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<h2 id="fallback">Fallback</h2>
<h3 id="concepts">Concepts</h3>
<p>A fallback policy is applied when a service request is abnormal.</p>
<p>There are three key concepts in fallback: isolation, circuit breaking, and fault tolerance:</p>
<ul>
<li><strong>Isolation</strong> is an exception detection mechanism. There are two common "exception"s: timeout and overload, which can be controlled by timeout duration and max concurrent requests.</li>
<li><strong>Circuit breaking</strong> is an exception response mechanism which depends on isolation. Circuit breaking is triggered by the error rate, like the number of bad requests, or the rate of invalid calls.</li>
<li><strong>Fault tolerance</strong> is an exception handling mechanism that depends on circuit breaking. Fault tolerance is called after a circuit breaking is triggered. Users can set the number of fault tolerance calls in the configuration.</li>
</ul>
<p>Let's combine the 3 concepts: the <strong>isolation</strong> mechanism detects there are M(the threshold) errors in N requests, the <strong>circuit breaking</strong> is triggered and make sure there are no more requests sent, and then <strong>fault tolerance</strong> method is called. Technically the concept definition is the same with Netflix Hystrix, making it easy to understand the config items(Reference: <a href="[https://github.com/Netflix/Hystrix/wiki/Configuration](https://github.com/Netflix/Hystrix/wiki/Configuration)">Hystrix Configuration</a>). ServiceComb provides 2 fault tolerance methods: returning null values and throwing exceptions.</p>
<h3 id="scenario">Scenario</h3>
<p>Users configure a fallback policy to handle microservices' exceptions.</p>
<h3 id="configuration">Configuration</h3>
<p>Configuration items can be set to be applied to all APIs or a particular method of a microservice.</p>
<h3 id="configuration-scope">Configuration Scope</h3>
<ul>
<li>Configuration by type: items can be applied to Providers and Consumers</li>
<li>Configuration by scope: items can be applied to a specific microservice, or [x-schema-id+operationId]</li>
</ul>
<p>All the items in this chapter can be configured in the following format:</p>
<pre><code>servicecomb.[namespace].[type].[MicroServiceName].[interface name].[property name]
</code></pre>
<p>The type can be Consumer or Provider. Specify the [MicroServiceName] to apply configuration to specific microservice. To make the configuration applied to API, we have to specify the API name in the format x-[schema-id+operationId]</p>
<p>The possible Isolation config items are as follows:</p>
<pre><code>servicecomb.isolation.Consumer.timeout.enabled
servicecomb.isolation.Consumer.DemoService.timeout.enabled
servicecomb.isolation.Consumer.DemoService.hello.sayHello.timeout.enabled
servicecomb.isolation.Provider.timeout.enabled
servicecomb.isolation.Provider.DemoService.timeout.enabled
servicecomb.isolation.Provider.DemoService.hello.sayHello.timeout.enabled
</code></pre>
<h3 id="configuration-items">Configuration Items</h3>
<p>For Providers, the configuration item should be: servicecomb.isolation.Consumer.timeout.enabled</p>
<p>For Consumers, the conriguration item should be servicecomb.isolation.Provider.timeout.enabled</p>
<p><strong>Table 1-1 The fallback policy config items</strong></p>
<table>
<thead>
<tr>
<th align="left">Configuration Item</th>
<th align="left">Default value</th>
<th align="left">Value Range</th>
<th align="left">Required</th>
<th align="left">Description</th>
<th align="left">Tips</th>
</tr>
</thead>
<tbody>
<tr>
<td align="left">servicecomb.isolation.[type].timeout.enabled</td>
<td align="left">FALSE</td>
<td align="left">-</td>
<td align="left">No</td>
<td align="left">Enable timeout detection or not.</td>
<td align="left"></td>
</tr>
<tr>
<td align="left">servicecomb.isolation.[type].timeoutInMilliseconds</td>
<td align="left">30000</td>
<td align="left">-</td>
<td align="left">No</td>
<td align="left">The timeout duration threshold.</td>
<td align="left"></td>
</tr>
<tr>
