-613%3A+Add+end-to-end+latency+metrics+to+Streams">KIP-613</a>
        for more details
    </li>
    <li>
        Kafka Streams added metrics reporting default RocksDB properties with
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-607%3A+Add+Metrics+to+Kafka+Streams+to+Report+Properties+of+RocksDB">KIP-607</a>
    </li>
    <li>
        Better Scala implicit Serdes support from
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-616%3A+Rename+implicit+Serdes+instances+in+kafka-streams-scala">KIP-616</a>
    </li>
</ul>
<h4><a id="upgrade_2_6_0" href="#upgrade_2_6_0">Upgrading to 2.6.0 from any version 0.8.x through 2.5.x</a></h4>

<p><b>If you are upgrading from a version prior to 2.1.x, please see the note below about the change to the schema used to store consumer offsets.
    Once you have changed the inter.broker.protocol.version to the latest version, it will not be possible to downgrade to a version prior to 2.1.</b></p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g., <code>2.5</code>, <code>2.4</code>, etc.)</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from version 0.11.0.x or above, and you have not overridden the message format, then you only need to override
        the inter-broker protocol version.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g., <code>2.5</code>, <code>2.4</code>, etc.)</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. Once you have done so, the
        brokers will be running the latest version and you can verify that the cluster's behavior and performance meets expectations.
        It is still possible to downgrade at this point if there are any problems.
    </li>
    <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
        <code>inter.broker.protocol.version</code> and setting it to <code>2.6</code>.
    </li>
    <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
        protocol version, it will no longer be possible to downgrade the cluster to an older version.
    </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.6 on each broker and restart them one by one. Note that the older Scala clients,
        which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs
        (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java clients must be used.
    </li>
</ol>

<h5 class="anchor-heading"><a id="upgrade_260_notable" class="anchor-link"></a><a href="#upgrade_260_notable">Notable changes in 2.6.0</a></h5>
<ul>
    <li>Kafka Streams adds a new processing mode (requires broker 2.5 or newer) that improves application
        scalability using exactly-once guarantees
        (cf. <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-447%3A+Producer+scalability+for+exactly+once+semantics">KIP-447</a>)
    </li>
    <li>TLSv1.3 has been enabled by default for Java 11 or newer. The client and server will negotiate TLSv1.3 if
        both support it and fallback to TLSv1.2 otherwise. See
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-573%3A+Enable+TLSv1.3+by+default">KIP-573</a> for more details.
    </li>
    <li>The default value for the <code>client.dns.lookup</code> configuration has been changed from <code>default</code>
        to <code>use_all_dns_ips</code>. If a hostname resolves to multiple IP addresses, clients and brokers will now
        attempt to connect to each IP in sequence until the connection is successfully established. See
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-602%3A+Change+default+value+for+client.dns.lookup">KIP-602</a>
        for more details.
    </li>
    <li><code>NotLeaderForPartitionException</code> has been deprecated and replaced with <code>NotLeaderOrFollowerException</code>.
        Fetch requests and other requests intended only for the leader or follower return NOT_LEADER_OR_FOLLOWER(6) instead of REPLICA_NOT_AVAILABLE(9)
        if the broker is not a replica, ensuring that this transient error during reassignments is handled by all clients as a retriable exception.
    </li>
</ul>

<h4><a id="upgrade_2_5_0" href="#upgrade_2_5_0">Upgrading to 2.5.0 from any version 0.8.x through 2.4.x</a></h4>

<p><b>If you are upgrading from a version prior to 2.1.x, please see the note below about the change to the schema used to store consumer offsets.
    Once you have changed the inter.broker.protocol.version to the latest version, it will not be possible to downgrade to a version prior to 2.1.</b></p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g., <code>2.4</code>, <code>2.3</code>, etc.)</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from version 0.11.0.x or above, and you have not overridden the message format, then you only need to override
        the inter-broker protocol version.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g., <code>2.4</code>, <code>2.3</code>, etc.)</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. Once you have done so, the
        brokers will be running the latest version and you can verify that the cluster's behavior and performance meets expectations.
        It is still possible to downgrade at this point if there are any problems.
    </li>
    <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
        <code>inter.broker.protocol.version</code> and setting it to <code>2.5</code>.
    </li>
    <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
        protocol version, it will no longer be possible to downgrade the cluster to an older version.
    </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.5 on each broker and restart them one by one. Note that the older Scala clients,
        which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs
        (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java clients must be used.
    </li>

    <li>There are several notable changes to the reassignment tool <code>kafka-reassign-partitions.sh</code>
        following the completion of
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-455%3A+Create+an+Administrative+API+for+Replica+Reassignment">KIP-455</a>.
        This tool now requires the <code>--additional</code> flag to be provided when changing the throttle of an
        active reassignment. Reassignment cancellation is now possible using the
        <code>--cancel</code> command. Finally, reassignment with <code>--zookeeper</code>
        has been deprecated in favor of <code>--bootstrap-server</code>. See the KIP for more detail.
    </li>
</ol>

<h5 class="anchor-heading"><a id="upgrade_250_notable" class="anchor-link"></a><a href="#upgrade_250_notable">Notable changes in 2.5.0</a></h5>
<ul>
    <li>When <code>RebalanceProtocol#COOPERATIVE</code> is used, <code>Consumer#poll</code> can still return data
        while it is in the middle of a rebalance for those partitions still owned by the consumer; in addition
        <code>Consumer#commitSync</code> now may throw a non-fatal <code>RebalanceInProgressException</code> to notify
        users of such an event, in order to distinguish from the fatal <code>CommitFailedException</code> and allow
        users to complete the ongoing rebalance and then reattempt committing offsets for those still-owned partitions.</li>
    <li>For improved resiliency in typical network environments, the default value of
        <code>zookeeper.session.timeout.ms</code> has been increased from 6s to 18s and
        <code>replica.lag.time.max.ms</code> from 10s to 30s.</li>
    <li>New DSL operator <code>cogroup()</code> has been added for aggregating multiple streams together at once.</li>
    <li>Added a new <code>KStream.toTable()</code> API to translate an input event stream into a KTable.</li>
    <li>Added a new Serde type <code>Void</code> to represent null keys or null values from input topic.</li>
    <li>Deprecated <code>UsePreviousTimeOnInvalidTimestamp</code> and replaced it with <code>UsePartitionTimeOnInvalidTimeStamp</code>.</li>
    <li>Improved exactly-once semantics by adding a pending offset fencing mechanism and stronger transactional commit
        consistency check, which greatly simplifies the implementation of a scalable exactly-once application.
        We also added a new exactly-once semantics code example under
        <a href="https://github.com/apache/kafka/tree/2.5/examples">examples</a> folder. Check out
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-447%3A+Producer+scalability+for+exactly+once+semantics">KIP-447</a>
        for the full details.</li>
    <li>Added a new public api <code>KafkaStreams.queryMetadataForKey(String, K, Serializer) to get detailed information on the key being queried.
        It provides information about the partition number where the key resides in addition to hosts containing the active and standby partitions for the key.</code></li>
    <li>Provided support to query stale stores (for high availability) and the stores belonging to a specific partition by deprecating <code>KafkaStreams.store(String, QueryableStoreType)</code> and replacing it with <code>KafkaStreams.store(StoreQueryParameters)</code>.</li>
    <li>Added a new public api to access lag information for stores local to an instance with <code>KafkaStreams.allLocalStorePartitionLags()</code>.</li>
    <li>Scala 2.11 is no longer supported. See
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-531%3A+Drop+support+for+Scala+2.11+in+Kafka+2.5">KIP-531</a>
        for details.</li>
    <li>All Scala classes from the package <code>kafka.security.auth</code> have been deprecated. See
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-504+-+Add+new+Java+Authorizer+Interface">KIP-504</a>
        for details of the new Java authorizer API added in 2.4.0.  Note that <code>kafka.security.auth.Authorizer</code>
        and <code>kafka.security.auth.SimpleAclAuthorizer</code> were deprecated in 2.4.0.
    </li>
    <li>TLSv1 and TLSv1.1 have been disabled by default since these have known security vulnerabilities. Only TLSv1.2 is now
        enabled by default. You can continue to use TLSv1 and TLSv1.1 by explicitly enabling these in the configuration options
        <code>ssl.protocol</code> and <code>ssl.enabled.protocols</code>.
    </li>
    <li>ZooKeeper has been upgraded to 3.5.7, and a ZooKeeper upgrade from 3.4.X to 3.5.7 can fail if there are no snapshot files in the 3.4 data directory.
        This usually happens in test upgrades where ZooKeeper 3.5.7 is trying to load an existing 3.4 data dir in which no snapshot file has been created.
        For more details about the issue please refer to <a href="https://issues.apache.org/jira/browse/ZOOKEEPER-3056">ZOOKEEPER-3056</a>.
        A fix is given in <a href="https://issues.apache.org/jira/browse/ZOOKEEPER-3056">ZOOKEEPER-3056</a>, which is to set <code>snapshot.trust.empty=true</code>
        config in <code>zookeeper.properties</code> before the upgrade.
    </li>
    <li>ZooKeeper version 3.5.7 supports TLS-encrypted connectivity to ZooKeeper both with or without client certificates,
        and additional Kafka configurations are available to take advantage of this.
        See <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-515%3A+Enable+ZK+client+to+use+the+new+TLS+supported+authentication">KIP-515</a> for details.
    </li>
</ul>

