DelayedOperationPurgatory机制（四）：DelayProduce之appendMesaages

生产者把消息发送到服务端时，KafkaApis会调用hadleProducerRequest方法，他会调用ReplicaManager.appendMessages方法将消息追加到Log中，生成相应的DelayedProduce对象并添加到delayedProducePurgatory中处理，下面介绍一下appendMesaages方法

  def appendMessages(timeout: Long,
                     requiredAcks: Short,
                     internalTopicsAllowed: Boolean,
                     messagesPerPartition: Map[TopicPartition, MessageSet],
                     responseCallback: Map[TopicPartition, PartitionResponse] => Unit) {

    if (isValidRequiredAcks(requiredAcks)) {
      val sTime = SystemTime.milliseconds
      // 将消息追加到Log中，同时还会检测delayedFetchPurgatory中相关的key对应的DelayedFetch，满足条件则将其执行完成
      val localProduceResults = appendToLocalLog(internalTopicsAllowed, messagesPerPartition, requiredAcks)
      debug("Produce to local log in %d ms".format(SystemTime.milliseconds - sTime))
      // 对追加的结果进行装换，注意ProducePartitionStateus的参数
      val produceStatus = localProduceResults.map { case (topicPartition, result) =>
        topicPartition ->
                ProducePartitionStatus(
                  result.info.lastOffset + 1, // required offset
                  new PartitionResponse(result.errorCode, result.info.firstOffset, result.info.timestamp)) // response status
      }
      // 下面检测是否生成DelayedProduce，其中一个条件就是检测ProducerRequest中acks的字段是否为-1
      if (delayedRequestRequired(requiredAcks, messagesPerPartition, localProduceResults)) {
        // create delayed produce operation
        val produceMetadata = ProduceMetadata(requiredAcks, produceStatus)
        // 创建DelayedProduce对象
        val delayedProduce = new DelayedProduce(timeout, produceMetadata, this, responseCallback)

        // create a list of (topic, partition) pairs to use as keys for this delayed produce operation
        val producerRequestKeys = messagesPerPartition.keys.map(new TopicPartitionOperationKey(_)).toSeq

        // try to complete the request immediately, otherwise put it into the purgatory
        // this is because while the delayed produce operation is being created, new
        // requests may arrive and hence make this operation completable.
        // 尝试完成DelayProduce，否则将DelayProduce天啊及到DelayProducePurgatory中管理
        delayedProducePurgatory.tryCompleteElseWatch(delayedProduce, producerRequestKeys)

      } else {
        // we can respond immediately
        val produceResponseStatus = produceStatus.mapValues(status => status.responseStatus)
        responseCallback(produceResponseStatus)
      }
    } else {
      // If required.acks is outside accepted range, something is wrong with the client
      // Just return an error and don't handle the request at all
      val responseStatus = messagesPerPartition.map {
        case (topicAndPartition, messageSet) =>
          (topicAndPartition -> new PartitionResponse(Errors.INVALID_REQUIRED_ACKS.code,
                                                      LogAppendInfo.UnknownLogAppendInfo.firstOffset,
                                                      Message.NoTimestamp))
      }
      responseCallback(responseStatus)
    }
  }