<td align="left">servicecomb.isolation.[type].maxConcurrentRequests</td>
<td align="left">10</td>
<td align="left">-</td>
<td align="left">No</td>
<td align="left">The maximum number of concurrent requests.</td>
<td align="left"></td>
</tr>
<tr>
<td align="left">servicecomb.circuitBreaker.[type].enabled</td>
<td align="left">TRUE</td>
<td align="left">-</td>
<td align="left">No</td>
<td align="left">Enable circuit breaking or not.</td>
<td align="left"></td>
</tr>
<tr>
<td align="left">servicecomb.circuitBreaker.[type].forceOpen</td>
<td align="left">FALSE</td>
<td align="left">-</td>
<td align="left">No</td>
<td align="left">Force circuit breaker to be enabled regardless of the number of errors.</td>
<td align="left"></td>
</tr>
<tr>
<td align="left">servicecomb.circuitBreaker.[type].forceClosed</td>
<td align="left">FALSE</td>
<td align="left">-</td>
<td align="left">No</td>
<td align="left">Force circuit breaker to be disabled.</td>
<td align="left">When forceOpen and forceClose are set at the same time, forceOpen will take effect.</td>
</tr>
<tr>
<td align="left">servicecomb.circuitBreaker.[type].sleepWindowInMilliseconds</td>
<td align="left">15000</td>
<td align="left">-</td>
<td align="left">No</td>
<td align="left">How long to recover from a circuit breaking.</td>
<td align="left">After the recovery, the number of failures will be reset. Note: If the call fails immediately after a recover, the circuit breaker is triggered immediately again.</td>
</tr>
<tr>
<td align="left">servicecomb.circuitBreaker.[type].requestVolumeThreshold</td>
<td align="left">20</td>
<td align="left">-</td>
<td align="left">No</td>
<td align="left">The threshold of failed requests within 10 seconds. If the threshold is reached, circuit breaker is triggered.</td>
<td align="left">The 10 seconds duration is splitted evenly into 10 segments for error calculation. The calculation will start after 1 second. So circuit breakers are triggered after at least 1 second.</td>
</tr>
<tr>
<td align="left">servicecomb.circuitBreaker.[type].errorThresholdPercentage</td>
<td align="left">50</td>
<td align="left">-</td>
<td align="left">No</td>
<td align="left">The threshold of error rate. If the threshold is reached, circuit breaker is triggered.</td>
<td align="left"></td>
</tr>
<tr>
<td align="left">servicecomb.fallback.[type].maxConcurrentRequests</td>
<td align="left">10</td>
<td align="left">-</td>
<td align="left">No</td>
<td align="left">The max number of concurrent fallback(specified by servicecomb.fallbackpolicy.policy) calls. When the threshold is reached, the fallback method is not called by return exception directly.</td>
<td align="left"></td>
</tr>
<tr>
<td align="left">servicecomb.fallbackpolicy.[type].policy</td>
<td align="left">throwException</td>
<td align="left">returnNull | throwException</td>
<td align="left">No</td>
<td align="left">The fallback policy when errors occurred.</td>
<td align="left"></td>
</tr>
</tbody>
</table>
<p><strong>Caution:</strong> Be cautious to set servicecomb.isolation.timeout.enabled to true. All handlers in the handler chain are asynchronously executed, the intermediate handlers' return will make the follow-up handlers processing abandoned. Therefore, we recommend to set servicecomb.isolation.timeout.enabled to be false(by default) and set the network timeout duration servicecomb.request.timeout to 30000.</p>
<h2 id="sample-code">Sample Code</h2>
<pre><code class="language-yaml">servicecomb:
handler:
chain:
Consumer:
default: bizkeeper-consumer
isolation:
Consumer:
timeout:
enabled: true
timeoutInMilliseconds: 30000
circuitBreaker:
Consumer:
sleepWindowInMilliseconds: 15000
requestVolumeThreshold: 20
fallback:
Consumer:
enabled: true
fallbackpolicy:
Consumer:
policy: throwException
</code></pre>
<blockquote>
<p><strong>NOTE:</strong></p>
<p>You need to enable service governance for fallback. The corresponding provider handler <code>bizkeeper-provider</code>, and the consumer handler is <code>bizkeeper-consumer</code>.</p>
</blockquote>
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