RocketMQ源码 — 四、 Consumer 接收消息过程

Consumer

consumer pull message

订阅

在Consumer启动之前先将自己放到一个本地的集合中，再以后获取消费者的时候会用到，同时会将自己订阅的信息告诉broker

接收消息

consumer启动的时候会启动两个service：
RebalanceService：主要实现consumer的负载均衡，但是并不会直接发送获取消息的请求，而是构造request之后放到PullMessageService中，等待PullMessageService的线程取出执行
PullMessageService：主要负责从broker获取message，包含一个需要获取消息的请求队列（是阻塞的），并不断依次从队列中取出请求向broker send Request

执行时序图太大，截屏截不全，所以放在git上，在这里:RocketMQ.asta

从Broker pullMessage

在PullMessageService中通过netty发送pull消息的请求之后，Broker的remoteServer会收到request，然后在PullMessageProcessor中的processRequest处理，先会解析requestHeader，request中带了读取MessageStore的参数：

• consumerGroup
• topic
• queueId
• queueOffset
• MaxMsgNums
• subscriptionData（ConsumerManager中获取）

processRequest处理流程

• 判断Broker当前是否允许接收消息
• 找到subscriptionGroupConfig，subscriptionGroupTable，如果不存在当前的group则新增一个
• 是否subscriptionGroupConfig.consumeEnable
• 获取从TopicConfigManager.topicConfigTable获取topicConfig
• topicConfig是否有读权限
• 校验queueId是否在范围内
• ConsumerManager.getConsumerGroupInfo获取ConsumerGroupInfo
• consumerGroupInfo.findSubscriptionData查找subscriptionData
• MessageStore.getMessage读取消息，从文件中读取消息，属于pullMessage的核心方法
• 判断brokerRole，master和slave工作模式的哪一种
• 判断MessageResult.status
• 如果responseCode是SUSCESS，判断是使用heap还是非heap方式传输数据
• 使用netty序列化response返回netty客户端

负载均衡

// AllocateMessageQueueAveragely
public List<MessageQueue> allocate(String consumerGroup, String currentCID, List<MessageQueue> mqAll,
                                       List<String> cidAll) {
    if (currentCID == null || currentCID.length() < 1) {
        throw new IllegalArgumentException("currentCID is empty");
    }
    if (mqAll == null || mqAll.isEmpty()) {
        throw new IllegalArgumentException("mqAll is null or mqAll empty");
    }
    if (cidAll == null || cidAll.isEmpty()) {
        throw new IllegalArgumentException("cidAll is null or cidAll empty");
    }

    List<MessageQueue> result = new ArrayList<MessageQueue>();
    if (!cidAll.contains(currentCID)) {
        log.info("[BUG] ConsumerGroup: {} The consumerId: {} not in cidAll: {}", //
                consumerGroup, //
                currentCID,//
                cidAll);
        return result;
    }

    int index = cidAll.indexOf(currentCID);
    int mod = mqAll.size() % cidAll.size();
    int averageSize =
            mqAll.size() <= cidAll.size() ? 1 : (mod > 0 && index < mod ? mqAll.size() / cidAll.size()
                    + 1 : mqAll.size() / cidAll.size());
    int startIndex = (mod > 0 && index < mod) ? index * averageSize : index * averageSize + mod;
    int range = Math.min(averageSize, mqAll.size() - startIndex);
    for (int i = 0; i < range; i++) {
        result.add(mqAll.get((startIndex + i) % mqAll.size()));
    }
    return result;
}

// RebalanceImpl
private boolean updateProcessQueueTableInRebalance(final String topic, final Set<MessageQueue> mqSet, final boolean isOrder) {
    boolean changed = false;

	// 去掉topic对应的无用MessageQueue（不包含在processQueueTable或者pullExpired）
    Iterator<Entry<MessageQueue, ProcessQueue>> it = this.processQueueTable.entrySet().iterator();
    while (it.hasNext()) {
        Entry<MessageQueue, ProcessQueue> next = it.next();
        MessageQueue mq = next.getKey();
        ProcessQueue pq = next.getValue();

        if (mq.getTopic().equals(topic)) {
            if (!mqSet.contains(mq)) {
                pq.setDropped(true);
                if (this.removeUnnecessaryMessageQueue(mq, pq)) {
                    it.remove();
                    changed = true;
                    log.info("doRebalance, {}, remove unnecessary mq, {}", consumerGroup, mq);
                }
            } else if (pq.isPullExpired()) {
                switch (this.consumeType()) {
                    case CONSUME_ACTIVELY:
                        break;
                    case CONSUME_PASSIVELY:
                        pq.setDropped(true);
                        if (this.removeUnnecessaryMessageQueue(mq, pq)) {
                            it.remove();
                            changed = true;
                            log.error("[BUG]doRebalance, {}, remove unnecessary mq, {}, because pull is pause, so try to fixed it",
                                    consumerGroup, mq);
                        }
                        break;
                    default:
                        break;
                }
            }
        }
    }

