zookeeper 的心跳

假定：主机 A, B 通过 tcp 连接发送数据，如果拔掉 A 主机的网线，B 是无法感知到的。但是如果 A 定时给 B 发送心跳，则能根据心跳的回复来判断连接的状态。

以 zookeeper 为例：zk client 会记录上一次发送数据的时间(lastSend)和上一次接收数据的时间(lastHeard)，zk client 给 server 发送心跳(ping)，这些心跳和其他命令一起发送给 zk server，如果 zk client 发现好长的时间没有接收到数据，认为超时，则断开与 server 的连接，并重连服务器。

// zookeeper 3.3.3
// void org.apache.zookeeper.ClientCnxn.SendThread.run()
public void run() {
    long now = System.currentTimeMillis();
    long lastHeard = now;
    long lastSend = now;
    
    // 这里的 zooKeeper 是客户端
    while (zooKeeper.state.isAlive()) {
        try {
            if (sockKey == null) {
                // don't re-establish connection if we are closing
                if (closing) {
                    break;
                }
　　　　　　　　   // 连接 zk server
                startConnect();
                lastSend = now;
                lastHeard = now;
            }
            int idleRecv = (int) (now - lastHeard);
            int idleSend = (int) (now - lastSend);
            int to = readTimeout - idleRecv;
            if (zooKeeper.state != States.CONNECTED) {
                to = connectTimeout - idleRecv;
            }
            // 接收数据超时，抛异常，异常会在后面的 catch 块中处理
            if (to <= 0) {
                throw new SessionTimeoutException(
                        "Client session timed out, have not heard from server in "
                        + idleRecv + "ms"
                        + " for sessionid 0x"
                        + Long.toHexString(sessionId));
            }
            if (zooKeeper.state == States.CONNECTED) {
                int timeToNextPing = readTimeout/2 - idleSend;
                // 发送 ping 命令（心跳），更新 lastSend
                if (timeToNextPing <= 0) {
                    sendPing();
                    lastSend = now;
                    enableWrite();
                } else {
                    if (timeToNextPing < to) {
                        to = timeToNextPing;
                    }
                }
            }

            selector.select(to);
            Set<SelectionKey> selected;
            synchronized (this) {
                selected = selector.selectedKeys();
            }
            // Everything below and until we get back to the select is
            // non blocking, so time is effectively a constant. That is
            // Why we just have to do this once, here
            now = System.currentTimeMillis();
            for (SelectionKey k : selected) {
                SocketChannel sc = ((SocketChannel) k.channel());
                if ((k.readyOps() & SelectionKey.OP_CONNECT) != 0) {
                    if (sc.finishConnect()) {
                        lastHeard = now;
                        lastSend = now;
                        primeConnection(k);
                    }
                } else if ((k.readyOps() & (SelectionKey.OP_READ | SelectionKey.OP_WRITE)) != 0) {
                    if (outgoingQueue.size() > 0) {
                        // We have something to send so it's the same
                        // as if we do the send now.
                        lastSend = now;
                    }
                    if (doIO()) {
                        lastHeard = now;
                    }
                }
            }
            if (zooKeeper.state == States.CONNECTED) {
                if (outgoingQueue.size() > 0) {
                    enableWrite();
                } else {
                    disableWrite();
                }
            }
            selected.clear();
        } catch (Exception e) {
            if (closing) {
                if (LOG.isDebugEnabled()) {
                    // closing so this is expected
                    LOG.debug("An exception was thrown while closing send thread for session 0x"
                            + Long.toHexString(getSessionId())
                            + " : " + e.getMessage());
                }
                break;
            } else {
                // this is ugly, you have a better way speak up
                if (e instanceof SessionExpiredException) {
                    LOG.info(e.getMessage() + ", closing socket connection");
                } else if (e instanceof SessionTimeoutException) {
                    LOG.info(e.getMessage() + RETRY_CONN_MSG);
                } else if (e instanceof EndOfStreamException) {
                    LOG.info(e.getMessage() + RETRY_CONN_MSG);
                } else {
                    LOG.warn("Session 0x"
                            + Long.toHexString(getSessionId())
                            + " for server "
                            + ((SocketChannel)sockKey.channel())
                                .socket().getRemoteSocketAddress()
                            + ", unexpected error"
                            + RETRY_CONN_MSG,
                            e);
                }
                // 断开连接
                cleanup();
                if (zooKeeper.state.isAlive()) {
                    eventThread.queueEvent(new WatchedEvent(
                            Event.EventType.None,
                            Event.KeeperState.Disconnected,
                            null));
                }

                now = System.currentTimeMillis();
                lastHeard = now;
                lastSend = now;
            }
        }
    }
    cleanup();
    try {
        selector.close();
    } catch (IOException e) {
        LOG.warn("Ignoring exception during selector close", e);
    }
    if (zooKeeper.state.isAlive()) {
        eventThread.queueEvent(new WatchedEvent(
                Event.EventType.None,
                Event.KeeperState.Disconnected,
                null));
    }
    ZooTrace.logTraceMessage(LOG, ZooTrace.getTextTraceLevel(),
                             "SendThread exitedloop.");
}

 zk server 对 session 也会有一个跟踪，它也会关掉超时的 session，具体逻辑在

void org.apache.zookeeper.server.SessionTrackerImpl.run()

zk server 每收到一个请求，就会触发 touchSession