系列文章
前言
生产者向消息队列里面写入消息，不同的业务场景会采用不同的写入策略，比如：同步发送，异步发送，延迟发送，事务消息等；本文首先从分析生产者发送消息的流程开始，然后再来介绍各种发送消息的策略。
生产者流程
1.流程概述
生产者首先需要设置namesrv，或者指定其他方式更新namesrv；然后从namesrv获取topic的路由信息，路由信息包括broker以及Message Queue等信息，同时将路由信息保存在本地内存中，方便下次使用；最后从Message Queue列表中选择合适的Queue发送消息，实现负载均衡；
2.启动过程
DefaultMQProducer实例化提供了两个参数分别是：生产者组名称以及RPCHook，RPCHook是一个接口，具体实现交由业务端实现，两个方法分别是：doBeforeRequest和doAfterResponse，表示在执行请求之前和接收返回之后分别执行相关逻辑；
接下来就是调用DefaultMQProducer的start方法，相关的初始化操作都在里面进行，内部其实调用的是DefaultMQProducerImpl的start方法，具体代码如下：public void start(final boolean startFactory) throws MQClientException { switch (this.serviceState) { case CREATE_JUST: this.serviceState = ServiceState.START_FAILED; this.checkConfig(); if (!this.defaultMQProducer.getProducerGroup().equals(MixAll.CLIENT_INNER_PRODUCER_GROUP)) { this.defaultMQProducer.changeInstanceNameToPID(); } this.mQClientFactory = MQClientManager.getInstance().getAndCreateMQClientInstance(this.defaultMQProducer, rpcHook); boolean registerOK = mQClientFactory.registerProducer(this.defaultMQProducer.getProducerGroup(), this); if (!registerOK) { this.serviceState = ServiceState.CREATE_JUST; throw new MQClientException("The producer group[" + this.defaultMQProducer.getProducerGroup() + "] has been created before, specify another name please." + FAQUrl.suggestTodo(FAQUrl.GROUP_NAME_DUPLICATE_URL), null); } this.topicPublishInfoTable.put(this.defaultMQProducer.getCreateTopicKey(), new TopicPublishInfo()); if (startFactory) { mQClientFactory.start(); } log.info("the producer [{}] start OK. sendMessageWithVIPChannel={}", this.defaultMQProducer.getProducerGroup(), this.defaultMQProducer.isSendMessageWithVIPChannel()); this.serviceState = ServiceState.RUNNING; break; case RUNNING: case START_FAILED: case SHUTDOWN_ALREADY: throw new MQClientException("The producer service state not OK, maybe started once, " + this.serviceState + FAQUrl.suggestTodo(FAQUrl.CLIENT_SERVICE_NOT_OK), null); default: break; } this.mQClientFactory.sendHeartbeatToAllBrokerWithLock(); } 默认serviceState的状态为CREATE_JUST，刚进入设置为START_FAILED，初始化完成之后设置为RUNNING，再次初始化时会直接报错，下面看一下具体初始化了哪些信息；
2.1检查配置
这里的检查其实就是对producerGroup进行合法性校验；
2.2设置instanceName
如果producerGroup不等于默认的"CLIENT_INNER_PRODUCER",则设置DefaultMQProducer的instanceName为进程的pid；
2.3创建MQClientInstance对象
首先检查 ConcurrentMap<String/ clientId /, MQClientInstance> factoryTable中是否已经存在已clientId为key的MQClientInstance，如果存在则取出，不存在则实例化；clientId是已ip地址，instanceName以及unitName组成的，例如：10.13.83.7@12500
2.4注册producer
将DefaultMQProducerImpl注册到MQClientInstance中，已producerGroup作为key，注册到ConcurrentMap<String/ group /, MQProducerInner> producerTable中，如果已经存在此Group，则抛出异常；
2.5初始化TopicPublishInfo
已topic名称为"TBW102"为key，实例化TopicPublishInfo作为value，存放在ConcurrentMap<String/ topic /, TopicPublishInfo> topicPublishInfoTable中,TopicPublishInfo用来存放topic的路由信息；
2.6启动MQClientInstance
MQClientInstance启动会启动很多相关服务，具体可以看如下代码：
