RocketMQ事务消息实现原理上篇

1 发送事务消息的入口为：TransactionMQProducer#sendMessageInTransaction：

    public TransactionSendResult sendMessageInTransaction(Message msg, Object arg) throws MQClientException {
        if (null == this.transactionListener) { // @1
            throw new MQClientException("TransactionListener is null", (Throwable)null);
        } else {
            return this.defaultMQProducerImpl.sendMessageInTransaction(msg, (LocalTransactionExecuter)null, arg);  // @2
        }
    }

　　代码@1：如果transactionListener为空，则直接抛出异常。
     代码@2：调用defaultMQProducerImpl的sendMessageInTransaction方法。

2. DefaultMQProducerImpl#sendMessageInTransaction

public TransactionSendResult sendMessageInTransaction(Message msg, LocalTransactionExecuter localTransactionExecuter, Object arg) throws MQClientException {
        TransactionListener transactionListener = this.getCheckListener();
        if (null == localTransactionExecuter && null == transactionListener) {
            throw new MQClientException("tranExecutor is null", (Throwable)null);
        } else {
            Validators.checkMessage(msg, this.defaultMQProducer);
            SendResult sendResult = null;
            MessageAccessor.putProperty(msg, "TRAN_MSG", "true");   // @1
            MessageAccessor.putProperty(msg, "PGROUP", this.defaultMQProducer.getProducerGroup());

            try {
                sendResult = this.send(msg);
            } catch (Exception var11) {
                throw new MQClientException("send message Exception", var11);
            }

            LocalTransactionState localTransactionState = LocalTransactionState.UNKNOW;
            Throwable localException = null;
            switch(sendResult.getSendStatus()) {
            case SEND_OK:
                try {
                    if (sendResult.getTransactionId() != null) {
                        msg.putUserProperty("__transactionId__", sendResult.getTransactionId());
                    }

                    String transactionId = msg.getProperty("UNIQ_KEY");
                    if (null != transactionId && !"".equals(transactionId)) {
                        msg.setTransactionId(transactionId);
                    }

                    if (null != localTransactionExecuter) {
                        localTransactionState = localTransactionExecuter.executeLocalTransactionBranch(msg, arg);
                    } else if (transactionListener != null) {
                        this.log.debug("Used new transaction API");
                        localTransactionState = transactionListener.executeLocalTransaction(msg, arg);
                    }

                    if (null == localTransactionState) {
                        localTransactionState = LocalTransactionState.UNKNOW;
                    }

                    if (localTransactionState != LocalTransactionState.COMMIT_MESSAGE) {
                        this.log.info("executeLocalTransactionBranch return {}", localTransactionState);
                        this.log.info(msg.toString());
                    }
                } catch (Throwable var10) {
                    this.log.info("executeLocalTransactionBranch exception", var10);
                    this.log.info(msg.toString());
                    localException = var10;
                }
                break;
            case FLUSH_DISK_TIMEOUT:
            case FLUSH_SLAVE_TIMEOUT:
            case SLAVE_NOT_AVAILABLE:
                localTransactionState = LocalTransactionState.ROLLBACK_MESSAGE;
            }

            try {
                this.endTransaction(sendResult, localTransactionState, localException);
            } catch (Exception var9) {
                this.log.warn("local transaction execute " + localTransactionState + ", but end broker transaction failed", var9);
            }

            TransactionSendResult transactionSendResult = new TransactionSendResult();
            transactionSendResult.setSendStatus(sendResult.getSendStatus());
            transactionSendResult.setMessageQueue(sendResult.getMessageQueue());
            transactionSendResult.setMsgId(sendResult.getMsgId());
            transactionSendResult.setQueueOffset(sendResult.getQueueOffset());
            transactionSendResult.setTransactionId(sendResult.getTransactionId());
            transactionSendResult.setLocalTransactionState(localTransactionState);
            return transactionSendResult;
        }
    }

　　Step1：首先先阐述一下参数含义。final Message msg：消息；TransactionListener tranExecuter：事务监听器； Object arg：其他附加参数，该参数会再TransactionListener 回调函数中原值传入。

