这篇文章主要介绍RocketMQ事务消息怎样实现，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！
RocketMQ4.3.0版本开始支持事务消息，后续分享将开始将剖析事务消息的实现原理。首先从官方给出的Demo实例入手，以此通往RocketMQ事务消息的世界中。
官方版本未发布之前，从apache rocketmq第一个版本上线后，代码中存在与事务消息相关的代码，例如COMMIT、ROLLBACK、PREPARED，在事务消息未开源之前网上对于事务消息的“声音”基本上是使用类似二阶段提交，主要是根据消息系统标志MessageSysFlag中定义来推测的：
TRANSACTION_PREPARED_TYPE
TRANSACTION_COMMIT_TYPE
TRANSACTION_ROLLBACK_TYPE
消息发送者首先发送TRANSACTION_PREPARED_TYPE类型的消息，然后根据事务状态来决定是提交或回滚事务发送commit请求或rollback请求，如果commit/rollback请求丢失后，rocketmq会在指定超时时间后回查事务状态来决定提交或回滚事务。
让我们各自带着自己的理解和猜测，从阅读RocketMQ官方提供的Demo程序入手，试图窥探一些大体的信息。
Demo示例程序位于：/rocketmq-example/src/main/java/org/apache/rocketmq/example/transaction包中。该包中未放置消息消费者，为了验证事务的消息消费情况，我们可以从其他包copy一个消费者，从而先运行生产者，然后运行消费者，判断事务消息的预发放、提交、回滚等效果，二话不说，先运行一下，看下效果再说：
消息发送端运行结果：
SendResult [sendStatus=SEND_OK, msgId=C0A8010518DC6D06D69C8D5767EC0000, offsetMsgId=null, messageQueue=MessageQueue [topic=transaction_topic_test, brokerName=broker-a, queueId=1], queueOffset=0] SendResult [sendStatus=SEND_OK, msgId=C0A8010518DC6D06D69C8D57680F0001, offsetMsgId=null, messageQueue=MessageQueue [topic=transaction_topic_test, brokerName=broker-a, queueId=2], queueOffset=1] SendResult [sendStatus=SEND_OK, msgId=C0A8010518DC6D06D69C8D57681E0002, offsetMsgId=null, messageQueue=MessageQueue [topic=transaction_topic_test, brokerName=broker-a, queueId=3], queueOffset=2] SendResult [sendStatus=SEND_OK, msgId=C0A8010518DC6D06D69C8D57682B0003, offsetMsgId=null, messageQueue=MessageQueue [topic=transaction_topic_test, brokerName=broker-a, queueId=0], queueOffset=3] SendResult [sendStatus=SEND_OK, msgId=C0A8010518DC6D06D69C8D5768380004, offsetMsgId=null, messageQueue=MessageQueue [topic=transaction_topic_test, brokerName=broker-a, queueId=1], queueOffset=4] SendResult [sendStatus=SEND_OK, msgId=C0A8010518DC6D06D69C8D5768490005, offsetMsgId=null, messageQueue=MessageQueue [topic=transaction_topic_test, brokerName=broker-a, queueId=2], queueOffset=5] SendResult [sendStatus=SEND_OK, msgId=C0A8010518DC6D06D69C8D5768560006, offsetMsgId=null, messageQueue=MessageQueue [topic=transaction_topic_test, brokerName=broker-a, queueId=3], queueOffset=6] SendResult [sendStatus=SEND_OK, msgId=C0A8010518DC6D06D69C8D5768640007, offsetMsgId=null, messageQueue=MessageQueue [topic=transaction_topic_test, brokerName=broker-a, queueId=0], queueOffset=7] SendResult [sendStatus=SEND_OK, msgId=C0A8010518DC6D06D69C8D5768730008, offsetMsgId=null, messageQueue=MessageQueue [topic=transaction_topic_test, brokerName=broker-a, queueId=1], queueOffset=8] SendResult [sendStatus=SEND_OK, msgId=C0A8010518DC6D06D69C8D5768800009, offsetMsgId=null, messageQueue=MessageQueue [topic=transaction_topic_test, brokerName=broker-a, queueId=2], queueOffset=9]
消息消费端效果：
Consumer Started.
