上一篇博客《RocketMQ原理学习---生产者普通消息发送》我们已经对生产者发送普通消息有了简单的了解，这篇博客我们来学习一下RocketMQ在发送事物消息时做了什么处理操作。
一、生产者发送消息
RocketMQ通过实现2PC协议来实现分布式事物，RocketMQ事物消息发送与消费流程图：
接下来我们通过源码看看RocketMQ生产者在发送事物消息的时候做了什么操作。
调用事物消息接口发送事物消息。
public TransactionSendResult sendMessageInTransaction(Message msg, LocalTransactionExecuter tranExecuter, Object arg) throws MQClientException {
if (null == this.transactionCheckListener) {
throw new MQClientException("localTransactionBranchCheckListener is null", (Throwable)null);
} else {
//发送事物消息
return this.defaultMQProducerImpl.sendMessageInTransaction(msg, tranExecuter, arg);
}
}在sendMessageInTransaction可以看到会进行如下操作：
（1）调用 sendResult = this.send(msg); 发送消息获取结果
（2）执行本地事物localTransactionState = tranExecuter.executeLocalTransactionBranch(msg, arg);
（3）将执行本地事物的结果再调用this.endTransaction(sendResult, localTransactionState, localException);
public TransactionSendResult sendMessageInTransaction(Message msg, LocalTransactionExecuter tranExecuter, Object arg) throws MQClientException {
if (null == tranExecuter) {
throw new MQClientException("tranExecutor is null", (Throwable)null);
} else {
Validators.checkMessage(msg, this.defaultMQProducer);
SendResult sendResult = null;
MessageAccessor.putProperty(msg, "TRAN_MSG", "true");
MessageAccessor.putProperty(msg, "PGROUP", this.defaultMQProducer.getProducerGroup());
try {
//发送消息
sendResult = this.send(msg);
} catch (Exception var10) {
throw new MQClientException("send message Exception", var10);
}
LocalTransactionState localTransactionState = LocalTransactionState.UNKNOW;
Throwable localException = null;
switch(sendResult.getSendStatus()) {
case SEND_OK:
try {
if (sendResult.getTransactionId() != null) {
msg.putUserProperty("__transactionId__", sendResult.getTransactionId());
}
//执行本地事物
localTransactionState = tranExecuter.executeLocalTransactionBranch(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 var9) {
this.log.info("executeLocalTransactionBranch exception", var9);
this.log.info(msg.toString());
localException = var9;
}
break;
case FLUSH_DISK_TIMEOUT:
case FLUSH_SLAVE_TIMEOUT:
case SLAVE_NOT_AVAILABLE:
localTransactionState = LocalTransactionState.ROLLBACK_MESSAGE;
}
try {
//提交执行结果
this.endTransaction(sendResult, localTransactionState, localException);
} catch (Exception var8) {
this.log.warn("local transaction execute " + localTransactionState + ", but end broker transaction failed", var8);
}
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;
}
}
在endTransaction方法中会根据本地事物执行结果的状态，将状态信息发送到broker
public void endTransaction(SendResult sendResult, LocalTransactionState localTransactionState, Throwable localException) throws RemotingException, MQBrokerException, InterruptedException, UnknownHostException {
MessageId id;
if (sendResult.getOffsetMsgId() != null) {
id = MessageDecoder.decodeMessageId(sendResult.getOffsetMsgId());
} else {
id = MessageDecoder.decodeMessageId(sendResult.getMsgId());
}
String transactionId = sendResult.getTransactionId();
String brokerAddr = this.mQClientFactory.findBrokerAddressInPublish(sendResult.getMessageQueue().getBrokerName());
EndTransactionRequestHeader requestHeader = new EndTransactionRequestHeader();
requestHeader.setTransactionId(transactionId);
requestHeader.setCommitLogOffset(id.getOffset());
switch(localTransactionState) {
case COMMIT_MESSAGE:
requestHeader.setCommitOrRollback(8);
break;
case ROLLBACK_MESSAGE:
requestHeader.setCommitOrRollback(12);
break;
case UNKNOW:
requestHeader.setCommitOrRollback(0);
}
requestHeader.setProducerGroup(this.defaultMQProducer.getProducerGroup());
requestHeader.setTranStateTableOffset(sendResult.getQueueOffset());
requestHeader.setMsgId(sendResult.getMsgId());
String remark = localException != null ? "executeLocalTransactionBranch exception: " + localException.toString() : null;
this.mQClientFactory.getMQClientAPIImpl().endTransactionOneway(brokerAddr, requestHeader, remark, (long)this.defaultMQProducer.getSendMsgTimeout());
}
二、Broker处理事物消息
一条完整消息的要经过如下处理：
（1）将消息持久化到commitlog文件中
（2）创建consumequeue文件
（3）创建index文件
1、普通消息在消息发送成功后会进行上面三步创建操作
2、事物消息prepare阶段只会进行第一步commitlog文件创建（此时消费者是无法消费消息的），当commit阶段时会进行第二部操作创建consumequeue文件，这样消费者才可以消费消息。
class CommitLogDispatcherBuildConsumeQueue implements CommitLogDispatcher {
@Override
public void dispatch(DispatchRequest request) {
final int tranType = MessageSysFlag.getTransactionValue(request.getSysFlag());
switch (tranType) {
//普通消息
case MessageSysFlag.TRANSACTION_NOT_TYPE:
//确认事物消息
case MessageSysFlag.TRANSACTION_COMMIT_TYPE:
DefaultMessageStore.this.putMessagePositionInfo(request);
break;
//准备事物消息
case MessageSysFlag.TRANSACTION_PREPARED_TYPE:
//回滚事物消息
case MessageSysFlag.TRANSACTION_ROLLBACK_TYPE:
break;
}
}
}