　　Step2：代码@1 在消息属性中，添加两个属性：TRAN_MSG，其值为true，表示为事务消息；PGROUP：消息所属发送者组，然后以同步方式发送消息。

  DefaultMQProducerImpl#sendKernelImpl

final String tranMsg = msg.getProperty(MessageConst.PROPERTY_TRANSACTION_PREPARED);
                if (tranMsg != null && Boolean.parseBoolean(tranMsg)) {
                    sysFlag |= MessageSysFlag.TRANSACTION_PREPARED_TYPE;
                }

　　在消息发送之前，会先检查消息的属性TRAN_MSG,如果存在并且值为true，则通过设置消息系统标记的方式，设置消息为MessageSysFlag.TRANSACTION_PREPARED_TYPE。

SendMessageProcessor#sendMessage

String traFlag = oriProps.get(MessageConst.PROPERTY_TRANSACTION_PREPARED);
if (traFlag != null && Boolean.parseBoolean(traFlag)) {
        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;
       }
      putMessageResult = this.brokerController.getTransactionalMessageService().prepareMessage(msgInner);
} else {
      putMessageResult = this.brokerController.getMessageStore().putMessage(msgInner);
}

　　Step3：Broker端首先客户发送消息请求后，判断消息类型，如果是事务消息，则调用TransactionalMessageService#prepareMessage方法，否则走原先的逻辑，调用MessageStore#putMessage方法。

org.apache.rocketmq.broker.transaction.queue.TransactionalMessageServiceImpl#prepareMessage

public PutMessageResult prepareMessage(MessageExtBrokerInner messageInner) {
        return transactionalMessageBridge.putHalfMessage(messageInner);
 }

　　step4：事务消息，将调用TransactionalMessageServiceImpl#prepareMessage方法，继而调用TransactionalMessageBridge#prepareMessage方法。

TransactionalMessageBridge#parseHalfMessageInner

public PutMessageResult putHalfMessage(MessageExtBrokerInner messageInner) {
        return store.putMessage(parseHalfMessageInner(messageInner));
    }

    private MessageExtBrokerInner parseHalfMessageInner(MessageExtBrokerInner msgInner) {
        MessageAccessor.putProperty(msgInner, MessageConst.PROPERTY_REAL_TOPIC, msgInner.getTopic());
        MessageAccessor.putProperty(msgInner, MessageConst.PROPERTY_REAL_QUEUE_ID,
            String.valueOf(msgInner.getQueueId()));
        msgInner.setSysFlag(
            MessageSysFlag.resetTransactionValue(msgInner.getSysFlag(), MessageSysFlag.TRANSACTION_NOT_TYPE));
        msgInner.setTopic(TransactionalMessageUtil.buildHalfTopic());
        msgInner.setQueueId(0);
        msgInner.setPropertiesString(MessageDecoder.messageProperties2String(msgInner.getProperties()));
        return msgInner;
    }

　　

 Step5：备份消息的原主题名称与原队列ID，然后取消是事务消息的消息标签，重新设置消息的主题为：RMQ_SYS_TRANS_HALF_TOPIC，队列ID固定为0。然后调用MessageStore#putMessage方法将消息持久化，这里TransactionalMessageBridge桥接类，就是封装事务消息的相关流程，最终调用MessageStore完成消息的持久化。消息入库后，会继续回到DefaultMQProducerImpl#sendMessageInTransaction，上文的Step2后面，也就是通过同步将消息发送到消息服务端。
DefaultMQProducerImpl#sendMessageInTransaction

switch (sendResult.getSendStatus()) {
            case SEND_OK: {
                try {
                    if (sendResult.getTransactionId() != null) {
                        msg.putUserProperty("__transactionId__", sendResult.getTransactionId());
                    }
                    String transactionId = msg.getProperty(MessageConst.PROPERTY_UNIQ_CLIENT_MESSAGE_ID_KEYIDX);
                    if (null != transactionId && !"".equals(transactionId)) {
                        msg.setTransactionId(transactionId);
                    }
                    localTransactionState = tranExecuter.executeLocalTransaction(msg, arg);
                    if (null == localTransactionState) {
                        localTransactionState = LocalTransactionState.UNKNOW;
                    }

                    if (localTransactionState != LocalTransactionState.COMMIT_MESSAGE) {
                        log.info("executeLocalTransactionBranch return {}", localTransactionState);
                        log.info(msg.toString());
                    }
                } catch (Throwable e) {
                    log.info("executeLocalTransactionBranch exception", e);
                    log.info(msg.toString());
                    localException = e;
                }
            }
            break;
            case FLUSH_DISK_TIMEOUT:
            case FLUSH_SLAVE_TIMEOUT:
            case SLAVE_NOT_AVAILABLE:
                localTransactionState = LocalTransactionState.ROLLBACK_MESSAGE;
                break;
            default:
                break;
        }