<h4><a id="upgrade_2_4_0" href="#upgrade_2_4_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x, 0.11.0.x, 1.0.x, 1.1.x, 2.0.x or 2.1.x or 2.2.x or 2.3.x to 2.4.0</a></h4>

<p><b>If you are upgrading from a version prior to 2.1.x, please see the note below about the change to the schema used to store consumer offsets.
    Once you have changed the inter.broker.protocol.version to the latest version, it will not be possible to downgrade to a version prior to 2.1.</b></p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.10.0, 0.11.0, 1.0, 2.0, 2.2).</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from version 0.11.0.x or above, and you have not overridden the message format, then you only need to override
        the inter-broker protocol version.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (0.11.0, 1.0, 1.1, 2.0, 2.1, 2.2, 2.3).</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. Once you have done so, the
        brokers will be running the latest version and you can verify that the cluster's behavior and performance meets expectations.
        It is still possible to downgrade at this point if there are any problems.
    </li>
    <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
        <code>inter.broker.protocol.version</code> and setting it to 2.4.
    </li>
    <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
        protocol version, it will no longer be possible to downgrade the cluster to an older version.
    </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.4 on each broker and restart them one by one. Note that the older Scala clients,
        which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs
        (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java clients must be used.
    </li>
</ol>

<p><b>Additional Upgrade Notes:</b></p>

<ol>
    <li>ZooKeeper has been upgraded to 3.5.6. ZooKeeper upgrade from 3.4.X to 3.5.6 can fail if there are no snapshot files in 3.4 data directory.
        This usually happens in test upgrades where ZooKeeper 3.5.6 is trying to load an existing 3.4 data dir in which no snapshot file has been created.
        For more details about the issue please refer to <a href="https://issues.apache.org/jira/browse/ZOOKEEPER-3056">ZOOKEEPER-3056</a>.
        A fix is given in <a href="https://issues.apache.org/jira/browse/ZOOKEEPER-3056">ZOOKEEPER-3056</a>, which is to set <code>snapshot.trust.empty=true</code>
        config in <code>zookeeper.properties</code> before the upgrade. But we have observed data loss in standalone cluster upgrades when using
        <code>snapshot.trust.empty=true</code> config. For more details about the issue please refer to <a href="https://issues.apache.org/jira/browse/ZOOKEEPER-3644">ZOOKEEPER-3644</a>.
        So we recommend the safe workaround of copying empty <a href="https://issues.apache.org/jira/secure/attachment/12928686/snapshot.0">snapshot</a> file to the 3.4 data directory,
        if there are no snapshot files in 3.4 data directory. For more details about the workaround please refer to <a href="https://cwiki.apache.org/confluence/display/ZOOKEEPER/Upgrade+FAQ">ZooKeeper Upgrade FAQ</a>.
    </li>
    <li>
        An embedded Jetty based <a href="http://zookeeper.apache.org/doc/r3.5.6/zookeeperAdmin.html#sc_adminserver">AdminServer</a> added in ZooKeeper 3.5.
        AdminServer is enabled by default in ZooKeeper and is started on port 8080.
        AdminServer is disabled by default in the ZooKeeper config (<code>zookeeper.properties</code>) provided by the Apache Kafka distribution.
        Make sure to update your local <code>zookeeper.properties</code> file with <code>admin.enableServer=false</code> if you wish to disable the AdminServer.
        Please refer <a href="http://zookeeper.apache.org/doc/r3.5.6/zookeeperAdmin.html#sc_adminserver">AdminServer config</a> to configure the AdminServer.
    </li>
</ol>

