源码分析Kafka之Producer

Kafka是一款很棒的消息系统，可以看看我之前写的 后端好书阅读与推荐来了解一下它的整体设计。今天我们就来深入了解一下它的实现细节（我fork了一份代码），首先关注Producer这一方。

要使用kafka首先要实例化一个KafkaProducer，需要有brokerIP、序列化器等必要Properties以及acks（0、1、n）、compression、retries、batch.size等非必要Properties，通过这个简单的接口可以控制Producer大部分行为，实例化后就可以调用send方法发送消息了。

核心实现是这个方法：

public Future send(ProducerRecord record, Callback callback) {
// intercept the record, which can be potentially modified; this method does not throw exceptions
ProducerRecord interceptedRecord = this.interceptors.onSend(record);//①
return doSend(interceptedRecord, callback);//②
}
通过不同的模式可以实现发送即忘（忽略返回结果）、同步发送（获取返回的future对象，回调函数置为null）、异步发送（设置回调函数）三种消息模式。

我们来看看消息类ProducerRecord有哪些属性：

private final String topic;//主题
private final Integer partition;//分区
private final Headers headers;//头
private final K key;//键
private final V value;//值
private final Long timestamp;//时间戳
它有多个构造函数，可以适应不同的消息类型：比如有无分区、有无key等。

①中ProducerInterceptors（有0 ~ 无穷多个，形成一个拦截链）对ProducerRecord进行拦截处理（比如打上时间戳，进行审计与统计等操作）

public ProducerRecord onSend(ProducerRecord record) {
ProducerRecord interceptRecord = record;
for (ProducerInterceptor interceptor : this.interceptors) {
try {
interceptRecord = interceptor.onSend(interceptRecord);
} catch (Exception e) {
// 不抛出异常，继续执行下一个拦截器
if (record != null)
log.warn("Error executing interceptor onSend callback for topic: {}, partition: {}", record.topic(), record.partition(), e);
else
log.warn("Error executing interceptor onSend callback", e);
}
}
return interceptRecord;
}
如果用户有定义就进行处理并返回处理后的ProducerRecord，否则直接返回本身。

然后②中doSend真正发送消息，并且是异步的（源码太长只保留关键）：

private Future doSend(ProducerRecord record, Callback callback) {
TopicPartition tp = null;
try {
// 序列化 key 和 value
byte[] serializedKey;
try {
serializedKey = keySerializer.serialize(record.topic(), record.headers(), record.key());
} catch (ClassCastException cce) {
}
byte[] serializedValue;
try {
serializedValue = valueSerializer.serialize(record.topic(), record.headers(), record.value());
} catch (ClassCastException cce) {
}
// 计算分区获得主题与分区
int partition = partition(record, serializedKey, serializedValue, cluster);
tp = new TopicPartition(record.topic(), partition);
// 回调与事务处理省略。
Header[] headers = record.headers().toArray();
// 消息追加到RecordAccumulator中
RecordAccumulator.RecordAppendResult result = accumulator.append(tp, timestamp, serializedKey,
serializedValue, headers, interceptCallback, remainingWaitMs);
// 该批次满了或者创建了新的批次就要唤醒IO线程发送该批次了，也就是sender的wakeup方法
if (result.batchIsFull || result.newBatchCreated) {
log.trace("Waking up the sender since topic {} partition {} is either full or getting a new batch", record.topic(), partition);
this.sender.wakeup();
}
return result.future;
} catch (Exception e) {
// 拦截异常并抛出
this.interceptors.onSendError(record, tp, e);
throw e;
}
}
下面是计算分区的方法：

private int partition(ProducerRecord record,
byte[] serializedKey, byte[] serializedValue, Cluster cluster) {
Integer partition = record.partition();
// 消息有分区就直接使用，否则就使用分区器计算
return partition != null ?
partition :
partitioner.partition(
record.topic(), record.key(), serializedKey,
record.value(), serializedValue, cluster);
}
默认的分区器DefaultPartitioner实现方式是如果partition存在就直接使用，否则根据key计算partition，如果key也不存在就使用round robin算法分配partition。

/**

• The default partitioning strategy:
• • If a partition is specified in the record, use it
  • If no partition is specified but a key is present choose a partition based on a hash of the key
  • If no partition or key is present choose a partition in a round-robin fashion
    */
    public class DefaultPartitioner implements Partitioner {
    private final ConcurrentMap topicCounterMap = new ConcurrentHashMap<>();
    public int partition(String topic, Object key, byte[] keyBytes, Object value, byte[] valueBytes, Cluster cluster) {
    List partitions = cluster.partitionsForTopic(topic);
    int numPartitions = partitions.size();
    if (keyBytes == null) {//key为空
    int nextValue = nextValue(topic);
    List availablePartitions = cluster.availablePartitionsForTopic(topic);//可用的分区
    if (availablePartitions.size() > 0) {//有分区，取模就行
    int part = Utils.toPositive(nextValue) % availablePartitions.size();
    return availablePartitions.get(part).partition();
    } else {// 无分区，
    return Utils.toPositive(nextValue) % numPartitions;
    }
    } else {// key 不为空，计算key的hash并取模获得分区
    return Utils.toPositive(Utils.murmur2(keyBytes)) % numPartitions;
    }
    }
    private int nextValue(String topic) {
    AtomicInteger counter = topicCounterMap.get(topic);
    if (null == counter) {
    counter = new AtomicInteger(ThreadLocalRandom.current().nextInt());
    AtomicInteger currentCounter = topicCounterMap.putIfAbsent(topic, counter);
    if (currentCounter != null) {
    counter = currentCounter;
    }
    }
    return counter.getAndIncrement();//返回并加一，在取模的配合下就是round robin
    }
    }
    以上就是发送消息的逻辑处理，接下来我们再看看消息发送的物理处理。

