KafkaConsumer 消费逻辑

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 expires
        long 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 immediately
    final 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 faster
    if (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 poll
    firePendingCompletedRequests();

    lock.lock();
    try {
        // note: 处理断开的连接 （pendingDisconnects队列）
        // Handle async disconnects prior to attempting any sends
        handlePendingDisconnects();

        // 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 now
        long 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 backoff
            if (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 status
        checkDisconnects(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 interrupted
        maybeThrowInterruptException();

        // 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 poll
        trySend(now);

        // fail requests that couldn't be sent if they have expired
        failExpiredRequests(now);

        // clean unsent requests collection to keep the map from growing indefinitely
        unsent.clean();
    } finally {
        lock.unlock();
    }

    // called without the lock to avoid deadlock potential if handlers need to acquire locks
    firePendingCompletedRequests();
}

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();
        }
    }