<h5><a id="upgrade_240_notable" href="#upgrade_240_notable">Notable changes in 2.4.0</a></h5>
<ul>
    <li>A new Admin API has been added for partition reassignments. Due to changing the way Kafka propagates reassignment information,
        it is possible to lose reassignment state in failure edge cases while upgrading to the new version. It is not recommended to start reassignments while upgrading.</li>
    <li>ZooKeeper has been upgraded from 3.4.14 to 3.5.6. TLS and dynamic reconfiguration are supported by the new version.</li>
    <li>The <code>bin/kafka-preferred-replica-election.sh</code> command line tool has been deprecated. It has been replaced by <code>bin/kafka-leader-election.sh</code>.</li>
    <li>The methods <code>electPreferredLeaders</code> in the Java <code>AdminClient</code> class have been deprecated in favor of the methods <code>electLeaders</code>.</li>
    <li>Scala code leveraging the <code>NewTopic(String, int, short)</code> constructor with literal values will need to explicitly call <code>toShort</code> on the second literal.</li>
    <li>The argument in the constructor <code>GroupAuthorizationException(String)</code> is now used to specify an exception message.
        Previously it referred to the group that failed authorization. This was done for consistency with other exception types and to
        avoid potential misuse. The constructor <code>TopicAuthorizationException(String)</code> which was previously used for a single
        unauthorized topic was changed similarly.
    </li>
    <li>The internal <code>PartitionAssignor</code> interface has been deprecated and replaced with a new <code>ConsumerPartitionAssignor</code> in the public API. Some
        methods/signatures are slightly different between the two interfaces. Users implementing a custom PartitionAssignor should migrate to the new interface as soon as possible.
    </li>
    <li>The <code>DefaultPartitioner</code> now uses a sticky partitioning strategy. This means that records for specific topic with null keys and no assigned partition 
        will be sent to the same partition until the batch is ready to be sent. When a new batch is created, a new partition is chosen. This decreases latency to produce, but
        it may result in uneven distribution of records across partitions in edge cases. Generally users will not be impacted, but this difference may be noticeable in tests and
        other situations producing records for a very short amount of time.
    </li>
    <li>The blocking <code>KafkaConsumer#committed</code> methods have been extended to allow a list of partitions as input parameters rather than a single partition.
        It enables fewer request/response iterations between clients and brokers fetching for the committed offsets for the consumer group.
        The old overloaded functions are deprecated and we would recommend users to make their code changes to leverage the new methods (details
        can be found in <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-520%3A+Add+overloaded+Consumer%23committed+for+batching+partitions">KIP-520</a>).
    </li>
    <li>We've introduced a new <code>INVALID_RECORD</code> error in the produce response to distinguish from the <code>CORRUPT_MESSAGE</code> error.
        To be more concrete, previously when a batch of records were sent as part of a single request to the broker and one or more of the records failed
        the validation due to various causes (mismatch magic bytes, crc checksum errors, null key for log compacted topics, etc), the whole batch would be rejected
        with the same and misleading <code>CORRUPT_MESSAGE</code>, and the caller of the producer client would see the corresponding exception from either
        the future object of <code>RecordMetadata</code> returned from the <code>send</code> call as well as in the <code>Callback#onCompletion(RecordMetadata metadata, Exception exception)</code>
        Now with the new error code and improved error messages of the exception, producer callers would be better informed about the root cause why their sent records were failed.
    </li>
    <li>We are introducing incremental cooperative rebalancing to the clients' group protocol, which allows consumers to keep all of their assigned partitions during a rebalance
        and at the end revoke only those which must be migrated to another consumer for overall cluster balance. The <code>ConsumerCoordinator</code> will choose the latest <code>RebalanceProtocol</code>
        that is commonly supported by all of the consumer's supported assignors. You can use the new built-in <code>CooperativeStickyAssignor</code> or plug in your own custom cooperative assignor. To do
        so you must implement the <code>ConsumerPartitionAssignor</code> interface and include <code>RebalanceProtocol.COOPERATIVE</code> in the list returned by <code>ConsumerPartitionAssignor#supportedProtocols</code>.
        Your custom assignor can then leverage the <code>ownedPartitions</code> field in each consumer's <code>Subscription</code> to give partitions back to their previous owners whenever possible. Note that when
        a partition is to be reassigned to another consumer, it <em>must</em> be removed from the new assignment until it has been revoked from its original owner. Any consumer that has to revoke a partition will trigger
        a followup rebalance to allow the revoked partition to safely be assigned to its new owner. See the
        <a href="https://kafka.apache.org/24/javadoc/index.html?org/apache/kafka/clients/consumer/ConsumerPartitionAssignor.RebalanceProtocol.html">ConsumerPartitionAssignor RebalanceProtocol javadocs</a> for more information.
        <br>
        To upgrade from the old (eager) protocol, which always revokes all partitions before rebalancing, to cooperative rebalancing, you must follow a specific upgrade path to get all clients on the same <code>ConsumerPartitionAssignor</code>
        that supports the cooperative protocol. This can be done with two rolling bounces, using the <code>CooperativeStickyAssignor</code> for the example: during the first one, add "cooperative-sticky" to the list of supported assignors
        for each member (without removing the previous assignor -- note that if previously using the default, you must include that explicitly as well). You then bounce and/or upgrade it.
        Once the entire group is on 2.4+ and all members have the "cooperative-sticky" among their supported assignors, remove the other assignor(s) and perform a second rolling bounce so that by the end all members support only the
        cooperative protocol. For further details on the cooperative rebalancing protocol and upgrade path, see <a href="https://cwiki.apache.org/confluence/x/vAclBg">KIP-429</a>.
    </li>
    <li>There are some behavioral changes to the <code>ConsumerRebalanceListener</code>, as well as a new API. Exceptions thrown during any of the listener's three callbacks will no longer be swallowed, and will instead be re-thrown
        all the way up to the <code>Consumer.poll()</code> call. The <code>onPartitionsLost</code> method has been added to allow users to react to abnormal circumstances where a consumer may have lost ownership of its partitions
        (such as a missed rebalance) and cannot commit offsets. By default, this will simply call the existing <code>onPartitionsRevoked</code> API to align with previous behavior. Note however that <code>onPartitionsLost</code> will not
        be called when the set of lost partitions is empty. This means that no callback will be invoked at the beginning of the first rebalance of a new consumer joining the group.
        <br>
        The semantics of the <code>ConsumerRebalanceListener's</code> callbacks are further changed when following the cooperative rebalancing protocol described above. In addition to <code>onPartitionsLost</code>, <code>onPartitionsRevoked</code>
        will also never be called when the set of revoked partitions is empty. The callback will generally be invoked only at the end of a rebalance, and only on the set of partitions that are being moved to another consumer. The
        <code>onPartitionsAssigned</code> callback will however always be called, even with an empty set of partitions, as a way to notify users of a rebalance event (this is true for both cooperative and eager). For details on
        the new callback semantics, see the <a href="https://kafka.apache.org/24/javadoc/index.html?org/apache/kafka/clients/consumer/ConsumerRebalanceListener.html">ConsumerRebalanceListener javadocs</a>.
    </li>
    <li>The Scala trait <code>kafka.security.auth.Authorizer</code> has been deprecated and replaced with a new Java API
        <code>org.apache.kafka.server.authorizer.Authorizer</code>. The authorizer implementation class
        <code>kafka.security.auth.SimpleAclAuthorizer</code> has also been deprecated and replaced with a new
        implementation <code>kafka.security.authorizer.AclAuthorizer</code>. <code>AclAuthorizer</code> uses features
        supported by the new API to improve authorization logging and is compatible with <code>SimpleAclAuthorizer</code>.
        For more details, see <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-504+-+Add+new+Java+Authorizer+Interface">KIP-504</a>.
    </li>
</ul>

<h4><a id="upgrade_2_3_0" href="#upgrade_2_3_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x, 0.11.0.x, 1.0.x, 1.1.x, 2.0.x or 2.1.x or 2.2.x to 2.3.0</a></h4>

<p><b>If you are upgrading from a version prior to 2.1.x, please see the note below about the change to the schema used to store consumer offsets.
    Once you have changed the inter.broker.protocol.version to the latest version, it will not be possible to downgrade to a version prior to 2.1.</b></p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.8.2, 0.9.0, 0.10.0, 0.10.1, 0.10.2, 0.11.0, 1.0, 1.1).</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from 0.11.0.x, 1.0.x, 1.1.x, 2.0.x, or 2.1.x, and you have not overridden the message format, then you only need to override
        the inter-broker protocol version.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (0.11.0, 1.0, 1.1, 2.0, 2.1, 2.2).</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. Once you have done so, the
        brokers will be running the latest version and you can verify that the cluster's behavior and performance meets expectations.
        It is still possible to downgrade at this point if there are any problems.
    </li>
    <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
        <code>inter.broker.protocol.version</code> and setting it to 2.3.
    </li>
    <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
        protocol version, it will no longer be possible to downgrade the cluster to an older version.
    </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.3 on each broker and restart them one by one. Note that the older Scala clients,
        which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs
        (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java clients must be used.
    </li>
</ol>

<h5><a id="upgrade_230_notable" href="#upgrade_230_notable">Notable changes in 2.3.0</a></h5>
<ul>
    <li> We are introducing a new rebalancing protocol for Kafka Connect based on
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-415%3A+Incremental+Cooperative+Rebalancing+in+Kafka+Connect">incremental cooperative rebalancing</a>.
        The new protocol does not require stopping all the tasks during a rebalancing phase between Connect workers. Instead, only the tasks that need to be exchanged
        between workers are stopped and they are started in a follow up rebalance. The new Connect protocol is enabled by default beginning with 2.3.0.
        For more details on how it works and how to enable the old behavior of eager rebalancing, checkout
        <a href="/{{version}}/documentation/#connect_administration">incremental cooperative rebalancing design</a>.
    </li>
    <li> We are introducing static membership towards consumer user. This feature reduces unnecessary rebalances during normal application upgrades or rolling bounces.
        For more details on how to use it, checkout <a href="/{{version}}/documentation/#static_membership">static membership design</a>.
    </li>
    <li> Kafka Streams DSL switches its used store types. While this change is mainly transparent to users, there are some corner cases that may require code changes.
        See the <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_230">Kafka Streams upgrade section</a> for more details.
    </li>
    <li>Kafka Streams 2.3.0 requires 0.11 message format or higher and does not work with older message format.</li>
</ul>