    List<PullRequest> pullRequestList = new ArrayList<PullRequest>();
    for (MessageQueue mq : mqSet) {
    	// 如果MessageQueue不在processQueueTable
        if (!this.processQueueTable.containsKey(mq)) {
            if (isOrder && !this.lock(mq)) {
                log.warn("doRebalance, {}, add a new mq failed, {}, because lock failed", consumerGroup, mq);
                continue;
            }

            this.removeDirtyOffset(mq);
            ProcessQueue pq = new ProcessQueue();
            long nextOffset = this.computePullFromWhere(mq);
            // 如果message还有数据需要读
            if (nextOffset >= 0) {
                ProcessQueue pre = this.processQueueTable.putIfAbsent(mq, pq);
                if (pre != null) {
                    log.info("doRebalance, {}, mq already exists, {}", consumerGroup, mq);
                } else {
                    log.info("doRebalance, {}, add a new mq, {}", consumerGroup, mq);
                    PullRequest pullRequest = new PullRequest();
                    pullRequest.setConsumerGroup(consumerGroup);
                    pullRequest.setNextOffset(nextOffset);
                    pullRequest.setMessageQueue(mq);
                    pullRequest.setProcessQueue(pq);
                    pullRequestList.add(pullRequest);
                    changed = true;
                }
            } else {
                log.warn("doRebalance, {}, add new mq failed, {}", consumerGroup, mq);
            }
        }
    }

	// 将pullRequest放在pullRequestQueue中等待去取数据
    this.dispatchPullRequest(pullRequestList);

    return changed;
}

issue

多个consumer读取同一个consumerQueue的时候怎么记录每个读取的进度

每个consumer只要记住自己读取哪一个队列，以及offset就可以了

pull消息过程中的关键类

DefaultMQPushConsumerImpl

供DefaultMQPushConsumer调用，作为consumer的默认实现：

AllocateMessageQueueAveragely

这个类主要是consumer消费的负载均衡算法

public List<MessageQueue> allocate(String consumerGroup, String currentCID, List<MessageQueue> mqAll,
                                       List<String> cidAll) {
    if (currentCID == null || currentCID.length() < 1) {
        throw new IllegalArgumentException("currentCID is empty");
    }
    if (mqAll == null || mqAll.isEmpty()) {
        throw new IllegalArgumentException("mqAll is null or mqAll empty");
    }
    if (cidAll == null || cidAll.isEmpty()) {
        throw new IllegalArgumentException("cidAll is null or cidAll empty");
    }

    List<MessageQueue> result = new ArrayList<MessageQueue>();
    if (!cidAll.contains(currentCID)) {
        log.info("[BUG] ConsumerGroup: {} The consumerId: {} not in cidAll: {}", //
                consumerGroup, //
                currentCID,//
                cidAll);
        return result;
    }

    // 基本原则，每个队列只能被一个consumer消费
    // 当messageQueue个数小于等于consume的时候，排在前面（在list中的顺序）的consumer消费一个queue，index大于messageQueue之后的consumer消费不到queue，也就是为0
    // 当messageQueue个数大于consumer的时候，分两种情况
    //     当有余数（mod > 0）并且index < mod的时候，当前comsumer可以消费的队列个数是 mqAll.size() / cidAll.size() + 1
    //     可以整除或者index 大于余数的时候，队列数为：mqAll.size() / cidAll.size()
    int index = cidAll.indexOf(currentCID);
    int mod = mqAll.size() % cidAll.size();
    int averageSize =
            mqAll.size() <= cidAll.size() ? 1 : (mod > 0 && index < mod ? mqAll.size() / cidAll.size()
                    + 1 : mqAll.size() / cidAll.size());
    int startIndex = (mod > 0 && index < mod) ? index * averageSize : index * averageSize + mod;
    int range = Math.min(averageSize, mqAll.size() - startIndex);
    for (int i = 0; i < range; i++) {
        result.add(mqAll.get((startIndex + i) % mqAll.size()));
    }
    return result;
}