ConsumeMessageThread_1 Receive New Messages: [MessageExt [queueId=0, storeSize=325, queueOffset=0, sysFlag=8, bornTimestamp=1532745715812, bornHost=/192.168.1.5:55482, storeTimestamp=1532745749010, storeHost=/192.168.1.5:10911, msgId=C0A8010500002A9F0000000000001DE8, commitLogOffset=7656, bodyCRC=988340972, reconsumeTimes=0, preparedTransactionOffset=5477, toString()=Message{topic='transaction_topic_test', flag=0, properties={MIN_OFFSET=0, REAL_TOPIC=transaction_topic_test, TRANSACTION_CHECK_TIMES=1, MAX_OFFSET=1, KEYS=KEY7, TRAN_MSG=true, CONSUME_START_TIME=1532746024360, UNIQ_KEY=C0A8010518DC6D06D69C8D5768640007, WAIT=true, PGROUP=please_rename_unique_group_name, TAGS=TagC, REAL_QID=0}, body=[72, 101, 108, 108, 111, 32, 82, 111, 99, 107, 101, 116, 77, 81, 32, 55], transactionId='C0A8010518DC6D06D69C8D5768640007'}]]
ConsumeMessageThread_2 Receive New Messages: [MessageExt [queueId=1, storeSize=325, queueOffset=0, sysFlag=8, bornTimestamp=1532745715768, bornHost=/192.168.1.5:55482, storeTimestamp=1532745749008, storeHost=/192.168.1.5:10911, msgId=C0A8010500002A9F0000000000001B91, commitLogOffset=7057, bodyCRC=601994070, reconsumeTimes=0, preparedTransactionOffset=4496, toString()=Message{topic='transaction_topic_test', flag=0, properties={MIN_OFFSET=0, REAL_TOPIC=transaction_topic_test, TRANSACTION_CHECK_TIMES=1, MAX_OFFSET=1, KEYS=KEY4, TRAN_MSG=true, CONSUME_START_TIME=1532746024361, UNIQ_KEY=C0A8010518DC6D06D69C8D5768380004, WAIT=true, PGROUP=please_rename_unique_group_name, TAGS=TagE, REAL_QID=1}, body=[72, 101, 108, 108, 111, 32, 82, 111, 99, 107, 101, 116, 77, 81, 32, 52], transactionId='C0A8010518DC6D06D69C8D5768380004'}]]
ConsumeMessageThread_3 Receive New Messages: [MessageExt [queueId=2, storeSize=325, queueOffset=0, sysFlag=8, bornTimestamp=1532745715727, bornHost=/192.168.1.5:55482, storeTimestamp=1532745748834, storeHost=/192.168.1.5:10911, msgId=C0A8010500002A9F000000000000193A, commitLogOffset=6458, bodyCRC=1401636825, reconsumeTimes=0, preparedTransactionOffset=3515, toString()=Message{topic='transaction_topic_test', flag=0, properties={MIN_OFFSET=0, REAL_TOPIC=transaction_topic_test, TRANSACTION_CHECK_TIMES=1, MAX_OFFSET=1, KEYS=KEY1, TRAN_MSG=true, CONSUME_START_TIME=1532746024368, UNIQ_KEY=C0A8010518DC6D06D69C8D57680F0001, WAIT=true, PGROUP=please_rename_unique_group_name, TAGS=TagB, REAL_QID=2}, body=[72, 101, 108, 108, 111, 32, 82, 111, 99, 107, 101, 116, 77, 81, 32, 49], transactionId='C0A8010518DC6D06D69C8D57680F0001'}]]综上所述，服务端发送了10条消息，而消费端只收到3条消息，应该是由于事务回滚，造成只提交了3条消息，为了更加严谨，可以安装一个rocketmq-consonse，更加直观的观察shangshagn's上述结果：cdn.com/97db9b9f7bc18478739f472e134cf48eb3af8cad.png">
接下来对示例代码进行解读：
1、生产者端代码解读：
public class TransactionProducer { public static void main(String[] args) throws MQClientException, InterruptedException {