<h4><a id="upgrade_2_2_0" href="#upgrade_2_2_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x, 0.11.0.x, 1.0.x, 1.1.x, 2.0.x or 2.1.x to 2.2.0</a></h4>

<p><b>If you are upgrading from a version prior to 2.1.x, please see the note below about the change to the schema used to store consumer offsets.
    Once you have changed the inter.broker.protocol.version to the latest version, it will not be possible to downgrade to a version prior to 2.1.</b></p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.8.2, 0.9.0, 0.10.0, 0.10.1, 0.10.2, 0.11.0, 1.0, 1.1).</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from 0.11.0.x, 1.0.x, 1.1.x, or 2.0.x and you have not overridden the message format, then you only need to override
        the inter-broker protocol version.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (0.11.0, 1.0, 1.1, 2.0).</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. Once you have done so, the
        brokers will be running the latest version and you can verify that the cluster's behavior and performance meets expectations.
        It is still possible to downgrade at this point if there are any problems.
    </li>
    <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
        <code>inter.broker.protocol.version</code> and setting it to 2.2.
    </li>
    <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
        protocol version, it will no longer be possible to downgrade the cluster to an older version.
    </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.2 on each broker and restart them one by one. Note that the older Scala clients,
        which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs
        (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java clients must be used.
    </li>
</ol>

<h5><a id="upgrade_221_notable" href="#upgrade_221_notable">Notable changes in 2.2.1</a></h5>
<ul>
    <li>Kafka Streams 2.2.1 requires 0.11 message format or higher and does not work with older message format.</li>
</ul>

<h5><a id="upgrade_220_notable" href="#upgrade_220_notable">Notable changes in 2.2.0</a></h5>
<ul>
    <li>The default consumer group id has been changed from the empty string (<code>""</code>) to <code>null</code>. Consumers who use the new default group id will not be able to subscribe to topics,
        and fetch or commit offsets. The empty string as consumer group id is deprecated but will be supported until a future major release. Old clients that rely on the empty string group id will now
        have to explicitly provide it as part of their consumer config. For more information see
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-289%3A+Improve+the+default+group+id+behavior+in+KafkaConsumer">KIP-289</a>.</li>
    <li>The <code>bin/kafka-topics.sh</code> command line tool is now able to connect directly to brokers with <code>--bootstrap-server</code> instead of zookeeper. The old <code>--zookeeper</code>
        option is still available for now. Please read <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-377%3A+TopicCommand+to+use+AdminClient">KIP-377</a> for more information.</li>
    <li>Kafka Streams depends on a newer version of RocksDBs that requires MacOS 10.13 or higher.</li>
</ul>

<h4><a id="upgrade_2_1_0" href="#upgrade_2_1_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x, 0.11.0.x, 1.0.x, 1.1.x, or 2.0.0 to 2.1.0</a></h4>

<p><b>Note that 2.1.x contains a change to the internal schema used to store consumer offsets. Once the upgrade is
  complete, it will not be possible to downgrade to previous versions. See the rolling upgrade notes below for more detail.</b></p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.8.2, 0.9.0, 0.10.0, 0.10.1, 0.10.2, 0.11.0, 1.0, 1.1).</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from 0.11.0.x, 1.0.x, 1.1.x, or 2.0.x and you have not overridden the message format, then you only need to override
        the inter-broker protocol version.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (0.11.0, 1.0, 1.1, 2.0).</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. Once you have done so, the
        brokers will be running the latest version and you can verify that the cluster's behavior and performance meets expectations.
        It is still possible to downgrade at this point if there are any problems. 
    </li>
    <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
        <code>inter.broker.protocol.version</code> and setting it to 2.1.
    </li>
    <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
        protocol version, it will no longer be possible to downgrade the cluster to an older version.
    </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.1 on each broker and restart them one by one. Note that the older Scala clients,
        which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs 
        (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java clients must be used.
    </li>
</ol>

<p><b>Additional Upgrade Notes:</b></p>

<ol>
    <li>Offset expiration semantics has slightly changed in this version. According to the new semantics, offsets of partitions in a group will
        not be removed while the group is subscribed to the corresponding topic and is still active (has active consumers). If group becomes
        empty all its offsets will be removed after default offset retention period (or the one set by broker) has passed (unless the group becomes
        active again). Offsets associated with standalone (simple) consumers, that do not use Kafka group management, will be removed after default
        offset retention period (or the one set by broker) has passed since their last commit.</li>
    <li>The default for console consumer's <code>enable.auto.commit</code> property when no <code>group.id</code> is provided is now set to <code>false</code>.
        This is to avoid polluting the consumer coordinator cache as the auto-generated group is not likely to be used by other consumers.</li>
    <li>The default value for the producer's <code>retries</code> config was changed to <code>Integer.MAX_VALUE</code>, as we introduced <code>delivery.timeout.ms</code>
        in <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-91+Provide+Intuitive+User+Timeouts+in+The+Producer">KIP-91</a>,
        which sets an upper bound on the total time between sending a record and receiving acknowledgement from the broker. By default,
        the delivery timeout is set to 2 minutes.</li>
    <li>By default, MirrorMaker now overrides <code>delivery.timeout.ms</code> to <code>Integer.MAX_VALUE</code> when
        configuring the producer. If you have overridden the value of <code>retries</code> in order to fail faster,
        you will instead need to override <code>delivery.timeout.ms</code>.</li>
    <li>The <code>ListGroup</code> API now expects, as a recommended alternative, <code>Describe Group</code> access to the groups a user should be able to list.
        Even though the old <code>Describe Cluster</code> access is still supported for backward compatibility, using it for this API is not advised.</li>
    <li><a href="https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=87298242">KIP-336</a> deprecates the ExtendedSerializer and ExtendedDeserializer interfaces and
        propagates the usage of Serializer and Deserializer. ExtendedSerializer and ExtendedDeserializer were introduced with
        <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-82+-+Add+Record+Headers">KIP-82</a> to provide record headers for serializers and deserializers
        in a Java 7 compatible fashion. Now we consolidated these interfaces as Java 7 support has been dropped since.</li>
</ol>

<h5><a id="upgrade_210_notable" href="#upgrade_210_notable">Notable changes in 2.1.0</a></h5>
<ul>
    <li>Jetty has been upgraded to 9.4.12, which excludes TLS_RSA_* ciphers by default because they do not support forward
        secrecy, see https://github.com/eclipse/jetty.project/issues/2807 for more information.</li>
    <li>Unclean leader election is automatically enabled by the controller when <code>unclean.leader.election.enable</code> config is dynamically updated by using per-topic config override.</li>
    <li>The <code>AdminClient</code> has added a method <code>AdminClient#metrics()</code>. Now any application using the <code>AdminClient</code> can gain more information
        and insight by viewing the metrics captured from the <code>AdminClient</code>. For more information
        see <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-324%3A+Add+method+to+get+metrics%28%29+in+AdminClient">KIP-324</a>
    </li>
    <li>Kafka now supports Zstandard compression from <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-110%3A+Add+Codec+for+ZStandard+Compression">KIP-110</a>.
        You must upgrade the broker as well as clients to make use of it. Consumers prior to 2.1.0 will not be able to read from topics which use
        Zstandard compression, so you should not enable it for a topic until all downstream consumers are upgraded. See the KIP for more detail.
    </li>
</ul>