　　

Step6：如果消息发送成功，会回调TransactionListener#executeLocalTransaction方法，执行本地事务，并且返回本地事务状态为：public enum LocalTransactionState {COMMIT_MESSAGE,ROLLBACK_MESSAGE,
UNKNOW,} 之一，注意：TransactionListener#executeLocalTransaction是在发送者成功发送PREPARED消息后，会执行本地事务方法，然后返回本地事务状态；如果PREPARED消息发送失败，则不会调用
TransactionListener#executeLocalTransaction，并且本地事务消息，设置为
LocalTransactionState.ROLLBACK_MESSAGE，表示消息需要被回滚。
   DefaultMQProducerImpl#sendMessageInTransaction

try {
this.endTransaction(sendResult, localTransactionState, localException);
} catch (Exception e) {
log.warn("local transaction execute " + localTransactionState + ", but end broker transaction failed", e);
}

　　step7：调用endTransaction方法结束事务（提交或回滚）。
   DefaultMQProducerImpl#endTransaction

EndTransactionRequestHeader requestHeader = new EndTransactionRequestHeader();
requestHeader.setTransactionId(transactionId);
requestHeader.setCommitLogOffset(id.getOffset());
switch (localTransactionState) {
    case COMMIT_MESSAGE:
         requestHeader.setCommitOrRollback(MessageSysFlag.TRANSACTION_COMMIT_TYPE);
         break;
    case ROLLBACK_MESSAGE:
         requestHeader.setCommitOrRollback(MessageSysFlag.TRANSACTION_ROLLBACK_TYPE);
         break;
     case UNKNOW:
         requestHeader.setCommitOrRollback(MessageSysFlag.TRANSACTION_NOT_TYPE);
         break;
     default:
         break;
}
requestHeader.setProducerGroup(this.defaultMQProducer.getProducerGroup());
requestHeader.setTranStateTableOffset(sendResult.getQueueOffset());
requestHeader.setMsgId(sendResult.getMsgId());

　　

 step8：组装结束事务请求，主要参数为：事务ID、事务操作（commitOrRollback)、消费组、消息队列偏移量、消息ID，fromTransactionCheck，从这里发出的请求，默认为false。Broker端的请求处理器为：EndTransactionProcessor。
   step9：EndTransactionProcessor根据事务提交类型：TRANSACTION_COMMIT_TYPE（提交事务）、TRANSACTION_ROLLBACK_TYPE（回滚事务）、TRANSACTION_NOT_TYPE、忽略该请求，会记录info级别的日志相关的代码将在下文详细分析，在这里，我们先大概梳理一条消息发送的路径TransactionMQProducer#sendMessageInTransaction的调用链来总结一下事务消息的发送流程。

本文到这里，初步展示了事务消息的发送流程，总的说来，RocketMQ的事务消息发送使用二阶段提交思路，首先，在消息发送时，先发送消息类型为Prepread类型的消息，然后在将该消息成功存入到消息服务器后，会回调   TransactionListener#executeLocalTransaction，执行本地事务状态回调函数，然后根据该方法的返回值，结束事务：
   1、COMMIT_MESSAGE ：提交事务。
   2、ROLLBACK_MESSAGE：回滚事务。
   3、UNKNOW：未知事务状态，此时消息服务器(Broker)收到EndTransaction命令时，将不对这种消息做处理，消息还处于Prepared类型，存储在主题为：RMQ_SYS_TRANS_HALF_TOPIC的队列中，然后消息发送流程将结束，那这些消息如何提交或回滚呢？为了实现避免客户端需要再次发送提交、回滚命令，RocketMQ会采取定时任务将RMQ_SYS_TRANS_HALF_TOPIC中的消息取出，然后回到客户端，判断该消息是否需要提交或回滚，来完成事务消息的声明周期，该部分内容将在下节重点探讨。

作者：唯有坚持不懈
来源：CSDN
原文：https://blog.csdn.net/prestigeding/article/details/81263833