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版本：kafka-clients-2.0.1.jar

之前想写个插件修改 kafkaConsumer 消费者的逻辑，根据 header 过滤一些消息。于是需要了解一下 kafkaConsumer 具体是如何拉取消费消息的，确认在消费之前过滤掉消息是否会有影响。
下面是相关的源码，并通过注释的方式进行说明。

先结论：kafkaConsumer 拉取消息的 offset 是存本地的，根据 offset 拉取消息。开启自动提交时，会自动提交 offset 到 broker（在一些场景下会手动检查是否需要提交），防止重启或reblance时 offset 丢失。而本地保存的 offset 是本地拉取到消息时就更新的，所以自动提交的场景下，在消费前过滤掉消息没有影响。

拉取消息

KafkaConsumer#poll

private ConsumerRecords<K, V> poll(final long timeoutMs, final boolean includeMetadataInTimeout) {// note： 获取轻锁同时检查非多线程环境，并检查 consumer 开启状态 （可以close的）acquireAndEnsureOpen();try {if (timeoutMs < 0) throw new IllegalArgumentException("Timeout must not be negative");// note: subscriptions:SubscriptionState  维护了当前消费者订阅的主题列表的状态信息(组、offset等)//   方法判断是否未订阅或未分配分区if (this.subscriptions.hasNoSubscriptionOrUserAssignment()) {throw new IllegalStateException("Consumer is not subscribed to any topics or assigned any partitions");}// poll for new data until the timeout expireslong elapsedTime = 0L;do {// note： 是否触发了唤醒操作 （调用了当前对象的 wakeup 方法） 通过抛异常的方式退出当前方法，（这里是while循环，可能一直在拉取消息,(无新消息时)）client.maybeTriggerWakeup();final long metadataEnd;if (includeMetadataInTimeout) {final long metadataStart = time.milliseconds();// note: 更新分区分配元数据以及offset， remain是用来算剩余时间的// 内部逻辑：//  1 协调器 ConsumerCoordinator.poll 拉取协调器事件(期间会发送心跳、自动提交)//  2 updateFetchPositions 更新positions，（但本地有positions数据就不更新，更新完pos后，如果还有缺的，就先使用reset策略，最后异步设置pos）if (!updateAssignmentMetadataIfNeeded(remainingTimeAtLeastZero(timeoutMs, elapsedTime))) {return ConsumerRecords.empty();}metadataEnd = time.milliseconds();elapsedTime += metadataEnd - metadataStart;} else {while (!updateAssignmentMetadataIfNeeded(Long.MAX_VALUE)) {log.warn("Still waiting for metadata");}metadataEnd = time.milliseconds();}//note: 这里终于开始拉取消息了，下面单独讲一下final Map<TopicPartition, List<ConsumerRecord<K, V>>> records = pollForFetches(remainingTimeAtLeastZero(timeoutMs, elapsedTime));if (!records.isEmpty()) {//note: 翻译：返回之前，发送下一个拉取的请求避免阻塞response// before returning the fetched records, we can send off the next round of fetches// and avoid block waiting for their responses to enable pipelining while the user// is handling the fetched records.//// NOTE: since the consumed position has already been updated, we must not allow// wakeups or any other errors to be triggered prior to returning the fetched records.if (fetcher.sendFetches() > 0 || client.hasPendingRequests()) {client.pollNoWakeup();}//note:  这里使用拦截器拦截一下，这里可以对消息进行修改或过滤，但需要注意commit的问题return this.interceptors.onConsume(new ConsumerRecords<>(records));}final long fetchEnd = time.milliseconds();elapsedTime += fetchEnd - metadataEnd;} while (elapsedTime < timeoutMs);return ConsumerRecords.empty();} finally {release();}
}