public void start() throws MQClientException { synchronized (this) { switch (this.serviceState) { case CREATE_JUST: this.serviceState = ServiceState.START_FAILED; // If not specified,looking address from name server if (null == this.clientConfig.getNamesrvAddr()) { this.mQClientAPIImpl.fetchNameServerAddr(); } // Start request-response channel this.mQClientAPIImpl.start(); // Start various schedule tasks this.startScheduledTask(); // Start pull service this.pullMessageService.start(); // Start rebalance service this.rebalanceService.start(); // Start push service this.defaultMQProducer.getDefaultMQProducerImpl().start(false); log.info("the client factory [{}] start OK", this.clientId); this.serviceState = ServiceState.RUNNING; break; case RUNNING: break; case SHUTDOWN_ALREADY: break; case START_FAILED: throw new MQClientException("The Factory object[" + this.getClientId() + "] has been created before, and failed.", null); default: break; } } } 默认serviceState的状态为CREATE_JUST，刚进入设置为START_FAILED，初始化完成之后设置为RUNNING，防止重复初始化；
2.6.1初始化NameServerAddr
首先判断DefaultMQProducer是否配置了NameServerAddr，如果没有配置会到一个地址下获取，地址默认为：:8080/rocketmq/nsaddr，相关的逻辑在MixAll类中，代码如下：
public static String getWSAddr() { String wsDomainName = System.getProperty("rocketmq.namesrv.domain", DEFAULT_NAMESRV_ADDR_LOOKUP); String wsDomainSubgroup = System.getProperty("rocketmq.namesrv.domain.subgroup", "nsaddr"); String wsAddr = "http://" + wsDomainName + ":8080/rocketmq/" + wsDomainSubgroup; if (wsDomainName.indexOf(":") > 0) { wsAddr = "http://" + wsDomainName + "/rocketmq/" + wsDomainSubgroup; } return wsAddr; } 正常情况下我们需要设置自己的地址，可以通过如下方式设置：
System.setProperty("rocketmq.namesrv.domain", "localhost"); 这种情况下就可以不用手动设置NameServerAddr；
2.6.2初始化RemotingClient
RemotingClient是一个接口类，底层使用的通讯框架是Netty，提供了实现类NettyRemotingClient，RemotingClient在初始化的时候实例化Bootstrap，方便后续用来创建Channel；
2.6.3启动定时器任务
总共启动了5个定时器任务，分别是：定时更新NameServerAddr信息，定时更新topic的路由信息，定时清理下线的broker，定时持久化Consumer的Offset信息，定时调整线程池；
2.6.3启动服务
pullMessageService和rebalanceService被用在消费端的两个服务类，分别是：从broker拉取消息的服务和均衡消息队列服务，负责分配消费者可消费的消息队列；
3.发送消息
相关发送消息的代码在DefaultMQProducerImpl的sendDefaultImpl方法中，部分代码如下所示：
private SendResult sendDefaultImpl( Message msg, final CommunicationMode communicationMode, final SendCallback sendCallback, final long timeout ) throws MQClientException, RemotingException, MQBrokerException, InterruptedException { this.makeSureStateOK(); Validators.checkMessage(msg, this.defaultMQProducer); final long invokeID = random.nextLong(); long beginTimestampFirst = System.currentTimeMillis(); long beginTimestampPrev = beginTimestampFirst; long endTimestamp = beginTimestampFirst; TopicPublishInfo topicPublishInfo = this.tryToFindTopicPublishInfo(msg.getTopic()); if (topicPublishInfo != null && topicPublishInfo.ok()) { boolean callTimeout = false; MessageQueue mq = null; Exception exception = null; SendResult sendResult = null; int timesTotal = communicationMode == CommunicationMode.SYNC ? 1 + this.defaultMQProducer.getRetryTimesWhenSendFailed() : 1; int times = 0; String[] brokersSent = new String[timesTotal]; for (; times < timesTotal; times++) { String lastBrokerName = null == mq ? null : mq.getBrokerName(); MessageQueue mqSelected = this.selectOneMessageQueue(topicPublishInfo, lastBrokerName); if (mqSelected != null) { mq = mqSelected; brokersSent[times] = mq.getBrokerName(); try { beginTimestampPrev = System.currentTimeMillis(); long costTime = beginTimestampPrev - beginTimestampFirst; if (timeout < costTime) { callTimeout = true; break; } sendResult = this.sendKernelImpl(msg, mq, communicationMode, sendCallback, topicPublishInfo, timeout - costTime); endTimestamp = System.currentTimeMillis(); this.updateFaultItem(mq.getBrokerName(), endTimestamp - beginTimestampPrev, false); switch (communicationMode) { case ASYNC: return null; case ONEWAY: return null; case SYNC: if (sendResult.getSendStatus() != SendStatus.SEND_OK) { if (this.defaultMQProducer.isRetryAnotherBrokerWhenNotStoreOK()) { continue; } } return sendResult; default: break; } ...以下代码省略... 此方法的四个参数分别是：msg为要发送的消息，communicationMode要使用的发送方式包括同步、异步、单向，sendCallback在异步情况下的回调函数，timeout发送消息的超时时间；
3.1获取topic的路由信息
通过msg中设置的topic获取路由信息，具体代码如下：
private TopicPublishInfo tryToFindTopicPublishInfo(final String topic) { TopicPublishInfo topicPublishInfo = this.topicPublishInfoTable.get(topic); if (null == topicPublishInfo || !topicPublishInfo.ok()) { this.topicPublishInfoTable.putIfAbsent(topic, new TopicPublishInfo()); this.mQClientFactory.updateTopicRouteInfoFromNameServer(topic); topicPublishInfo = this.topicPublishInfoTable.get(topic); } if (topicPublishInfo.isHaveTopicRouterInfo() || topicPublishInfo.ok()) { return topicPublishInfo; } else { this.mQClientFactory.updateTopicRouteInfoFromNameServer(topic, true, this.defaultMQProducer); topicPublishInfo = this.topicPublishInfoTable.get(topic); return topicPublishInfo; } } 首先从变量ConcurrentMap<String/ topic /, TopicPublishInfo> topicPublishInfoTable中获取是否存在指定topic的路由信息，如果获取不到则使用topic去nameServer获取路由信息，如果还是获取不到则使用默认的topic名称为"TBW102"去获取路由信息，需要使用默认名称去获取的情况是没有创建topic，需要broker自动创建topic的情况；获取路由信息使用的是MQClientInstance中的updateTopicRouteInfoFromNameServer方法，此方法根据topic获取路由信息，具体连接哪台nameServer，会从列表中顺序的选择nameServer，实现负载均衡；
注：名称为"TBW102"的topic是系统自动创建的；3.2选择MessageQueue
成功获取到路由信息之后，需要从MessageQueue列表中选择一个，在这之前获取了默认发送消息失败的重试次数，默认为3次(只有发送模式是同步的情况下)，代码如下：
public MessageQueue selectOneMessageQueue(final TopicPublishInfo tpInfo, final String lastBrokerName) { if (this.sendLatencyFaultEnable) { try { int index = tpInfo.getSendWhichQueue().getAndIncrement(); for (int i = 0; i < tpInfo.getMessageQueueList().size(); i++) { int pos = Math.abs(index++) % tpInfo.getMessageQueueList().size(); if (pos < 0) pos = 0; MessageQueue mq = tpInfo.getMessageQueueList().get(pos); if (latencyFaultTolerance.isAvailable(mq.getBrokerName())) { if (null == lastBrokerName || mq.getBrokerName().equals(lastBrokerName)) return mq; } } final String notBestBroker = latencyFaultTolerance.pickOneAtLeast(); int writeQueueNums = tpInfo.getQueueIdByBroker(notBestBroker); if (writeQueueNums > 0) { final MessageQueue mq = tpInfo.selectOneMessageQueue(); if (notBestBroker != null) { mq.setBrokerName(notBestBroker); mq.setQueueId(tpInfo.getSendWhichQueue().getAndIncrement() % writeQueueNums); } return mq; } else { latencyFaultTolerance.remove(notBestBroker); } } catch (Exception e) { log.error("Error occurred when selecting message queue", e); } return tpInfo.selectOneMessageQueue(); } return tpInfo.selectOneMessageQueue(lastBrokerName); } 以上代码在MQFaultStrategy，此类提供了选择MessageQueue的策略，相关策略可以看类变量：
