在Broker中，事务消息的初始化是通过BrokerController.initialTransaction()方法执行的。

private void initialTransaction() {
    this.transactionalMessageService = ServiceProvider.loadClass(ServiceProvider.TRANSACTION_SERVICE_ID, TransactionalMessageService.class);
    if (null == this.transactionalMessageService) {
        this.transactionalMessageService = new TransactionalMessageServiceImpl(new TransactionalMessageBridge(this, this.getMessageStore()));
        LOG.warn("Load default transaction message hook service: {}", TransactionalMessageServiceImpl.class.getSimpleName());
    }
    this.transactionalMessageCheckListener = ServiceProvider.loadClass(ServiceProvider.TRANSACTION_LISTENER_ID, AbstractTransactionalMessageCheckListener.class);
    if (null == this.transactionalMessageCheckListener) {
        this.transactionalMessageCheckListener = new DefaultTransactionalMessageCheckListener();
        LOG.warn("Load default discard message hook service: {}", DefaultTransactionalMessageCheckListener.class.getSimpleName());
    }
    this.transactionalMessageCheckListener.setBrokerController(this);
    this.transactionalMessageCheckService = new TransactionalMessageCheckService(this);
}
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这里有三个核心的初始化变量

TransactionalMessageService

事务消息主要处理服务。默认实现类是TransactionalMessageServiceImpl也可以自己定义事务消息处理实现类，通过ServiceProvider.loadClass()方法进行加载。TransactionalMessageService类定义如下。内部属性已加注释标明。

public interface TransactionalMessageService {
    //用于保存Half事务消息
    PutMessageResult prepareMessage(MessageExtBrokerInner messageInner);
    CompletableFuture asyncPrepareMessage(MessageExtBrokerInner messageInner);
    //删除事务消息
    boolean deletePrepareMessage(MessageExt messageExt);
    //提交事务消息
    OperationResult commitMessage(EndTransactionRequestHeader requestHeader);
    //回滚事务消息
    OperationResult rollbackMessage(EndTransactionRequestHeader requestHeader);
    void check(long transactionTimeout, int transactionCheckMax, AbstractTransactionalMessageCheckListener listener);
    //打开事务消息
    boolean open();
    //关闭事务消息
    void close();
}
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transactionalMessageCheckListener

事务消息回查监听器

transactionalMessageCheckService

事务消息回查服务，启动一个线程定时检查超时的Half消息是否需要回查。

处理事务消息

当初始化完成之后，Broker就可以处理事务消息了。

Broker存储事务消息的是org.apache.rocketmq.broker.processor.SendMessageProcessor，这和普通消息其实是一样的。
但是有两点针对事务消息的特殊处理：

第一处:

在org.apache.rocketmq.broker.processor.SendMessageProcessor#sendMessage中：

//获取扩展字段的值，若是该值为true则为事务消息
String traFlag = oriProps.get(MessageConst.PROPERTY_TRANSACTION_PREPARED);
boolean sendTransactionPrepareMessage = false;
if (Boolean.parseBoolean(traFlag)
    && !(msgInner.getReconsumeTimes() > 0 && msgInner.getDelayTimeLevel() > 0)) { 
    //判断当前Broker配置是否支持事务消息
    if (this.brokerController.getBrokerConfig().isRejectTransactionMessage()) {
        response.setCode(ResponseCode.NO_PERMISSION);
        response.setRemark(
            "the broker[" + this.brokerController.getBrokerConfig().getBrokerIP1()
                + "] sending transaction message is forbidden");
        return response;
    }
    sendTransactionPrepareMessage = true;
}
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if (sendTransactionPrepareMessage) {
    //保存Half信息
    putMessageResult = this.brokerController.getTransactionalMessageService().prepareMessage(msgInner);
} else {
    putMessageResult = this.brokerController.getMessageStore().putMessage(msgInner);
}
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第二处：

存储事务消息前的预处理，对应方法是org.apache.rocketmq.broker.transaction.queue.TransactionalMessageBridge#parseHalfMessageInner

private MessageExtBrokerInner parseHalfMessageInner(MessageExtBrokerInner msgInner) {
    //将原消息的topic保存在扩展字段中
    MessageAccessor.putProperty(msgInner, MessageConst.PROPERTY_REAL_TOPIC, msgInner.getTopic());
    //将原消息的QueueId保存在扩展字段中
    MessageAccessor.putProperty(msgInner, MessageConst.PROPERTY_REAL_QUEUE_ID,
        String.valueOf(msgInner.getQueueId()));
    //将原消息的SysFlag保存在扩展字段中
    msgInner.setSysFlag(
        MessageSysFlag.resetTransactionValue(msgInner.getSysFlag(), MessageSysFlag.TRANSACTION_NOT_TYPE));
    //修改topic的值为RMQ_SYS_TRANS_HALF_TOPIC
    msgInner.setTopic(TransactionalMessageUtil.buildHalfTopic());
    //修改Queueid为0
    msgInner.setQueueId(0);
    msgInner.setPropertiesString(MessageDecoder.messageProperties2String(msgInner.getProperties()));
    return msgInner;
}
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完成上述步骤之后，调用DefaultMessageStole.putMessage()方法将其保存到CommitLog中。

CommitLog存储成功之后，通过org.apache.rocketmq.store.CommitLog.DefaultAppendMessageCallback#doAppend()方法对其进行处理。

final int tranType = MessageSysFlag.getTransactionValue(msgInner.getSysFlag());
switch (tranType) {
    // Prepared and Rollback message is not consumed, will not enter the consume queue
    case MessageSysFlag.TRANSACTION_PREPARED_TYPE:
    case MessageSysFlag.TRANSACTION_ROLLBACK_TYPE:
        queueOffset = 0L;
        break;
    case MessageSysFlag.TRANSACTION_NOT_TYPE:
    case MessageSysFlag.TRANSACTION_COMMIT_TYPE:
    default:
        break;
}
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这里的逻辑是这样的，当读到的消息类型为事务消息时，设置当前消息的位点值为0，而不是设置真实的位点。这样该位点就不会建立ConsumeQueue索引，也不会被消费。