def readFromLocalLog()

in core/src/main/scala/kafka/server/ReplicaManager.scala [1068:1194]
108 lines of code
11 McCabe index (conditional complexity)

  def readFromLocalLog(replicaId: Int,
                       fetchOnlyFromLeader: Boolean,
                       fetchIsolation: FetchIsolation,
                       fetchMaxBytes: Int,
                       hardMaxBytesLimit: Boolean,
                       readPartitionInfo: Seq[(TopicPartition, PartitionData)],
                       quota: ReplicaQuota,
                       clientMetadata: Option[ClientMetadata]): Seq[(TopicPartition, LogReadResult)] = {
    val traceEnabled = isTraceEnabled

    def read(tp: TopicPartition, fetchInfo: PartitionData, limitBytes: Int, minOneMessage: Boolean): LogReadResult = {
      val offset = fetchInfo.fetchOffset
      val partitionFetchSize = fetchInfo.maxBytes
      val followerLogStartOffset = fetchInfo.logStartOffset

      val adjustedMaxBytes = math.min(fetchInfo.maxBytes, limitBytes)
      try {
        if (traceEnabled)
          trace(s"Fetching log segment for partition $tp, offset $offset, partition fetch size $partitionFetchSize, " +
            s"remaining response limit $limitBytes" +
            (if (minOneMessage) s", ignoring response/partition size limits" else ""))

        val partition = getPartitionOrException(tp)
        val fetchTimeMs = time.milliseconds

        // If we are the leader, determine the preferred read-replica
        val preferredReadReplica = clientMetadata.flatMap(
          metadata => findPreferredReadReplica(partition, metadata, replicaId, fetchInfo.fetchOffset, fetchTimeMs))

        if (preferredReadReplica.isDefined) {
          replicaSelectorOpt.foreach { selector =>
            debug(s"Replica selector ${selector.getClass.getSimpleName} returned preferred replica " +
              s"${preferredReadReplica.get} for $clientMetadata")
          }
          // If a preferred read-replica is set, skip the read
          val offsetSnapshot = partition.fetchOffsetSnapshot(fetchInfo.currentLeaderEpoch, fetchOnlyFromLeader = false)
          LogReadResult(info = FetchDataInfo(LogOffsetMetadata.UnknownOffsetMetadata, MemoryRecords.EMPTY),
            highWatermark = offsetSnapshot.highWatermark.messageOffset,
            leaderLogStartOffset = offsetSnapshot.logStartOffset,
            leaderLogEndOffset = offsetSnapshot.logEndOffset.messageOffset,
            followerLogStartOffset = followerLogStartOffset,
            fetchTimeMs = -1L,
            lastStableOffset = Some(offsetSnapshot.lastStableOffset.messageOffset),
            preferredReadReplica = preferredReadReplica,
            exception = None)
        } else {
          // Try the read first, this tells us whether we need all of adjustedFetchSize for this partition
          val readInfo: LogReadInfo = partition.readRecords(
            fetchOffset = fetchInfo.fetchOffset,
            currentLeaderEpoch = fetchInfo.currentLeaderEpoch,
            maxBytes = adjustedMaxBytes,
            fetchIsolation = fetchIsolation,
            fetchOnlyFromLeader = fetchOnlyFromLeader,
            minOneMessage = minOneMessage)

          val fetchDataInfo = if (shouldLeaderThrottle(quota, partition, replicaId)) {
            // If the partition is being throttled, simply return an empty set.
            FetchDataInfo(readInfo.fetchedData.fetchOffsetMetadata, MemoryRecords.EMPTY)
          } else if (!hardMaxBytesLimit && readInfo.fetchedData.firstEntryIncomplete) {
            // For FetchRequest version 3, we replace incomplete message sets with an empty one as consumers can make
            // progress in such cases and don't need to report a `RecordTooLargeException`
            FetchDataInfo(readInfo.fetchedData.fetchOffsetMetadata, MemoryRecords.EMPTY)
          } else {
            readInfo.fetchedData
          }

          LogReadResult(info = fetchDataInfo,
            highWatermark = readInfo.highWatermark,
            leaderLogStartOffset = readInfo.logStartOffset,
            leaderLogEndOffset = readInfo.logEndOffset,
            followerLogStartOffset = followerLogStartOffset,
            fetchTimeMs = fetchTimeMs,
            lastStableOffset = Some(readInfo.lastStableOffset),
            preferredReadReplica = preferredReadReplica,
            exception = None)
        }
      } catch {
        // NOTE: Failed fetch requests metric is not incremented for known exceptions since it
        // is supposed to indicate un-expected failure of a broker in handling a fetch request
        case e@ (_: UnknownTopicOrPartitionException |
                 _: NotLeaderOrFollowerException |
                 _: UnknownLeaderEpochException |
                 _: FencedLeaderEpochException |
                 _: ReplicaNotAvailableException |
                 _: KafkaStorageException |
                 _: OffsetOutOfRangeException) =>
          LogReadResult(info = FetchDataInfo(LogOffsetMetadata.UnknownOffsetMetadata, MemoryRecords.EMPTY),
            highWatermark = Log.UnknownOffset,
            leaderLogStartOffset = Log.UnknownOffset,
            leaderLogEndOffset = Log.UnknownOffset,
            followerLogStartOffset = Log.UnknownOffset,
            fetchTimeMs = -1L,
            lastStableOffset = None,
            exception = Some(e))
        case e: Throwable =>
          brokerTopicStats.topicStats(tp.topic).failedFetchRequestRate.mark()
          brokerTopicStats.allTopicsStats.failedFetchRequestRate.mark()

          val fetchSource = Request.describeReplicaId(replicaId)
          error(s"Error processing fetch with max size $adjustedMaxBytes from $fetchSource " +
            s"on partition $tp: $fetchInfo", e)

          LogReadResult(info = FetchDataInfo(LogOffsetMetadata.UnknownOffsetMetadata, MemoryRecords.EMPTY),
            highWatermark = Log.UnknownOffset,
            leaderLogStartOffset = Log.UnknownOffset,
            leaderLogEndOffset = Log.UnknownOffset,
            followerLogStartOffset = Log.UnknownOffset,
            fetchTimeMs = -1L,
            lastStableOffset = None,
            exception = Some(e))
      }
    }

    var limitBytes = fetchMaxBytes
    val result = new mutable.ArrayBuffer[(TopicPartition, LogReadResult)]
    var minOneMessage = !hardMaxBytesLimit
    readPartitionInfo.foreach { case (tp, fetchInfo) =>
      val readResult = read(tp, fetchInfo, limitBytes, minOneMessage)
      val recordBatchSize = readResult.info.records.sizeInBytes
      // Once we read from a non-empty partition, we stop ignoring request and partition level size limits
      if (recordBatchSize > 0)
        minOneMessage = false
      limitBytes = math.max(0, limitBytes - recordBatchSize)
      result += (tp -> readResult)
    }
    result
  }