dubbo的心跳检测机制

• 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： ```java 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：
```java
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();
        }
    }

  我们可以看到dubbo的心跳检测，服务端会发送发送心跳包，客户端也会发送心跳包，与一般只有客户端发送心跳包，服务端接受心跳是有所不同的。