<h4><a id="upgrade_2_0_0" href="#upgrade_2_0_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x, 0.11.0.x, 1.0.x, or 1.1.x to 2.0.0</a></h4>
<p>Kafka 2.0.0 introduces wire protocol changes. By following the recommended rolling upgrade plan below,
    you guarantee no downtime during the upgrade. However, please review the <a href="#upgrade_200_notable">notable changes in 2.0.0</a> before upgrading.
</p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.8.2, 0.9.0, 0.10.0, 0.10.1, 0.10.2, 0.11.0, 1.0, 1.1).</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from 0.11.0.x, 1.0.x, or 1.1.x and you have not overridden the message format, then you only need to override
        the inter-broker protocol format.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (0.11.0, 1.0, 1.1).</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. </li>
    <li> Once the entire cluster is upgraded, bump the protocol version by editing <code>inter.broker.protocol.version</code> and setting it to 2.0.
    <li> Restart the brokers one by one for the new protocol version to take effect.</li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 2.0 on each broker and restart them one by one. Note that the older Scala consumer
        does not support the new message format introduced in 0.11, so to avoid the performance cost of down-conversion (or to
        take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>), the newer Java consumer must be used.</li>
</ol>

<p><b>Additional Upgrade Notes:</b></p>

<ol>
    <li>If you are willing to accept downtime, you can simply take all the brokers down, update the code and start them back up. They will start
        with the new protocol by default.</li>
    <li>Bumping the protocol version and restarting can be done any time after the brokers are upgraded. It does not have to be immediately after.
        Similarly for the message format version.</li>
    <li>If you are using Java8 method references in your Kafka Streams code you might need to update your code to resolve method ambiguities.
        Hot-swapping the jar-file only might not work.</li>
    <li>ACLs should not be added to prefixed resources,
        (added in <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-290%3A+Support+for+Prefixed+ACLs">KIP-290</a>),
        until all brokers in the cluster have been updated.
        <p><b>NOTE:</b> any prefixed ACLs added to a cluster, even after the cluster is fully upgraded, will be ignored should the cluster be downgraded again.
    </li>
</ol>

<h5><a id="upgrade_200_notable" href="#upgrade_200_notable">Notable changes in 2.0.0</a></h5>
<ul>
    <li><a href="https://cwiki.apache.org/confluence/x/oYtjB">KIP-186</a> increases the default offset retention time from 1 day to 7 days. This makes it less likely to "lose" offsets in an application that commits infrequently. It also increases the active set of offsets and therefore can increase memory usage on the broker. Note that the console consumer currently enables offset commit by default and can be the source of a large number of offsets which this change will now preserve for 7 days instead of 1. You can preserve the existing behavior by setting the broker config <code>offsets.retention.minutes</code> to 1440.</li>
    <li>Support for Java 7 has been dropped, Java 8 is now the minimum version required.</li>
    <li> The default value for <code>ssl.endpoint.identification.algorithm</code> was changed to <code>https</code>, which performs hostname verification (man-in-the-middle attacks are possible otherwise). Set <code>ssl.endpoint.identification.algorithm</code> to an empty string to restore the previous behaviour. </li>
    <li><a href="https://issues.apache.org/jira/browse/KAFKA-5674">KAFKA-5674</a> extends the lower interval of <code>max.connections.per.ip</code> minimum to zero and therefore allows IP-based filtering of inbound connections.</li>
    <li><a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-272%3A+Add+API+version+tag+to+broker%27s+RequestsPerSec+metric">KIP-272</a>
        added API version tag to the metric <code>kafka.network:type=RequestMetrics,name=RequestsPerSec,request={Produce|FetchConsumer|FetchFollower|...}</code>.
        This metric now becomes <code>kafka.network:type=RequestMetrics,name=RequestsPerSec,request={Produce|FetchConsumer|FetchFollower|...},version={0|1|2|3|...}</code>. This will impact
        JMX monitoring tools that do not automatically aggregate. To get the total count for a specific request type, the tool needs to be
        updated to aggregate across different versions.
    </li>
    <li><a href="https://cwiki.apache.org/confluence/x/uaBzB">KIP-225</a> changed the metric "records.lag" to use tags for topic and partition. The original version with the name format "{topic}-{partition}.records-lag" has been removed.</li>
    <li>The Scala consumers, which have been deprecated since 0.11.0.0, have been removed. The Java consumer has been the recommended option
        since 0.10.0.0. Note that the Scala consumers in 1.1.0 (and older) will continue to work even if the brokers are upgraded to 2.0.0.</li>
    <li>The Scala producers, which have been deprecated since 0.10.0.0, have been removed. The Java producer has been the recommended option
        since 0.9.0.0. Note that the behaviour of the default partitioner in the Java producer differs from the default partitioner
        in the Scala producers. Users migrating should consider configuring a custom partitioner that retains the previous behaviour.
        Note that the Scala producers in 1.1.0 (and older) will continue to work even if the brokers are upgraded to 2.0.0.</li>
    <li>MirrorMaker and ConsoleConsumer no longer support the Scala consumer, they always use the Java consumer.</li>
    <li>The ConsoleProducer no longer supports the Scala producer, it always uses the Java producer.</li>
    <li>A number of deprecated tools that rely on the Scala clients have been removed: ReplayLogProducer, SimpleConsumerPerformance, SimpleConsumerShell, ExportZkOffsets, ImportZkOffsets, UpdateOffsetsInZK, VerifyConsumerRebalance.</li>
    <li>The deprecated kafka.tools.ProducerPerformance has been removed, please use org.apache.kafka.tools.ProducerPerformance.</li>
    <li>New Kafka Streams configuration parameter <code>upgrade.from</code> added that allows rolling bounce upgrade from older version. </li>
    <li><a href="https://cwiki.apache.org/confluence/x/DVyHB">KIP-284</a> changed the retention time for Kafka Streams repartition topics by setting its default value to <code>Long.MAX_VALUE</code>.</li>
    <li>Updated <code>ProcessorStateManager</code> APIs in Kafka Streams for registering state stores to the processor topology. For more details please read the Streams <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_200">Upgrade Guide</a>.</li>
    <li>
        In earlier releases, Connect's worker configuration required the <code>internal.key.converter</code> and <code>internal.value.converter</code> properties.
        In 2.0, these are <a href="https://cwiki.apache.org/confluence/x/AZQ7B">no longer required</a> and default to the JSON converter.
        You may safely remove these properties from your Connect standalone and distributed worker configurations:<br />
        <code>internal.key.converter=org.apache.kafka.connect.json.JsonConverter</code>
        <code>internal.key.converter.schemas.enable=false</code>
        <code>internal.value.converter=org.apache.kafka.connect.json.JsonConverter</code>
        <code>internal.value.converter.schemas.enable=false</code>
    </li>
    <li><a href="https://cwiki.apache.org/confluence/x/5kiHB">KIP-266</a> adds a new consumer configuration <code>default.api.timeout.ms</code>
        to specify the default timeout to use for <code>KafkaConsumer</code> APIs that could block. The KIP also adds overloads for such blocking
        APIs to support specifying a specific timeout to use for each of them instead of using the default timeout set by <code>default.api.timeout.ms</code>.
        In particular, a new <code>poll(Duration)</code> API has been added which does not block for dynamic partition assignment.
        The old <code>poll(long)</code> API has been deprecated and will be removed in a future version. Overloads have also been added
        for other <code>KafkaConsumer</code> methods like <code>partitionsFor</code>, <code>listTopics</code>, <code>offsetsForTimes</code>,
        <code>beginningOffsets</code>, <code>endOffsets</code> and <code>close</code> that take in a <code>Duration</code>.</li>
    <li>Also as part of KIP-266, the default value of <code>request.timeout.ms</code> has been changed to 30 seconds.
        The previous value was a little higher than 5 minutes to account for maximum time that a rebalance would take.
        Now we treat the JoinGroup request in the rebalance as a special case and use a value derived from
        <code>max.poll.interval.ms</code> for the request timeout. All other request types use the timeout defined
        by <code>request.timeout.ms</code></li>
    <li>The internal method <code>kafka.admin.AdminClient.deleteRecordsBefore</code> has been removed. Users are encouraged to migrate to <code>org.apache.kafka.clients.admin.AdminClient.deleteRecords</code>.</li>
    <li>The AclCommand tool <code>--producer</code> convenience option uses the <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-277+-+Fine+Grained+ACL+for+CreateTopics+API">KIP-277</a> finer grained ACL on the given topic. </li>
    <li><a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-176%3A+Remove+deprecated+new-consumer+option+for+tools">KIP-176</a> removes
        the <code>--new-consumer</code> option for all consumer based tools. This option is redundant since the new consumer is automatically
        used if --bootstrap-server is defined.
    </li>
    <li><a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-290%3A+Support+for+Prefixed+ACLs">KIP-290</a> adds the ability
        to define ACLs on prefixed resources, e.g. any topic starting with 'foo'.</li>
    <li><a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-283%3A+Efficient+Memory+Usage+for+Down-Conversion">KIP-283</a> improves message down-conversion
        handling on Kafka broker, which has typically been a memory-intensive operation. The KIP adds a mechanism by which the operation becomes less memory intensive
        by down-converting chunks of partition data at a time which helps put an upper bound on memory consumption. With this improvement, there is a change in
        <code>FetchResponse</code> protocol behavior where the broker could send an oversized message batch towards the end of the response with an invalid offset.
        Such oversized messages must be ignored by consumer clients, as is done by <code>KafkaConsumer</code>.
    <p>KIP-283 also adds new topic and broker configurations <code>message.downconversion.enable</code> and <code>log.message.downconversion.enable</code> respectively
       to control whether down-conversion is enabled. When disabled, broker does not perform any down-conversion and instead sends an <code>UNSUPPORTED_VERSION</code>
       error to the client.</p></li>
    <li>Dynamic broker configuration options can be stored in ZooKeeper using kafka-configs.sh before brokers are started.
        This option can be used to avoid storing clear passwords in server.properties as all password configs may be stored encrypted in ZooKeeper.</li>
    <li>ZooKeeper hosts are now re-resolved if connection attempt fails. But if your ZooKeeper host names resolve
    to multiple addresses and some of them are not reachable, then you may need to increase the connection timeout
    <code>zookeeper.connection.timeout.ms</code>.</li>
</ul>