  Sender（是一个Runnable，被包含在一个IO线程ioThread中，该线程不断从RecordAccumulator队列中的读取消息并通过Selector将数据发送给Broker）的wakeup方法，实际上是KafkaClient接口的wakeup方法，由NetworkClient类实现，采用了NIO，也就是java.nio.channels.Selector.wakeup()方法实现。

  Sender的run中主要逻辑是不停执行准备消息和等待消息：

  long pollTimeout = sendProducerData(now);//③
  client.poll(pollTimeout, now);//④
  ③完成消息设置并保存到信道中，然后监听感兴趣的key，由KafkaChannel实现。

  public void setSend(Send send) {
  if (this.send != null)
  throw new IllegalStateException("Attempt to begin a send operation with prior send operation still in progress, connection id is " + id);
  this.send = send;
  this.transportLayer.addInterestOps(SelectionKey.OP_WRITE);
  }
  // transportLayer的一种实现中的相关方法
  public void addInterestOps(int ops) {
  key.interestOps(key.interestOps() | ops);
  }
  ④主要是Selector的poll，其select被wakeup唤醒：

  public void poll(long timeout) throws IOException {
  / check ready keys /
  long startSelect = time.nanoseconds();
  int numReadyKeys = select(timeout);//wakeup使其停止阻塞
  long endSelect = time.nanoseconds();
  this.sensors.selectTime.record(endSelect - startSelect, time.milliseconds());
  if (numReadyKeys > 0 || !immediatelyConnectedKeys.isEmpty() || dataInBuffers) {
  Set readyKeys = this.nioSelector.selectedKeys();
  // Poll from channels that have buffered data (but nothing more from the underlying socket)
  if (dataInBuffers) {
  keysWithBufferedRead.removeAll(readyKeys); //so no channel gets polled twice
  Set toPoll = keysWithBufferedRead;
  keysWithBufferedRead = new HashSet<>(); //poll() calls will repopulate if needed
  pollSelectionKeys(toPoll, false, endSelect);
  }
  // Poll from channels where the underlying socket has more data
  pollSelectionKeys(readyKeys, false, endSelect);
  // Clear all selected keys so that they are included in the ready count for the next select
  readyKeys.clear();
  pollSelectionKeys(immediatelyConnectedKeys, true, endSelect);
  immediatelyConnectedKeys.clear();
  } else {
  madeReadProgressLastPoll = true; //no work is also "progress"
  }
  long endIo = time.nanoseconds();
  this.sensors.ioTime.record(endIo - endSelect, time.milliseconds());
  }
  其中pollSelectionKeys方法会调用如下方法完成消息发送：

  public Send write() throws IOException {
  Send result = null;
  if (send != null && send(send)) {
  result = send;
  send = null;
  }
  return result;
  }
  private boolean send(Send send) throws IOException {
  send.writeTo(transportLayer);
  if (send.completed())
  transportLayer.removeInterestOps(SelectionKey.OP_WRITE);
  return send.completed();
  }
  Send是一次数据发包，一般由ByteBufferSend或者MultiRecordsSend实现，其writeTo调用transportLayer的write方法，一般由PlaintextTransportLayer或者SslTransportLayer实现，区分是否使用ssl：

  public long writeTo(GatheringByteChannel channel) throws IOException {
  long written = channel.write(buffers);
  if (written < 0)
  throw new EOFException("Wrote negative bytes to channel. This shouldn't happen.");
  remaining -= written;
  pending = TransportLayers.hasPendingWrites(channel);
  return written;
  }
  public int write(ByteBuffer src) throws IOException {
  return socketChannel.write(src);
  }
  到此就把Producer的业务相关逻辑处理和非业务相关的网络 2方面的主要流程梳理清楚了。其他额外的功能是通过一些配置保证的。

  比如顺序保证就是max.in.flight.requests.per.connection，InFlightRequests的doSend会进行判断（由NetworkClient的canSendRequest调用），只要该参数设为1即可保证当前包未确认就不能发送下一个包从而实现有序性

  public boolean canSendMore(String node) {
  Deque queue = requests.get(node);
  return queue == null || queue.isEmpty() ||
  (queue.peekFirst().send.completed() && queue.size() < this.maxInFlightRequestsPerConnection);
  }
  再比如可靠性，通过设置acks，Sender中sendProduceRequest的clientRequest加入了回调函数：

  RequestCompletionHandler callback = new RequestCompletionHandler() {
  public void onComplete(ClientResponse response) {
  handleProduceResponse(response, recordsByPartition, time.milliseconds());//调用completeBatch
  }
  };
  /**

  • 完成或者重试投递，这里如果acks不对就会重试
  • @param batch The record batch
  • @param response The produce response
  • @param correlationId The correlation id for the request
  • @param now The current POSIX timestamp in milliseconds
    */
    private void completeBatch(ProducerBatch batch, ProduceResponse.PartitionResponse response, long correlationId,
    long now, long throttleUntilTimeMs) {
    }
    public class ProduceResponse extends AbstractResponse {
    /**
  • Possible error code:
  • INVALID_REQUIRED_ACKS (21)
    */
    }
    kafka源码一层一层包装很多，错综复杂，如有错误请大家不吝赐教。

  

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