dubbo心跳检测机制

• 目的：
  
  • 维持provider和consumer之间的长连接
• 实现：
  
  • dubbo心跳时间heartbeat默认是60s，超过heartbeat时间没有收到消息，就发送心跳消息(provider，consumer一样),如果连着3次(heartbeatTimeout为heartbeat*3)没有收到心跳响应，provider会关闭channel，而consumer会进行重连;不论是provider还是consumer的心跳检测都是通过启动定时任务的方式实现；
• provider绑定和consumer连接的入口：

public class HeaderExchanger implements Exchanger {      public static final String NAME = "header";      @Override     public ExchangeClient connect(URL url, ExchangeHandler handler) throws RemotingException {         return new HeaderExchangeClient(Transporters.connect(url, new DecodeHandler(new HeaderExchangeHandler(handler))), true);     }      @Override     public ExchangeServer bind(URL url, ExchangeHandler handler) throws RemotingException {         return new HeaderExchangeServer(Transporters.bind(url, new DecodeHandler(new HeaderExchangeHandler(handler))));     }  }

• provider启动心跳检测

public HeaderExchangeServer(Server server) {         if (server == null) {             throw new IllegalArgumentException("server == null");         }         this.server = server;         this.heartbeat = server.getUrl().getParameter(Constants.HEARTBEAT_KEY, 0);         //心跳超时时间默认为心跳时间的3倍         this.heartbeatTimeout = server.getUrl().getParameter(Constants.HEARTBEAT_TIMEOUT_KEY, heartbeat * 3);         //如果心跳超时时间小于心跳时间的两倍则抛异常         if (heartbeatTimeout < heartbeat * 2) {             throw new IllegalStateException("heartbeatTimeout < heartbeatInterval * 2");         }         startHeatbeatTimer();     }

• startHeatbeatTimer的实现
  
  • 先停止已有的定时任务，启动新的定时任务

private void startHeatbeatTimer() {         // 停止原有定时任务         stopHeartbeatTimer();         // 发起新的定时任务         if (heartbeat > 0) {             heatbeatTimer = scheduled.scheduleWithFixedDelay(                     new HeartBeatTask(new HeartBeatTask.ChannelProvider() {                         public Collection<Channel> getChannels() {                             return Collections.unmodifiableCollection(HeaderExchangeServer.this.getChannels());                         }                     }, heartbeat, heartbeatTimeout),                     heartbeat, heartbeat, TimeUnit.MILLISECONDS);         }     }

• HeartBeatTask的实现
  
  • 遍历所有的channel，检测心跳间隔,如果超过心跳间隔没有读或写，则发送需要回复的心跳消息,最有判断是否心跳超时(heartbeatTimeout),如果超时，provider关闭channel，consumer进行重连

public void run() {         try {             long now = System.currentTimeMillis();             for (Channel channel : channelProvider.getChannels()) {                 if (channel.isClosed()) {                     continue;                 }                 try {                     Long lastRead = (Long) channel.getAttribute(HeaderExchangeHandler.KEY_READ_TIMESTAMP);                     Long lastWrite = (Long) channel.getAttribute(HeaderExchangeHandler.KEY_WRITE_TIMESTAMP);                     // 读写的时间，任一超过心跳间隔，发送心跳                     if ((lastRead != null && now - lastRead > heartbeat)                             || (lastWrite != null && now - lastWrite > heartbeat)) {                         Request req = new Request();                         req.setVersion("2.0.0");                         req.setTwoWay(true); // 需要响应的心跳事件                         req.setEvent(Request.HEARTBEAT_EVENT);                         channel.send(req);                         if (logger.isDebugEnabled()) {                             logger.debug("Send heartbeat to remote channel " + channel.getRemoteAddress()                                     + ", cause: The channel has no data-transmission exceeds a heartbeat period: " + heartbeat + "ms");                         }                     }                     // 最后读的时间，超过心跳超时时间                     if (lastRead != null && now - lastRead > heartbeatTimeout) {                         logger.warn("Close channel " + channel                                 + ", because heartbeat read idle time out: " + heartbeatTimeout + "ms");                         // 客户端侧，重新连接服务端                         if (channel instanceof Client) {                             try {                                 ((Client) channel).reconnect();                             } catch (Exception e) {                                 //do nothing                             }                         // 服务端侧，关闭客户端连接                         } else {                             channel.close();                         }                     }                 } catch (Throwable t) {                     logger.warn("Exception when heartbeat to remote channel " + channel.getRemoteAddress(), t);                 }             }         } catch (Throwable t) {             logger.warn("Unhandled exception when heartbeat, cause: " + t.getMessage(), t);         }     }

• consumer端的实现
  
  • 默认需要心跳检测

public HeaderExchangeClient(Client client, boolean needHeartbeat) {         if (client == null) {             throw new IllegalArgumentException("client == null");         }         this.client = client;         // 创建 HeaderExchangeChannel 对象         this.channel = new HeaderExchangeChannel(client);         // 读取心跳相关配置         String dubbo = client.getUrl().getParameter(Constants.DUBBO_VERSION_KEY);         this.heartbeat = client.getUrl().getParameter(Constants.HEARTBEAT_KEY, dubbo != null && dubbo.startsWith("1.0.") ? Constants.DEFAULT_HEARTBEAT : 0);         this.heartbeatTimeout = client.getUrl().getParameter(Constants.HEARTBEAT_TIMEOUT_KEY, heartbeat * 3);         if (heartbeatTimeout < heartbeat * 2) { // 避免间隔太短             throw new IllegalStateException("heartbeatTimeout < heartbeatInterval * 2");         }         // 发起心跳定时器         if (needHeartbeat) {             startHeatbeatTimer();         }     }

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