关于 pollForFetches 的逻辑

pollForFetches

private Map<TopicPartition, List<ConsumerRecord<K, V>>> pollForFetches(final long timeoutMs) {final long startMs = time.milliseconds();long pollTimeout = Math.min(coordinator.timeToNextPoll(startMs), timeoutMs);// note: 先获取已经拉取了的消息，存在就直接返回//  fetcher 内部有一个 completedFetches 暂存预拉取的请求，可解析出 nextLineRecords 用于暂存预拉取的消息//    从 nextLineRecords 获取消息时，先判断一下状态（如assigned、paused、position），//      然后获取到消息后，再更新 subscriptions 中的 position 位置（值为下一个的offset）， 注意这个时候还没commit// if data is available already, return it immediatelyfinal Map<TopicPartition, List<ConsumerRecord<K, V>>> records = fetcher.fetchedRecords();if (!records.isEmpty()) {return records;}// note: 没有预拉取的消息，发送拉取请求(实际没发) //  先找到partition的leader，检查可用，检查没有待处理的请求，然后从 subscriptions 获取 position，构建ClientRequest暂存//  以及设置listener (成功则处理结果入队列completedFetches)// send any new fetches (won't resend pending fetches)fetcher.sendFetches();// We do not want to be stuck blocking in poll if we are missing some positions// since the offset lookup may be backing off after a failure// NOTE: the use of cachedSubscriptionHashAllFetchPositions means we MUST call// updateAssignmentMetadataIfNeeded before this method.if (!cachedSubscriptionHashAllFetchPositions && pollTimeout > retryBackoffMs) {pollTimeout = retryBackoffMs;}// note: 轮询等待，详见下文client.poll(pollTimeout, startMs, () -> {// since a fetch might be completed by the background thread, we need this poll condition// to ensure that we do not block unnecessarily in poll()return !fetcher.hasCompletedFetches();});// after the long poll, we should check whether the group needs to rebalance// prior to returning data so that the group can stabilize fasterif (coordinator.rejoinNeededOrPending()) {return Collections.emptyMap();}return fetcher.fetchedRecords();
}

ConsumerNetworkClient#poll

/*** Poll for any network IO.* @param timeout timeout in milliseconds* @param now current time in milliseconds* @param disableWakeup If TRUE disable triggering wake-ups*/
public void poll(long timeout, long now, PollCondition pollCondition, boolean disableWakeup) {// note: 触发已完成的请求的回调处理器  （有一个pendingCompletion的队列）// there may be handlers which need to be invoked if we woke up the previous call to pollfirePendingCompletedRequests();lock.lock();try {// note: 处理断开的连接 （pendingDisconnects队列）// Handle async disconnects prior to attempting any sendshandlePendingDisconnects();// note: 实际上这里才真正发出请求。。 前面那个feature只是构建request//  前面准备的 ClientRequest 放在一个 UnsentRequests （内部map, key：Node，val: requests）中//  这里面取出来进行发送， kafkaClient.ready -> send// send all the requests we can send nowlong pollDelayMs = trySend(now);timeout = Math.min(timeout, pollDelayMs);// note: 这里主要是判断是否需要阻塞 poll （timeout是否为0） 如果没有待完成且判断应该阻塞（completedFetches为空）则阻塞//  poll 里面是从 sockets 里面读写数据// check whether the poll is still needed by the caller. Note that if the expected completion// condition becomes satisfied after the call to shouldBlock() (because of a fired completion// handler), the client will be woken up.if (pendingCompletion.isEmpty() && (pollCondition == null || pollCondition.shouldBlock())) {// if there are no requests in flight, do not block longer than the retry backoffif (client.inFlightRequestCount() == 0)timeout = Math.min(timeout, retryBackoffMs);client.poll(Math.min(maxPollTimeoutMs, timeout), now);now = time.milliseconds();} else {client.poll(0, now);}// note: 检查断开的链接，判断node连接是否断开，是则从unset中取出对应requests，构建response加到completedFetches中// handle any disconnects by failing the active requests. note that disconnects must// be checked immediately following poll since any subsequent call to client.ready()// will reset the disconnect statuscheckDisconnects(now);if (!disableWakeup) {// trigger wakeups after checking for disconnects so that the callbacks will be ready// to be fired on the next call to poll()maybeTriggerWakeup();}// throw InterruptException if this thread is interruptedmaybeThrowInterruptException();// note: 再发一次请求，推测是可能部分 node 的连接在第一次没有ready （没ready会进行初始化，并返回false）// try again to send requests since buffer space may have been// cleared or a connect finished in the polltrySend(now);// fail requests that couldn't be sent if they have expiredfailExpiredRequests(now);// clean unsent requests collection to keep the map from growing indefinitelyunsent.clean();} finally {lock.unlock();}// called without the lock to avoid deadlock potential if handlers need to acquire locksfirePendingCompletedRequests();
}

自动提交

提交 offset 是为了防止重启或 rebalance 后，导致本地 position 丢失无法正常拉取后面的消息。

入口是 ConsumerCoordinator#maybeAutoCommitOffsetsAsync

触发逻辑主要是

• KafkaConsumer#poll 拉消息
• -> KafkaConsumer#updateAssignmentMetadataIfNeeded
• -> ConsumerCoordinator#poll -> maybeAutoCommitOffsetsAsync （也是先构建请求存 unset 里面，等拉消息的时候再发出去）

    public void maybeAutoCommitOffsetsAsync(long now) {// 这里用来判断是否满足自动提交的间隔if (autoCommitEnabled && now >= nextAutoCommitDeadline) {this.nextAutoCommitDeadline = now + autoCommitIntervalMs;doAutoCommitOffsetsAsync();}}