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
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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
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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);
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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;
}
}
