KafkaConsumer分析

一 重要的字段

String clientId：Consumer惟一标识

ConsumerCoordinator coordinator： 控制Consumer与服务器端GroupCoordinator之间的通讯逻辑

Fetcher<K, V> fetcher： 负责从服务器端获取消息的组件，而且更新partition的offset

ConsumerNetworkClient client:  负责和服务器端通讯

SubscriptionState subscriptions: 便于快速获取topic partition等状态，维护了消费者消费状态

Metadata metadata: 集群元数据信息

AtomicLong currentThread: 当前使用KafkaConsumer的线程id

AtomicInteger refcount: 重入次数



二 核心的方法

2.1 subscribe 订阅主题

订阅给定的主题列表，以得到动态分配的分区

主题的订阅不是增量的，这个列表将会代替当前的分配。注意，不可能将主题订阅与组管理与手动分区分配相结合

做为组管理的一部分，消费者将会跟踪属于某一个特殊组的消费者列表，若是知足在下列条件，将会触发再平衡操做:

1 订阅的主题列表的那些分区数量的改变

2 主题建立或者删除

3 消费者组的成员挂了

4 经过join api将一个新的消费者添加到一个存在的消费者组

public void subscribe(Collection<String> topics, ConsumerRebalanceListener listener) {
    // 取得一把锁
    acquire();
    try {
        if (topics == null) { // 主题列表为null，抛出异常
            throw new IllegalArgumentException("Topiccollection to subscribe to cannot be null");
        } else if (topics.isEmpty()) {// 主题列表为空，取消订阅
            this.unsubscribe();
        } else {
            for (String topic : topics) {
                if (topic == null || topic.trim().isEmpty())
                    throw new IllegalArgumentException("Topic collection to subscribe to cannot contain null or emptytopic");
            }
            log.debug("Subscribed to topic(s):{}", Utils.join(topics, ", "));
            this.subscriptions.subscribe(new HashSet<>(topics), listener);
            // 用新提供的topic集合替换当前的topic集合，若是启用了主题过时，主题的过时时间将在下一次更新中从新设置。
            metadata.setTopics(subscriptions.groupSubscription());
        }
    } finally {
        // 释放锁
        release();
    }
}



2.2 assign 手动分配分区

对于用户手动指定topic的订阅模式，经过此方法能够分配分区列表给一个消费者：

public void assign(Collection<TopicPartition> partitions) {
    acquire();
    try {
        if (partitions == null) {
            throw new IllegalArgumentException("Topic partition collection to assign to cannot be null");
        } else if (partitions.isEmpty()) {// partition为空取消订阅
            this.unsubscribe();
        } else {
            Set<String> topics = new HashSet<>();
            // 遍历TopicPartition，把topic添加到一个集合里
            for (TopicPartition tp : partitions) {
                String topic = (tp != null) ? tp.topic() : null;
                if (topic == null || topic.trim().isEmpty())
                    throw new IllegalArgumentException("Topic partitions to assign to cannot have null or empty topic");
                topics.add(topic);
            }

            // 进行一次自动提交
            this.coordinator.maybeAutoCommitOffsetsNow();

            log.debug("Subscribed to partition(s): {}", Utils.join(partitions, ", "));
            // 根据用户提供的指定的partitions 改变assignment
            this.subscriptions.assignFromUser(new HashSet<>(partitions));
            metadata.setTopics(topics);// 更新metatdata topic
        }
    } finally {
        release();
    }
}


2.3 commitSync & commitAsync 提交消费者已经消费完的消息的offset,为指定已订阅的主题和分区列表返回最后一次poll返回的offset

public void commitSync(final Map<TopicPartition, OffsetAndMetadata> offsets) {
    acquire();
    try {
        coordinator.commitOffsetsSync(offsets);
    } finally {
        release();
    }
}
 
public void commitAsync(final Map<TopicPartition, OffsetAndMetadata> offsets, OffsetCommitCallback callback) {
    acquire();
    try {
        log.debug("Committing offsets: {} ", offsets);
        coordinator.commitOffsetsAsync(new HashMap<>(offsets), callback);
    } finally {
        release();
    }
}


2.4 seek 指定消费者消费的起始位置

public void seek(TopicPartition partition, long offset) {
    if (offset < 0) {
        throw new IllegalArgumentException("seek offset must not be a negative number");
    }
    acquire();
    try {
        log.debug("Seeking to offset {} for partition {}", offset, partition);
        this.subscriptions.seek(partition, offset);
    } finally {
        release();
    }
}
// 为指定的分区查找第一个offset
public void seekToBeginning(Collection<TopicPartition> partitions) {
    acquire();
    try {
        Collection<TopicPartition> parts = partitions.size() == 0 ? this.subscriptions.assignedPartitions() : partitions;
        for (TopicPartition tp : parts) {
            log.debug("Seeking to beginning of partition {}", tp);
            subscriptions.needOffsetReset(tp, OffsetResetStrategy.EARLIEST);
        }
    } finally {
        release();
    }
}
// 为指定的分区查找最后的offset
public void seekToEnd(Collection<TopicPartition> partitions) {
    acquire();
    try {
        Collection<TopicPartition> parts = partitions.size() == 0 ? this.subscriptions.assignedPartitions() : partitions;
        for (TopicPartition tp : parts) {
            log.debug("Seeking to end of partition {}", tp);
            subscriptions.needOffsetReset(tp, OffsetResetStrategy.LATEST);
        }
    } finally {
        release();
    }
}