private final LatencyFaultTolerancelatencyFaultTolerance = new LatencyFaultToleranceImpl(); private boolean sendLatencyFaultEnable = false; private long[] latencyMax = {50L, 100L, 550L, 1000L, 2000L, 3000L, 15000L}; private long[] notAvailableDuration = {0L, 0L, 30000L, 60000L, 120000L, 180000L, 600000L};
latencyFaultTolerance：延迟容错对象，维护brokers的延迟信息；
sendLatencyFaultEnable：延迟容错开关，默认不开启；latencyMax：延迟级别数组；notAvailableDuration ：根据延迟级别，对应broker不可用的时长；这样上面的这段代码就好理解了，首先判定是否开启开关，如果开启首先获取index，index初始为一个随机值，后面每次+1，这样在后面与MessageQueue长度取模的时候每个MessageQueue可以按顺序获取；获取
MessageQueue之后需要判定其对应的Broker是否可用，同时也需要和当前指定的brokerName进行匹配；如果没有获取到就选择一个延迟相对小的，pickOneAtLeast会做排序处理；如果都不行就直接获取一个MessageQueue，不管其他条件了；3.3发送消息
在发送之前会做超时检测，如果前面两步已经超时了，则直接报超时，默认超时时间是3秒；部分代码如下：
private SendResult sendKernelImpl(final Message msg, final MessageQueue mq, final CommunicationMode communicationMode, final SendCallback sendCallback, final TopicPublishInfo topicPublishInfo, final long timeout) throws MQClientException, RemotingException, MQBrokerException, InterruptedException { long beginStartTime = System.currentTimeMillis(); String brokerAddr = this.mQClientFactory.findBrokerAddressInPublish(mq.getBrokerName()); if (null == brokerAddr) { tryToFindTopicPublishInfo(mq.getTopic()); brokerAddr = this.mQClientFactory.findBrokerAddressInPublish(mq.getBrokerName()); } SendMessageContext context = null; if (brokerAddr != null) { brokerAddr = MixAll.brokerVIPChannel(this.defaultMQProducer.isSendMessageWithVIPChannel(), brokerAddr); byte[] prevBody = msg.getBody(); try { //for MessageBatch,ID has been set in the generating process if (!(msg instanceof MessageBatch)) { MessageClientIDSetter.setUniqID(msg); } int sysFlag = 0; boolean msgBodyCompressed = false; if (this.tryToCompressMessage(msg)) { sysFlag |= MessageSysFlag.COMPRESSED_FLAG; msgBodyCompressed = true; } final String tranMsg = msg.getProperty(MessageConst.PROPERTY_TRANSACTION_PREPARED); if (tranMsg != null && Boolean.parseBoolean(tranMsg)) { sysFlag |= MessageSysFlag.TRANSACTION_PREPARED_TYPE; } if (hasCheckForbiddenHook()) { CheckForbiddenContext checkForbiddenContext = new CheckForbiddenContext(); checkForbiddenContext.setNameSrvAddr(this.defaultMQProducer.getNamesrvAddr()); checkForbiddenContext.setGroup(this.defaultMQProducer.getProducerGroup()); checkForbiddenContext.setCommunicationMode(communicationMode); checkForbiddenContext.setBrokerAddr(brokerAddr); checkForbiddenContext.setMessage(msg); checkForbiddenContext.setMq(mq); checkForbiddenContext.setUnitMode(this.isUnitMode()); this.executeCheckForbiddenHook(checkForbiddenContext); } if (this.hasSendMessageHook()) { context = new SendMessageContext(); context.setProducer(this); context.setProducerGroup(this.defaultMQProducer.getProducerGroup()); context.setCommunicationMode(communicationMode); context.setBornHost(this.defaultMQProducer.getClientIP()); context.setBrokerAddr(brokerAddr); context.setMessage(msg); context.setMq(mq); String isTrans = msg.getProperty(MessageConst.PROPERTY_TRANSACTION_PREPARED); if (isTrans != null && isTrans.equals("true")) { context.setMsgType(MessageType.Trans_Msg_Half); } if (msg.getProperty("__STARTDELIVERTIME") != null || msg.getProperty(MessageConst.PROPERTY_DELAY_TIME_LEVEL) != null) { context.setMsgType(MessageType.Delay_Msg); } this.executeSendMessageHookBefore(context); } SendMessageRequestHeader requestHeader = new SendMessageRequestHeader(); requestHeader.setProducerGroup(this.defaultMQProducer.getProducerGroup()); requestHeader.setTopic(msg.getTopic()); requestHeader.setDefaultTopic(this.defaultMQProducer.getCreateTopicKey()); requestHeader.setDefaultTopicQueueNums(this.defaultMQProducer.getDefaultTopicQueueNums()); requestHeader.setQueueId(mq.getQueueId()); requestHeader.setSysFlag(sysFlag); requestHeader.setBornTimestamp(System.currentTimeMillis()); requestHeader.setFlag(msg.getFlag()); requestHeader.setProperties(MessageDecoder.messageProperties2String(msg.getProperties())); requestHeader.setReconsumeTimes(0); requestHeader.setUnitMode(this.isUnitMode()); requestHeader.setBatch(msg instanceof MessageBatch); if (requestHeader.getTopic().startsWith(MixAll.RETRY_GROUP_TOPIC_PREFIX)) { String reconsumeTimes = MessageAccessor.getReconsumeTime(msg); if (reconsumeTimes != null) { requestHeader.setReconsumeTimes(Integer.valueOf(reconsumeTimes)); MessageAccessor.clearProperty(msg, MessageConst.PROPERTY_RECONSUME_TIME); } String maxReconsumeTimes = MessageAccessor.getMaxReconsumeTimes(msg); if (maxReconsumeTimes != null) { requestHeader.setMaxReconsumeTimes(Integer.valueOf(maxReconsumeTimes)); MessageAccessor.clearProperty(msg, MessageConst.PROPERTY_MAX_RECONSUME_TIMES); } } SendResult sendResult = null; switch (communicationMode) { case ASYNC: Message tmpMessage = msg; if (msgBodyCompressed) { //If msg body was compressed, msgbody should be reset using prevBody. //Clone new message using commpressed message body and recover origin massage. //Fix bug:https://github.com/apache/rocketmq-externals/issues/66 tmpMessage = MessageAccessor.cloneMessage(msg); msg.setBody(prevBody); } long costTimeAsync = System.currentTimeMillis() - beginStartTime; if (timeout < costTimeAsync) { throw new RemotingTooMuchRequestException("sendKernelImpl call timeout"); } sendResult = this.mQClientFactory.getMQClientAPIImpl().sendMessage( brokerAddr, mq.getBrokerName(), tmpMessage, requestHeader, timeout - costTimeAsync, communicationMode, sendCallback, topicPublishInfo, this.mQClientFactory, this.defaultMQProducer.getRetryTimesWhenSendAsyncFailed(), context, this); break; case ONEWAY: case SYNC: long costTimeSync = System.currentTimeMillis() - beginStartTime; if (timeout < costTimeSync) { throw new RemotingTooMuchRequestException("sendKernelImpl call timeout"); } sendResult = this.mQClientFactory.getMQClientAPIImpl().sendMessage( brokerAddr, mq.getBrokerName(), msg, requestHeader, timeout - costTimeSync, communicationMode, context, this); break; default: assert false; break; } if (this.hasSendMessageHook()) { context.setSendResult(sendResult); this.executeSendMessageHookAfter(context); } return sendResult; 此处的6个参数分别是：msg消息本身，mq要发送到的队列，communicationMode发送策略，sendCallback异步回调函数，topicPublishInfo路由信息，timeout发送超时时间(3秒-前2步消耗的时间)；
首先需要获取指定broker的地址，这要才能创建channel与broker连接；然后就是一些hock处理；接下来就是准备发送的消息头SendMessageRequestHeader，最后根据不同的发送策略执行发送消息，此处就不在进入更加深入的分析；总结
本文重点介绍了发送者的启动，以及发送消息的大概流程；关于消息的发送策略，以及消息的类型包括：顺序消息，事务消息等，将在后面的文章介绍。