<h5><a id="upgrade_200_new_protocols" href="#upgrade_200_new_protocols">New Protocol Versions</a></h5>
<ul>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-279%3A+Fix+log+divergence+between+leader+and+follower+after+fast+leader+fail+over">KIP-279</a>: OffsetsForLeaderEpochResponse v1 introduces a partition-level <code>leader_epoch</code> field. </li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-219+-+Improve+quota+communication">KIP-219</a>: Bump up the protocol versions of non-cluster action requests and responses that are throttled on quota violation.</li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-290%3A+Support+for+Prefixed+ACLs">KIP-290</a>: Bump up the protocol versions ACL create, describe and delete requests and responses.</li>
</ul>


<h5><a id="upgrade_200_streams_from_11" href="#upgrade_200_streams_from_11">Upgrading a 1.1 Kafka Streams Application</a></h5>
<ul>
    <li> Upgrading your Streams application from 1.1 to 2.0 does not require a broker upgrade.
         A Kafka Streams 2.0 application can connect to 2.0, 1.1, 1.0, 0.11.0, 0.10.2 and 0.10.1 brokers (it is not possible to connect to 0.10.0 brokers though). </li>
    <li> Note that in 2.0 we have removed the public APIs that are deprecated prior to 1.0; users leveraging on those deprecated APIs need to make code changes accordingly.
         See <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_200">Streams API changes in 2.0.0</a> for more details. </li>
</ul>

<h4><a id="upgrade_1_1_0" href="#upgrade_1_1_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x, 0.11.0.x, or 1.0.x to 1.1.x</a></h4>
<p>Kafka 1.1.0 introduces wire protocol changes. By following the recommended rolling upgrade plan below,
    you guarantee no downtime during the upgrade. However, please review the <a href="#upgrade_110_notable">notable changes in 1.1.0</a> before upgrading.
</p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.8.2, 0.9.0, 0.10.0, 0.10.1, 0.10.2, 0.11.0, 1.0).</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
                following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
        If you are upgrading from 0.11.0.x or 1.0.x and you have not overridden the message format, then you only need to override
        the inter-broker protocol format.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (0.11.0 or 1.0).</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. </li>
    <li> Once the entire cluster is upgraded, bump the protocol version by editing <code>inter.broker.protocol.version</code> and setting it to 1.1.
    <li> Restart the brokers one by one for the new protocol version to take effect. </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 1.1 on each broker and restart them one by one. Note that the older Scala consumer
        does not support the new message format introduced in 0.11, so to avoid the performance cost of down-conversion (or to
        take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>), the newer Java consumer must be used.</li>
</ol>

<p><b>Additional Upgrade Notes:</b></p>

<ol>
    <li>If you are willing to accept downtime, you can simply take all the brokers down, update the code and start them back up. They will start
        with the new protocol by default.</li>
    <li>Bumping the protocol version and restarting can be done any time after the brokers are upgraded. It does not have to be immediately after.
        Similarly for the message format version.</li>
    <li>If you are using Java8 method references in your Kafka Streams code you might need to update your code to resolve method ambiguties.
        Hot-swapping the jar-file only might not work.</li>
</ol>

<h5><a id="upgrade_111_notable" href="#upgrade_111_notable">Notable changes in 1.1.1</a></h5>
<ul>
    <li> New Kafka Streams configuration parameter <code>upgrade.from</code> added that allows rolling bounce upgrade from version 0.10.0.x </li>
    <li> See the <a href="/{{version}}/documentation/streams/upgrade-guide.html"><b>Kafka Streams upgrade guide</b></a> for details about this new config.
</ul>

<h5><a id="upgrade_110_notable" href="#upgrade_110_notable">Notable changes in 1.1.0</a></h5>
<ul>
    <li>The kafka artifact in Maven no longer depends on log4j or slf4j-log4j12. Similarly to the kafka-clients artifact, users
        can now choose the logging back-end by including the appropriate slf4j module (slf4j-log4j12, logback, etc.). The release
        tarball still includes log4j and slf4j-log4j12.</li>
    <li><a href="https://cwiki.apache.org/confluence/x/uaBzB">KIP-225</a> changed the metric "records.lag" to use tags for topic and partition. The original version with the name format "{topic}-{partition}.records-lag" is deprecated and will be removed in 2.0.0.</li>
    <li>Kafka Streams is more robust against broker communication errors. Instead of stopping the Kafka Streams client with a fatal exception,
	Kafka Streams tries to self-heal and reconnect to the cluster. Using the new <code>AdminClient</code> you have better control of how often
	Kafka Streams retries and can <a href="/{{version}}/documentation/streams/developer-guide/config-streams">configure</a>
	fine-grained timeouts (instead of hard coded retries as in older version).</li>
    <li>Kafka Streams rebalance time was reduced further making Kafka Streams more responsive.</li>
    <li>Kafka Connect now supports message headers in both sink and source connectors, and to manipulate them via simple message transforms. Connectors must be changed to explicitly use them. A new <code>HeaderConverter</code> is introduced to control how headers are (de)serialized, and the new "SimpleHeaderConverter" is used by default to use string representations of values.</li>
    <li>kafka.tools.DumpLogSegments now automatically sets deep-iteration option if print-data-log is enabled
        explicitly or implicitly due to any of the other options like decoder.</li>
</ul>

