2021-03-26 kafka生产者RecordAccumul

RecordAccumulator

This class acts as a queue that accumulates records into MemoryRecords instances to be sent to the server.
The accumulator uses a bounded amount of memory and append calls will block when that memory is exhausted, unless this behavior is explicitly disabled.
累加器，就是kafka的客户端缓存。所有send的ProducerRecord存放在这里，等待发送到远程Broker中。
缓存空间是一定的，缓存满了之后，新的ProducerRecord在send时会被阻塞。

缓存池

• RecordAccumulator

  RecordAccumulator中用ConcurrentMap保存当前缓存的partition和每个partition的数据，每个partition数据保存在双端队列中：ConcurrentMap<TopicPartition, Deque<RecordBatch>> batches

ConcurrentMap实际上是一个自定义的CopyOnWriteMap

• RecordBatch

  Deque中保存着所有待发送的消息，某些消息又组成了一个RecordBatch

• ByteBuffer

  RecordBatch中保存着实际的数据，每个数据序列化成二进制数组后，与其他kafka消息协议字段一起保存在ByteBuffer中

• Compressor

  Compressor用来把数据写入到ByteBuffer中。它像一支笔将kafka数据写到ByteBuffer里。

  private final ByteBufferOutputStream bufferStream;
  private final DataOutputStream appendStream;
  
  bufferStream = new ByteBufferOutputStream(buffer);
  appendStream = wrapForOutput(bufferStream, type, COMPRESSION_DEFAULT_BUFFER_SIZE);
  

• Trunk

  trunk用来关联每个record，用于消息发出后回调发送方当前record是否发送成功

/**
 * A callback and the associated FutureRecordMetadata argument to pass to it.
 */
final private static class Thunk {
    final Callback callback;
    //记录了record的相关信息，比如key、value、partition、exception等
    final FutureRecordMetadata future;

    public Thunk(Callback callback, FutureRecordMetadata future) {
        this.callback = callback;
        this.future = future;
    }
}