TransactionListener transactionListener = new TransactionListenerImpl(); // @1
TransactionMQProducer producer = new TransactionMQProducer("please_rename_unique_group_name");
producer.setNamesrvAddr("127.0.0.1:9876");
ExecutorService executorService = new ThreadPoolExecutor(2, 5, 100, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(2000), new ThreadFactory() { @Override
public Thread newThread(Runnable r) {
Thread thread = new Thread(r);
thread.setName("client-transaction-msg-check-thread"); return thread;
}
}); // @2
producer.setExecutorService(executorService); // @3
producer.setTransactionListener(transactionListener); // @4
producer.start();
String[] tags = new String[] {"TagA", "TagB", "TagC", "TagD", "TagE"}; for (int i = 0; i < 10; i++) { // @5
try {
Message msg = new Message("transaction_topic_test", tags[i % tags.length], "KEY" + i,
("Hello RocketMQ " + i).getBytes(RemotingHelper.DEFAULT_CHARSET));
SendResult sendResult = producer.sendMessageInTransaction(msg, null);
System.out.printf("%s%n", sendResult);
Thread.sleep(10);
} catch (MQClientException | UnsupportedEncodingException e) {
e.printStackTrace();
}
} for (int i = 0; i < 100000; i++) { //这里只是阻止生产者过早退出，导致事务消息的相关机制无法运行
Thread.sleep(1000);
}
producer.shutdown();
}
}代码@1：创建TransactionListener 实例，字面理解为事务消息事件监听器，下文详细对其进行展开。
代码@2：ExecutorService
executorService，创建一个线程池，其线程的名称前缀”client-transaction-msg-check-thread“，从字面理解为客户端事务消息状态检测线程，我们可以大胆的猜测一下是不是这个线程池调用TransactionListener方法，完成对事务消息的检测呢？【这里只是作者的猜测，大家不能当真，在作者后续文章发布后，如果该观点错误，会加以修复，这里写出来，主要是想分享一下我读源码的方法】。
代码@3：为事务消息发送者设置线程池。
代码@4：为事务消息发送者设置事务监听器。
代码@5：发送10条消息。
2、TransactionListener代码解读
public class TransactionListenerImpl implements TransactionListener { private AtomicInteger transactionIndex = new AtomicInteger(0); private ConcurrentHashMap<String, Integer> localTrans = new ConcurrentHashMap<>(); @Override
public LocalTransactionState executeLocalTransaction(Message msg, Object arg) { int value = transactionIndex.getAndIncrement(); int status = value % 3;
localTrans.put(msg.getTransactionId(), status); return LocalTransactionState.UNKNOW;
} @Override
public LocalTransactionState checkLocalTransaction(MessageExt msg) {
Integer status = localTrans.get(msg.getTransactionId()); if (null != status) { switch (status) { case 0: return LocalTransactionState.UNKNOW; case 1: return LocalTransactionState.COMMIT_MESSAGE; case 2: return LocalTransactionState.ROLLBACK_MESSAGE;
}
} return LocalTransactionState.COMMIT_MESSAGE;
}
}executeLocalTransaction方法：记录本地事务的事务状态，这里其实现就是循环设置事务消息的状态为0,1,2，demo中是把消息的状态数据存放在一个Map中。实际应用时通常会持久化消息的事务状态，例如数据库或缓存。
checkLocalTransaction方法，事务回查业务实现，查本地事务表，判断事务的状态如为0：UNKNOW，1：COMMIT_MESSAGE；ROLLBACK_MESSAGE。这里就能解释，生产者连续发10条消息，因为只有3条消息的事务状态为COMMIT_MESSAGE，故消息消费者只能消费3条。
以上是“RocketMQ事务消息怎样实现”这篇文章的所有内容，感谢各位的阅读！希望分享的内容对大家有帮助，更多相关知识，欢迎关注亿速云行业资讯频道！
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