这篇文章主要介绍RocketMQ中push consumer启动之触发消息拉取的示例代码，文中介绍的非常详细，具有一定的参考价值，感兴趣的小伙伴们一定要看完！

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一、consumer端数据存储结构

push consumer启动方法DefaultMQPushConsumerImpl.start最后一步会触发MQClientInstance.rebalanceImmediately，该调用最终会进入到RebalanceImpl.doRebalance中，它会根据topic当前的实际consumer数量（从nameserver获取）通过负载均衡原则来决定自己所要订阅的message queue。然后在本地创建对应的消息缓存队列（ProcessQueue），并触发消息拉取操作。

RebalanceImpl是整个consumer的核心，它即保存本消费者订阅的topic信息，又缓存了topic中的message数据。RebalanceImpl相关的几个核心类如下：

• MessageQueue代表的是远端broker上一个topic下的某个message queue

• ProcessQueue是对远端message queue的一个本地缓存，拉取下来的消息都存在一个TreeMap中，其中key是commitlog中的offset

• RebalanceImpl中保存了三种关系：message queue和process queue的映射关系；topic和message queue的映射关系；topic的订阅关系

二、订阅端的负载均衡策略

doRebalance方法会调用rebalanceByTopic来决定本消费者具体要订阅一个topic下的哪些message queue，以达到负载均衡的效果。

    private void rebalanceByTopic(final String topic, final boolean isOrder) {
        switch (messageModel) {
            case BROADCASTING: {
                // 广播模式，订阅所有message queue
                Set mqSet = this.topicSubscribeInfoTable.get(topic);
                if (mqSet != null) {
                    boolean changed = this.updateProcessQueueTableInRebalance(topic, mqSet, isOrder);
                    if (changed) {
                        this.messageQueueChanged(topic, mqSet, mqSet);
                        log.info("messageQueueChanged {} {} {} {}",
                            consumerGroup,
                            topic,
                            mqSet,
                            mqSet);
                    }
                } else {
                    log.warn("doRebalance, {}, but the topic[{}] not exist.", consumerGroup, topic);
                }
                break;
            }
            case CLUSTERING: {
                // 集群模式，获取该topic下所有的message queue + 该topic所有的consumer
                Set mqSet = this.topicSubscribeInfoTable.get(topic);
                List cidAll = this.mQClientFactory.findConsumerIdList(topic, consumerGroup);
                if (null == mqSet) {
                    if (!topic.startsWith(MixAll.RETRY_GROUP_TOPIC_PREFIX)) {
                        log.warn("doRebalance, {}, but the topic[{}] not exist.", consumerGroup, topic);
                    }
                }

                if (null == cidAll) {
                    log.warn("doRebalance, {} {}, get consumer id list failed", consumerGroup, topic);
                }

                if (mqSet != null && cidAll != null) {
                    List mqAll = new ArrayList();
                    mqAll.addAll(mqSet);

                    Collections.sort(mqAll);
                    Collections.sort(cidAll);

                    AllocateMessageQueueStrategy strategy = this.allocateMessageQueueStrategy;

                    List allocateResult = null;
                    try {
                        // 通过负载均衡策略计算出当前消费者所需订阅的message queue子集
                        allocateResult = strategy.allocate(
                            this.consumerGroup,
                            this.mQClientFactory.getClientId(),
                            mqAll,
                            cidAll);
                    } catch (Throwable e) {
                        log.error("AllocateMessageQueueStrategy.allocate Exception. allocateMessageQueueStrategyName={}", strategy.getName(),
                            e);
                        return;
                    }

                    Set allocateResultSet = new HashSet();
                    if (allocateResult != null) {
                        allocateResultSet.addAll(allocateResult);
                    }

                    // 更新Process Queue缓存列表
                    boolean changed = this.updateProcessQueueTableInRebalance(topic, allocateResultSet, isOrder);
                    if (changed) {
                        log.info(
                            "rebalanced result changed. allocateMessageQueueStrategyName={}, group={}, topic={}, clientId={}, mqAllSize={}, cidAllSize={}, rebalanceResultSize={}, rebalanceResultSet={}",
                            strategy.getName(), consumerGroup, topic, this.mQClientFactory.getClientId(), mqSet.size(), cidAll.size(),
                            allocateResultSet.size(), allocateResultSet);
                        this.messageQueueChanged(topic, mqSet, allocateResultSet);
                    }
                }
                break;
            }
            default:
                break;
        }
    }

三、创建本地缓存并启动消息拉取

rebalanceByTopic中通过负载均衡策略计算出当前消费者对于一个topic实际订阅的message queue子集之后，就会在updateProcessQueueTableInBalance方法中创建ProcessQueue，并启动消息拉取。

    private boolean updateProcessQueueTableInRebalance(final String topic, final Set mqSet,
        final boolean isOrder) {
        boolean changed = false;

        Iterator> it = this.processQueueTable.entrySet().iterator();
        while (it.hasNext()) {
            Entry next = it.next();
            MessageQueue mq = next.getKey();
            ProcessQueue pq = next.getValue();

            if (mq.getTopic().equals(topic)) {
                if (!mqSet.contains(mq)) {
                    // 本地Process Queue存在，但不再订阅，则废弃改process queue
                    pq.setDropped(true);
                    if (this.removeUnnecessaryMessageQueue(mq, pq)) {
                        it.remove();
                        changed = true;
                        log.info("doRebalance, {}, remove unnecessary mq, {}", consumerGroup, mq);
                    }
                } else if (pq.isPullExpired()) { // process queue过期，也废弃，等待新建
                    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 pullRequestList = new ArrayList();
        for (MessageQueue mq : mqSet) {
            if (!this.processQueueTable.containsKey(mq)) {
                // 新订阅，本地不存在对应的process queue，则新建
                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);
                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);
                        // 初始化首次拉取请求
                        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);
                }
            }
        }

        // 批量触发首次拉取请求
        this.dispatchPullRequest(pullRequestList);

        return changed;
    }

四、整体流程

消息拉取的初始化过程如下图：

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