RocketMQ——消费过程

PullMessageService负责拉取消息，从远端服务器拉取消息后存储到ProcessQueue中，然后调用ConsumeMessageService#submitConsumeRequest 方法进行消费，适应小城池来消费消息，确保消息拉取与消息消费的解耦。

1.jpg

消息消费

ConsumeMessageConcurrentlyService#submitConsumeRequest 负责提交消费请求

 @Override
    public void submitConsumeRequest(
        final List msgs,
        final ProcessQueue processQueue,
        final MessageQueue messageQueue,
        final boolean dispatchToConsume) {
        //获取消费批次
        final int consumeBatchSize = this.defaultMQPushConsumer.getConsumeMessageBatchMaxSize();
        //小于最大消息数则直接提交
        if (msgs.size() <= consumeBatchSize) {
            ConsumeRequest consumeRequest = new ConsumeRequest(msgs, processQueue, messageQueue);
                this.consumeExecutor.submit(consumeRequest);
        } else {
        //消息切割提交
            for (int total = 0; total < msgs.size(); ) {
                ConsumeRequest consumeRequest = new ConsumeRequest(msgThis, processQueue, messageQueue);
                try {
                    this.consumeExecutor.submit(consumeRequest);
                } catch (RejectedExecutionException e) {
                //失败后重试
                    this.submitConsumeRequestLater(consumeRequest);
                }
            }
        }
    }

消息消费只是先提交到线程池中，每一个ConsumeRequest 为一个线程。
ConsumeMessageConcurrentlyService$ConsumeRequest#run 整体流程如下

2.jpg

消费进度管理

• 集群模式：消费进度保存在broker，需要每个消费端都可以访问到
• 广播模式：消息进度存储在本地，只需要本机访问即可。

核心类图

3.jpg

广播模式消息进度存储

org.apache.rocketmq.client.consumer.store.LocalFileOffsetStore
存储位置

 public final static String LOCAL_OFFSET_STORE_DIR = System.getProperty(
        "rocketmq.client.localOffsetStoreDir",
        System.getProperty("user.home") + File.separator + ".rocketmq_offsets");

MQClientInstance#startScheduledTask

       this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {

            @Override
            public void run() {
                try {
                    MQClientInstance.this.persistAllConsumerOffset();
                } catch (Exception e) {
                    log.error("ScheduledTask persistAllConsumerOffset exception", e);
                }
            }
        }, 1000 * 10, this.clientConfig.getPersistConsumerOffsetInterval(), TimeUnit.MILLISECONDS);

每隔十秒持久化一次。

集群模式消息进度存储

org.apache.rocketmq.client.consumer.store.RemoteBrokerOffsetStore

    private long fetchConsumeOffsetFromBroker(MessageQueue mq) throws RemotingException, MQBrokerException,
        InterruptedException, MQClientException {
        FindBrokerResult findBrokerResult = this.mQClientFactory.findBrokerAddressInAdmin(mq.getBrokerName());
        if (null == findBrokerResult) {

            this.mQClientFactory.updateTopicRouteInfoFromNameServer(mq.getTopic());
            findBrokerResult = this.mQClientFactory.findBrokerAddressInAdmin(mq.getBrokerName());
        }

        if (findBrokerResult != null) {
            QueryConsumerOffsetRequestHeader requestHeader = new QueryConsumerOffsetRequestHeader();
            requestHeader.setTopic(mq.getTopic());
            requestHeader.setConsumerGroup(this.groupName);
            requestHeader.setQueueId(mq.getQueueId());

            return this.mQClientFactory.getMQClientAPIImpl().queryConsumerOffset(
                findBrokerResult.getBrokerAddr(), requestHeader, 1000 * 5);
        } else {
            throw new MQClientException("The broker[" + mq.getBrokerName() + "] not exist", null);
        }
    }

所以mq一个队列同一时刻也只允许一个消费者消费，也是避免这里更新的并发问题了。