2.5 poll方法 获取消息

从指定的主题或者分区获取数据，在poll以前，你没有订阅任何主题或分区是不行的,每一次poll,消费者都会尝试使用最后一次消费的offset做为接下来获取数据的start offset，最后一次消费的offset也能够经过seek(TopicPartition, long)设置或者自动设置

public ConsumerRecords<K, V> poll(long timeout) {
    acquire();
    try {
        if (timeout < 0)
            throw new IllegalArgumentException("Timeout must not be negative");
        // 若是没有任何订阅，抛出异常
        if (this.subscriptions.hasNoSubscriptionOrUserAssignment())
            throw new IllegalStateException("Consumer is not subscribed to any topics or assigned any partitions");

        // 一直poll新数据直到超时
        long start = time.milliseconds();
        // 距离超时还剩余多少时间
        long remaining = timeout;
        do {
            // 获取数据，若是自动提交，则进行偏移量自动提交，若是设置offset重置，则进行offset重置
            Map<TopicPartition, List<ConsumerRecord<K, V>>> records = pollOnce(remaining);
            if (!records.isEmpty()) {
                // 再返回结果以前，咱们能够进行下一轮的fetch请求，避免阻塞等待
                fetcher.sendFetches();
                client.pollNoWakeup();
                // 若是有拦截器进行拦截，没有直接返回
                if (this.interceptors == null)
                    return new ConsumerRecords<>(records);
                else
                    return this.interceptors.onConsume(new ConsumerRecords<>(records));
            }

            long elapsed = time.milliseconds() - start;
            remaining = timeout - elapsed;
        } while (remaining > 0);

        return ConsumerRecords.empty();
    } finally {
        release();
    }
}


private Map<TopicPartition, List<ConsumerRecord<K, V>>> pollOnce(long timeout) {
    // 轮询coordinator事件，处理周期性的offset提交
    coordinator.poll(time.milliseconds());

    // fetch positions if we have partitions we're subscribed to that we
    // don't know the offset for
    // 判断上一次消费的位置是否为空,若是不为空，则
    if (!subscriptions.hasAllFetchPositions())
        // 更新fetch position
        updateFetchPositions(this.subscriptions.missingFetchPositions());

    // 数据你准备好了就当即返回，也就是还有可能没有准备好
    Map<TopicPartition, List<ConsumerRecord<K, V>>> records = fetcher.fetchedRecords();
    if (!records.isEmpty())
        return records;

    // 咱们须要发送新fetch请求
    fetcher.sendFetches();

    long now = time.milliseconds();
    long pollTimeout = Math.min(coordinator.timeToNextPoll(now), timeout);

    client.poll(pollTimeout, now, new PollCondition() {
        @Override
        public boolean shouldBlock() {
            // 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();
        }
    });
    // 早长时间的poll以后，咱们应该在返回数据以前检查是否这个组须要从新平衡，以致于这个组可以迅速的稳定
    if (coordinator.needRejoin())
        return Collections.emptyMap();
    // 获取返回的消息
    return fetcher.fetchedRecords();
}


2.6 pause 暂停消费者，暂停后poll返回空

public void pause(Collection<TopicPartition> partitions) {
    acquire();
    try {
        for (TopicPartition partition: partitions) {
            log.debug("Pausing partition {}", partition);
            subscriptions.pause(partition);
        }
    } finally {
        release();
    }
}
// 返回暂停的分区

public Set<TopicPartition> paused() {
    acquire();
    try {
        return Collections.unmodifiableSet(subscriptions.pausedPartitions());
    } finally {
        release();
    }
}


2.7 resume 恢复消费者

public void resume(Collection<TopicPartition> partitions) {
    acquire();
    try {
        for (TopicPartition partition: partitions) {
            log.debug("Resuming partition {}", partition);
            subscriptions.resume(partition);
        }
    } finally {
        release();
    }
}


2.8 position方法 获取下一个消息的offset

// 获取下一个record的offset
public long position(TopicPartition partition) {
    acquire();
    try {
        if (!this.subscriptions.isAssigned(partition))
            throw new IllegalArgumentException("You can only check the position for partitions assigned to this consumer.");
        Long offset = this.subscriptions.position(partition);
        if (offset == null) {
            updateFetchPositions(Collections.singleton(partition));
            offset = this.subscriptions.position(partition);
        }
        return offset;
    } finally {
        release();
    }
}