<h5><a id="upgrade_110_new_protocols" href="#upgrade_110_new_protocols">New Protocol Versions</a></h5>
<ul>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-226+-+Dynamic+Broker+Configuration">KIP-226</a> introduced DescribeConfigs Request/Response v1.</li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-227%3A+Introduce+Incremental+FetchRequests+to+Increase+Partition+Scalability">KIP-227</a> introduced Fetch Request/Response v7.</li>
</ul>

<h5><a id="upgrade_110_streams_from_10" href="#upgrade_110_streams_from_10">Upgrading a 1.0 Kafka Streams Application</a></h5>
<ul>
    <li> Upgrading your Streams application from 1.0 to 1.1 does not require a broker upgrade.
        A Kafka Streams 1.1 application can connect to 1.0, 0.11.0, 0.10.2 and 0.10.1 brokers (it is not possible to connect to 0.10.0 brokers though). </li>
    <li> See <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_110">Streams API changes in 1.1.0</a> for more details. </li>
</ul>

<h4><a id="upgrade_1_0_0" href="#upgrade_1_0_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x or 0.11.0.x to 1.0.0</a></h4>
<p>Kafka 1.0.0 introduces wire protocol changes. By following the recommended rolling upgrade plan below,
    you guarantee no downtime during the upgrade. However, please review the <a href="#upgrade_100_notable">notable changes in 1.0.0</a> before upgrading.
</p>

<p><b>For a rolling upgrade:</b></p>

<ol>
    <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
        are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
        overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
        to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
        <ul>
            <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.8.2, 0.9.0, 0.10.0, 0.10.1, 0.10.2, 0.11.0).</li>
            <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION  (See <a href="#upgrade_10_performance_impact">potential performance impact
		following the upgrade</a> for the details on what this configuration does.)</li>
        </ul>
	If you are upgrading from 0.11.0.x and you have not overridden the message format, you must set
	both the message format version and the inter-broker protocol version to 0.11.0.
        <ul>
            <li>inter.broker.protocol.version=0.11.0</li>
            <li>log.message.format.version=0.11.0</li>
        </ul>
    </li>
    <li> Upgrade the brokers one at a time: shut down the broker, update the code, and restart it. </li>
    <li> Once the entire cluster is upgraded, bump the protocol version by editing <code>inter.broker.protocol.version</code> and setting it to 1.0.
    <li> Restart the brokers one by one for the new protocol version to take effect. </li>
    <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
        upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
        change log.message.format.version to 1.0 on each broker and restart them one by one. If you are upgrading from
        0.11.0 and log.message.format.version is set to 0.11.0, you can update the config and skip the rolling restart.
        Note that the older Scala consumer does not support the new message format introduced in 0.11, so to avoid the
        performance cost of down-conversion (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
        the newer Java consumer must be used.</li>
</ol>

<p><b>Additional Upgrade Notes:</b></p>

<ol>
    <li>If you are willing to accept downtime, you can simply take all the brokers down, update the code and start them back up. They will start
        with the new protocol by default.</li>
    <li>Bumping the protocol version and restarting can be done any time after the brokers are upgraded. It does not have to be immediately after.
        Similarly for the message format version.</li>
</ol>

<h5><a id="upgrade_102_notable" href="#upgrade_102_notable">Notable changes in 1.0.2</a></h5>
<ul>
    <li> New Kafka Streams configuration parameter <code>upgrade.from</code> added that allows rolling bounce upgrade from version 0.10.0.x </li>
    <li> See the <a href="/{{version}}/documentation/streams/upgrade-guide.html"><b>Kafka Streams upgrade guide</b></a> for details about this new config.
</ul>

<h5><a id="upgrade_101_notable" href="#upgrade_101_notable">Notable changes in 1.0.1</a></h5>
<ul>
    <li>Restored binary compatibility of AdminClient's Options classes (e.g. CreateTopicsOptions, DeleteTopicsOptions, etc.) with
        0.11.0.x. Binary (but not source) compatibility had been broken inadvertently in 1.0.0.</li>
</ul>

<h5><a id="upgrade_100_notable" href="#upgrade_100_notable">Notable changes in 1.0.0</a></h5>
<ul>
    <li>Topic deletion is now enabled by default, since the functionality is now stable. Users who wish to
        to retain the previous behavior should set the broker config <code>delete.topic.enable</code> to <code>false</code>. Keep in mind that topic deletion removes data and the operation is not reversible (i.e. there is no "undelete" operation)</li>
    <li>For topics that support timestamp search if no offset can be found for a partition, that partition is now included in the search result with a null offset value. Previously, the partition was not included in the map.
        This change was made to make the search behavior consistent with the case of topics not supporting timestamp search.
    <li>If the <code>inter.broker.protocol.version</code> is 1.0 or later, a broker will now stay online to serve replicas
        on live log directories even if there are offline log directories. A log directory may become offline due to IOException
        caused by hardware failure. Users need to monitor the per-broker metric <code>offlineLogDirectoryCount</code> to check
        whether there is offline log directory. </li>
    <li>Added KafkaStorageException which is a retriable exception. KafkaStorageException will be converted to NotLeaderForPartitionException in the response
        if the version of client's FetchRequest or ProducerRequest does not support KafkaStorageException. </li>
    <li>-XX:+DisableExplicitGC was replaced by -XX:+ExplicitGCInvokesConcurrent in the default JVM settings. This helps
        avoid out of memory exceptions during allocation of native memory by direct buffers in some cases.</li>
    <li>The overridden <code>handleError</code> method implementations have been removed from the following deprecated classes in
        the <code>kafka.api</code> package: <code>FetchRequest</code>, <code>GroupCoordinatorRequest</code>, <code>OffsetCommitRequest</code>,
        <code>OffsetFetchRequest</code>, <code>OffsetRequest</code>, <code>ProducerRequest</code>, and <code>TopicMetadataRequest</code>.
        This was only intended for use on the broker, but it is no longer in use and the implementations have not been maintained.
        A stub implementation has been retained for binary compatibility.</li>
    <li>The Java clients and tools now accept any string as a client-id.</li>
    <li>The deprecated tool <code>kafka-consumer-offset-checker.sh</code> has been removed. Use <code>kafka-consumer-groups.sh</code> to get consumer group details.</li>
    <li>SimpleAclAuthorizer now logs access denials to the authorizer log by default.</li>
    <li>Authentication failures are now reported to clients as one of the subclasses of <code>AuthenticationException</code>.
        No retries will be performed if a client connection fails authentication.</li>
    <li>Custom <code>SaslServer</code> implementations may throw <code>SaslAuthenticationException</code> to provide an error
        message to return to clients indicating the reason for authentication failure. Implementors should take care not to include
        any security-critical information in the exception message that should not be leaked to unauthenticated clients.</li>
    <li>The <code>app-info</code> mbean registered with JMX to provide version and commit id will be deprecated and replaced with
        metrics providing these attributes.</li>
    <li>Kafka metrics may now contain non-numeric values. <code>org.apache.kafka.common.Metric#value()</code> has been deprecated and
        will return <code>0.0</code> in such cases to minimise the probability of breaking users who read the value of every client
        metric (via a <code>MetricsReporter</code> implementation or by calling the <code>metrics()</code> method).
        <code>org.apache.kafka.common.Metric#metricValue()</code> can be used to retrieve numeric and non-numeric metric values.</li>
    <li>Every Kafka rate metric now has a corresponding cumulative count metric with the suffix <code>-total</code>
        to simplify downstream processing. For example, <code>records-consumed-rate</code> has a corresponding
        metric named <code>records-consumed-total</code>.</li>
    <li>Mx4j will only be enabled if the system property <code>kafka_mx4jenable</code> is set to <code>true</code>. Due to a logic
        inversion bug, it was previously enabled by default and disabled if <code>kafka_mx4jenable</code> was set to <code>true</code>.</li>
    <li>The package <code>org.apache.kafka.common.security.auth</code> in the clients jar has been made public and added to the javadocs.
        Internal classes which had previously been located in this package have been moved elsewhere.</li>
    <li>When using an Authorizer and a user doesn't have required permissions on a topic, the broker
        will return TOPIC_AUTHORIZATION_FAILED errors to requests irrespective of topic existence on broker.
        If the user have required permissions and the topic doesn't exists, then the UNKNOWN_TOPIC_OR_PARTITION
        error code will be returned. </li>
    <li>config/consumer.properties file updated to use new consumer config properties.</li>
</ul>

