kafka 心跳和 reblance

kafka 的心跳是 kafka consumer 和 broker 之间的健康检查，只有当 broker coordinator 正常时，consumer 才会发送心跳。

consumer 和 rebalance 相关的 2 个配置参数：

参数名                --> MemberMetadata 字段
session.timeout.ms   --> MemberMetadata.sessionTimeoutMs
max.poll.interval.ms --> MemberMetadata.rebalanceTimeoutMs

broker 端，sessionTimeoutMs 参数

broker 处理心跳的逻辑在 GroupCoordinator 类中：如果心跳超期， broker coordinator 会把消费者从 group 中移除，并触发 rebalance。

   private def completeAndScheduleNextHeartbeatExpiration(group: GroupMetadata, member: MemberMetadata) {
     // complete current heartbeat expectation
     member.latestHeartbeat = time.milliseconds()
     val memberKey = MemberKey(member.groupId, member.memberId)
     heartbeatPurgatory.checkAndComplete(memberKey)

     // reschedule the next heartbeat expiration deadline
     // 计算心跳截止时刻
     val newHeartbeatDeadline = member.latestHeartbeat + member.sessionTimeoutMs
     val delayedHeartbeat = new DelayedHeartbeat(this, group, member, newHeartbeatDeadline, member.sessionTimeoutMs)
     heartbeatPurgatory.tryCompleteElseWatch(delayedHeartbeat, Seq(memberKey))
   } 

   // 心跳过期
   def onExpireHeartbeat(group: GroupMetadata, member: MemberMetadata, heartbeatDeadline: Long) {
     group.inLock {
       if (!shouldKeepMemberAlive(member, heartbeatDeadline)) {
         info(s"Member ${member.memberId} in group ${group.groupId} has failed, removing it from the group")
         removeMemberAndUpdateGroup(group, member)
       }
     }
   }

   private def shouldKeepMemberAlive(member: MemberMetadata, heartbeatDeadline: Long) =
     member.awaitingJoinCallback != null ||
       member.awaitingSyncCallback != null ||
       member.latestHeartbeat + member.sessionTimeoutMs > heartbeatDeadline

consumer 端：sessionTimeoutMs，rebalanceTimeoutMs 参数

如果客户端发现心跳超期，客户端会标记 coordinator 为不可用，并阻塞心跳线程；如果超过了 poll 消息的间隔超过了 rebalanceTimeoutMs，则 consumer 告知 broker 主动离开消费组，也会触发 rebalance

org.apache.kafka.clients.consumer.internals.AbstractCoordinator.HeartbeatThread 代码片段：

if (coordinatorUnknown()) {
    if (findCoordinatorFuture != null || lookupCoordinator().failed())
        // the immediate future check ensures that we backoff properly in the case that no
        // brokers are available to connect to.
        AbstractCoordinator.this.wait(retryBackoffMs);
} else if (heartbeat.sessionTimeoutExpired(now)) {
    // the session timeout has expired without seeing a successful heartbeat, so we should
    // probably make sure the coordinator is still healthy.
    markCoordinatorUnknown();
} else if (heartbeat.pollTimeoutExpired(now)) {
    // the poll timeout has expired, which means that the foreground thread has stalled
    // in between calls to poll(), so we explicitly leave the group.
    maybeLeaveGroup();
} else if (!heartbeat.shouldHeartbeat(now)) {
    // poll again after waiting for the retry backoff in case the heartbeat failed or the
    // coordinator disconnected
    AbstractCoordinator.this.wait(retryBackoffMs);
} else {
    heartbeat.sentHeartbeat(now);

    sendHeartbeatRequest().addListener(new RequestFutureListener<Void>() {
        @Override
        public void onSuccess(Void value) {
            synchronized (AbstractCoordinator.this) {
                heartbeat.receiveHeartbeat(time.milliseconds());
            }
        }

        @Override
        public void onFailure(RuntimeException e) {
            synchronized (AbstractCoordinator.this) {
                if (e instanceof RebalanceInProgressException) {
                    // it is valid to continue heartbeating while the group is rebalancing. This
                    // ensures that the coordinator keeps the member in the group for as long
                    // as the duration of the rebalance timeout. If we stop sending heartbeats,
                    // however, then the session timeout may expire before we can rejoin.
                    heartbeat.receiveHeartbeat(time.milliseconds());
                } else {
                    heartbeat.failHeartbeat();

                    // wake up the thread if it's sleeping to reschedule the heartbeat
                    AbstractCoordinator.this.notify();
                }
            }
        }
    });
}

 /**
  * A helper class for managing the heartbeat to the coordinator
  */
 public final class Heartbeat {
     private final long sessionTimeout;
     private final long heartbeatInterval;
     private final long maxPollInterval;
     private final long retryBackoffMs;

     private volatile long lastHeartbeatSend; // volatile since it is read by metrics
     private long lastHeartbeatReceive;
     private long lastSessionReset;
     private long lastPoll;
     private boolean heartbeatFailed;

     public Heartbeat(long sessionTimeout,
                      long heartbeatInterval,
                      long maxPollInterval,
                      long retryBackoffMs) {
         if (heartbeatInterval >= sessionTimeout)
             throw new IllegalArgumentException("Heartbeat must be set lower than the session timeout");

         this.sessionTimeout = sessionTimeout;
         this.heartbeatInterval = heartbeatInterval;
         this.maxPollInterval = maxPollInterval;
         this.retryBackoffMs = retryBackoffMs;
     }

     public void poll(long now) {
         this.lastPoll = now;
     }

     public void sentHeartbeat(long now) {
         this.lastHeartbeatSend = now;
         this.heartbeatFailed = false;
     }

     public void failHeartbeat() {
         this.heartbeatFailed = true;
     }

     public void receiveHeartbeat(long now) {
         this.lastHeartbeatReceive = now;
     }

     public boolean shouldHeartbeat(long now) {
         return timeToNextHeartbeat(now) == 0;
     }

     public long lastHeartbeatSend() {
         return this.lastHeartbeatSend;
     }

     public long timeToNextHeartbeat(long now) {
         long timeSinceLastHeartbeat = now - Math.max(lastHeartbeatSend, lastSessionReset);
         final long delayToNextHeartbeat;
         if (heartbeatFailed)
             delayToNextHeartbeat = retryBackoffMs;
         else
             delayToNextHeartbeat = heartbeatInterval;

         if (timeSinceLastHeartbeat > delayToNextHeartbeat)
             return 0;
         else
             return delayToNextHeartbeat - timeSinceLastHeartbeat;
     }

     public boolean sessionTimeoutExpired(long now) {
         return now - Math.max(lastSessionReset, lastHeartbeatReceive) > sessionTimeout;
     }

     public long interval() {
         return heartbeatInterval;
     }

     public void resetTimeouts(long now) {
         this.lastSessionReset = now;
         this.lastPoll = now;
         this.heartbeatFailed = false;
     }

     public boolean pollTimeoutExpired(long now) {
         return now - lastPoll > maxPollInterval;
     }

 }

join group 的处理逻辑：kafka.coordinator.group.GroupCoordinator#onCompleteJoin