十九、RocketMQ 主从同步读写分离机制

---

关于主从同步最新理解：RocketMQ 主从同步若干问题答疑

RocketMQ在消息拉取时是如何根据消息消费队列MessageQueue来选择Broker的呢？消息消费队列如图所示：

RocketMQ根据MessageQueue查找Broker地址的唯一依据便是brokerName，从RocketMQ的Broker组织实现来看，同一组Broker(M-S)服务器，其brokerName相同，主服务器的brokerId为0，从服务器的brokerId大于0，那RocketMQ根据brokerName如何定位到哪一台Broker上来呢？

PullAPIWrapper#pullKernelImpl

FindBrokerResult findBrokerResult =
            this.mQClientFactory.findBrokerAddressInSubscribe(mq.getBrokerName(),
                this.recalculatePullFromWhichNode(mq), false);

RocketMQ的MQClientInstance类提供了根据brokerName、brokerId查找Broker地址的方法，返回值如图：

MQClientInstance#findBrokerAddressInSubscribe

public FindBrokerResult findBrokerAddressInSubscribe(
        final String brokerName,
        final long brokerId,
        final boolean onlyThisBroker
    ) {
        String brokerAddr = null;
        boolean slave = false;
        boolean found = false;
        HashMap<Long/* brokerId */, String/* address */> map = this.brokerAddrTable.get(brokerName);
        if (map != null && !map.isEmpty()) {
            brokerAddr = map.get(brokerId);
            slave = brokerId != MixAll.MASTER_ID;
            found = brokerAddr != null;
            if (!found && !onlyThisBroker) {
                Entry<Long, String> entry = map.entrySet().iterator().next();
                brokerAddr = entry.getValue();
                slave = entry.getKey() != MixAll.MASTER_ID;
                found = true;
            }
        }
        if (found) {
            return new FindBrokerResult(brokerAddr, slave, findBrokerVersion(brokerName, brokerAddr));
        }
        return null;
    }