<h5><a id="upgrade_100_new_protocols" href="#upgrade_100_new_protocols">New Protocol Versions</a></h5>
<ul>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-112%3A+Handle+disk+failure+for+JBOD">KIP-112</a>: LeaderAndIsrRequest v1 introduces a partition-level <code>is_new</code> field. </li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-112%3A+Handle+disk+failure+for+JBOD">KIP-112</a>: UpdateMetadataRequest v4 introduces a partition-level <code>offline_replicas</code> field. </li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-112%3A+Handle+disk+failure+for+JBOD">KIP-112</a>: MetadataResponse v5 introduces a partition-level <code>offline_replicas</code> field. </li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-112%3A+Handle+disk+failure+for+JBOD">KIP-112</a>: ProduceResponse v4 introduces error code for KafkaStorageException. </li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-112%3A+Handle+disk+failure+for+JBOD">KIP-112</a>: FetchResponse v6 introduces error code for KafkaStorageException. </li>
    <li> <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-152+-+Improve+diagnostics+for+SASL+authentication+failures">KIP-152</a>:
         SaslAuthenticate request has been added to enable reporting of authentication failures. This request will
         be used if the SaslHandshake request version is greater than 0. </li>
</ul>

<h5><a id="upgrade_100_streams_from_0110" href="#upgrade_100_streams_from_0110">Upgrading a 0.11.0 Kafka Streams Application</a></h5>
<ul>
    <li> Upgrading your Streams application from 0.11.0 to 1.0 does not require a broker upgrade.
         A Kafka Streams 1.0 application can connect to 0.11.0, 0.10.2 and 0.10.1 brokers (it is not possible to connect to 0.10.0 brokers though).
         However, Kafka Streams 1.0 requires 0.10 message format or newer and does not work with older message formats. </li>
    <li> If you are monitoring on streams metrics, you will need make some changes to the metrics names in your reporting and monitoring code, because the metrics sensor hierarchy was changed. </li>
    <li> There are a few public APIs including <code>ProcessorContext#schedule()</code>, <code>Processor#punctuate()</code> and <code>KStreamBuilder</code>, <code>TopologyBuilder</code> are being deprecated by new APIs.
         We recommend making corresponding code changes, which should be very minor since the new APIs look quite similar, when you upgrade.
    <li> See <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_100">Streams API changes in 1.0.0</a> for more details. </li>
</ul>

<h5><a id="upgrade_100_streams_from_0102" href="#upgrade_100_streams_from_0102">Upgrading a 0.10.2 Kafka Streams Application</a></h5>
<ul>
    <li> Upgrading your Streams application from 0.10.2 to 1.0 does not require a broker upgrade.
         A Kafka Streams 1.0 application can connect to 1.0, 0.11.0, 0.10.2 and 0.10.1 brokers (it is not possible to connect to 0.10.0 brokers though). </li>
    <li> If you are monitoring on streams metrics, you will need make some changes to the metrics names in your reporting and monitoring code, because the metrics sensor hierarchy was changed. </li>
    <li> There are a few public APIs including <code>ProcessorContext#schedule()</code>, <code>Processor#punctuate()</code> and <code>KStreamBuilder</code>, <code>TopologyBuilder</code> are being deprecated by new APIs.
         We recommend making corresponding code changes, which should be very minor since the new APIs look quite similar, when you upgrade.
    <li> If you specify customized <code>key.serde</code>, <code>value.serde</code> and <code>timestamp.extractor</code> in configs, it is recommended to use their replaced configure parameter as these configs are deprecated. </li>
    <li> See <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_0110">Streams API changes in 0.11.0</a> for more details. </li>
</ul>

<h5><a id="upgrade_100_streams_from_0101" href="#upgrade_1100_streams_from_0101">Upgrading a 0.10.1 Kafka Streams Application</a></h5>
<ul>
    <li> Upgrading your Streams application from 0.10.1 to 1.0 does not require a broker upgrade.
         A Kafka Streams 1.0 application can connect to 1.0, 0.11.0, 0.10.2 and 0.10.1 brokers (it is not possible to connect to 0.10.0 brokers though). </li>
    <li> You need to recompile your code. Just swapping the Kafka Streams library jar file will not work and will break your application. </li>
    <li> If you are monitoring on streams metrics, you will need make some changes to the metrics names in your reporting and monitoring code, because the metrics sensor hierarchy was changed. </li>
    <li> There are a few public APIs including <code>ProcessorContext#schedule()</code>, <code>Processor#punctuate()</code> and <code>KStreamBuilder</code>, <code>TopologyBuilder</code> are being deprecated by new APIs.
         We recommend making corresponding code changes, which should be very minor since the new APIs look quite similar, when you upgrade.
    <li> If you specify customized <code>key.serde</code>, <code>value.serde</code> and <code>timestamp.extractor</code> in configs, it is recommended to use their replaced configure parameter as these configs are deprecated. </li>
    <li> If you use a custom (i.e., user implemented) timestamp extractor, you will need to update this code, because the <code>TimestampExtractor</code> interface was changed. </li>
    <li> If you register custom metrics, you will need to update this code, because the <code>StreamsMetric</code> interface was changed. </li>
    <li> See <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_100">Streams API changes in 1.0.0</a>,
         <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_0110">Streams API changes in 0.11.0</a> and
         <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_0102">Streams API changes in 0.10.2</a> for more details. </li>
</ul>

<h5><a id="upgrade_100_streams_from_0100" href="#upgrade_100_streams_from_0100">Upgrading a 0.10.0 Kafka Streams Application</a></h5>
<ul>
    <li> Upgrading your Streams application from 0.10.0 to 1.0 does require a <a href="#upgrade_10_1">broker upgrade</a> because a Kafka Streams 1.0 application can only connect to 0.1, 0.11.0, 0.10.2, or 0.10.1 brokers. </li>
    <li> There are couple of API changes, that are not backward compatible (cf. <a href="/{{version}}/documentation/streams/upgrade-guide#streams_api_changes_100">Streams API changes in 1.0.0</a>,
         <a href="/{{version}}/documentation/streams#streams_api_changes_0110">Streams API changes in 0.11.0</a>,
         <a href="/{{version}}/documentation/streams#streams_api_changes_0102">Streams API changes in 0.10.2</a>, and
         <a href="/{{version}}/documentation/streams#streams_api_changes_0101">Streams API changes in 0.10.1</a> for more details).
         Thus, you need to update and recompile your code. Just swapping the Kafka Streams library jar file will not work and will break your application. </li>
    <li> Upgrading from 0.10.0.x to 1.0.2 requires two rolling bounces with config <code>upgrade.from="0.10.0"</code> set for first upgrade phase
        (cf. <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-268%3A+Simplify+Kafka+Streams+Rebalance+Metadata+Upgrade">KIP-268</a>).
        As an alternative, an offline upgrade is also possible.
        <ul>
            <li> prepare your application instances for a rolling bounce and make sure that config <code>upgrade.from</code> is set to <code>"0.10.0"</code> for new version 0.11.0.3 </li>
            <li> bounce each instance of your application once </li>
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