一. 背景

最近的一个项目：我们的系统接收到上游系统的派单任务后，会推送到指定的门店的相关设备，并进行相应的业务处理。

二. Netty 的使用

在接收到派单任务之后，通过 Netty 推送到指定门店相关的设备。在我们的系统中 Netty 实现了消息推送、长连接以及心跳机制。

2.1 Netty Server 端:

每个 Netty 服务端通过 ConcurrentHashMap 保存了客户端的 clientId 以及它连接的 SocketChannel。

服务器端向客户端发送消息时，只要获取 clientId 对应的 SocketChannel，往 SocketChannel 里写入相应的 message 即可。

        EventLoopGroup boss = new NioEventLoopGroup(1);
        EventLoopGroup worker = new NioEventLoopGroup();
        ServerBootstrap bootstrap = new ServerBootstrap();
        bootstrap.group(boss, worker)
                .channel(NioServerSocketChannel.class)
                .option(ChannelOption.SO_BACKLOG, 128)
                .option(ChannelOption.TCP_NODELAY, true)
                .childOption(ChannelOption.SO_KEEPALIVE, true)
                .childHandler(new ChannelInitializer() {
                    @Override
                    protected void initChannel(Channel channel) throws Exception {
                        ChannelPipeline p = channel.pipeline();
                        p.addLast(new MessageEncoder());
                        p.addLast(new MessageDecoder());
                        p.addLast(new PushServerHandler());
                    }
                });

        ChannelFuture future = bootstrap.bind(host,port).sync();
        if (future.isSuccess()) {
            logger.info("server start...");
        }

2.2 Netty Client 端:

客户端用于接收服务端的消息，随即进行业务处理。客户端还有心跳机制，它通过 IdleEvent 事件定时向服务端放送 Ping 消息以此来检测 SocketChannel 是否中断。

    public PushClientBootstrap(String host, int port) throws InterruptedException {

        this.host = host;
        this.port = port;

        start(host,port);
    }

    private void start(String host, int port) throws InterruptedException {

        bootstrap = new Bootstrap();
        bootstrap.channel(NioSocketChannel.class)
                .option(ChannelOption.SO_KEEPALIVE, true)
                .group(workGroup)
                .remoteAddress(host, port)
                .handler(new ChannelInitializer(){

                    @Override
                    protected void initChannel(Channel channel) throws Exception {
                        ChannelPipeline p = channel.pipeline();
                        p.addLast(new IdleStateHandler(20, 10, 0));  // IdleStateHandler 用于检测心跳
                        p.addLast(new MessageDecoder());
                        p.addLast(new MessageEncoder());
                        p.addLast(new PushClientHandler());
                    }
                });
        doConnect(port, host);
    }

    /**
     * 建立连接，并且可以实现自动重连.
     * @param port port.
     * @param host host.
     * @throws InterruptedException InterruptedException.
     */
    private void doConnect(int port, String host) throws InterruptedException {

        if (socketChannel != null && socketChannel.isActive()) {
            return;
        }

        final int portConnect = port;
        final String hostConnect = host;

        ChannelFuture future = bootstrap.connect(host, port);

        future.addListener(new ChannelFutureListener() {

            @Override
            public void operationComplete(ChannelFuture futureListener) throws Exception {
                if (futureListener.isSuccess()) {
                    socketChannel = (SocketChannel) futureListener.channel();
                    logger.info("Connect to server successfully!");
                } else {
                    logger.info("Failed to connect to server, try connect after 10s");

                    futureListener.channel().eventLoop().schedule(new Runnable() {
                        @Override
                        public void run() {
                            try {
                                doConnect(portConnect, hostConnect);
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                        }
                    }, 10, TimeUnit.SECONDS);
                }
            }
        }).sync();
    }

三. 借助 ZooKeeper 实现简单的服务注册与发现

3.1 服务注册

服务注册本质上是为了解耦服务提供者和服务消费者。服务注册是一个高可用强一致性的服务发现存储仓库，主要用来存储服务的api和地址对应关系。为了高可用，服务注册中心一般为一个集群，并且能够保证分布式一致性。目前常用的有 ZooKeeper、Etcd 等等。

在我们项目中采用了 ZooKeeper 实现服务注册。

public class ServiceRegistry {

    private static final Logger logger = LoggerFactory.getLogger(ServiceRegistry.class);

    private CountDownLatch latch = new CountDownLatch(1);

    private String registryAddress;

    public ServiceRegistry(String registryAddress) {
        this.registryAddress = registryAddress;
    }

    public void register(String data) {
        if (data != null) {
            ZooKeeper zk = connectServer();
            if (zk != null) {
                createNode(zk, data);
            }
        }
    }

    /**
     * 连接 zookeeper 服务器
     * @return
     */
    private ZooKeeper connectServer() {
        ZooKeeper zk = null;
        try {
            zk = new ZooKeeper(registryAddress, Constants.ZK_SESSION_TIMEOUT, new Watcher() {
                @Override
                public void process(WatchedEvent event) {
                    if (event.getState() == Event.KeeperState.SyncConnected) {
                        latch.countDown();
                    }
                }
            });
            latch.await();
        } catch (IOException | InterruptedException e) {
            logger.error("", e);
        }
        return zk;
    }

    /**
     * 创建节点
     * @param zk
     * @param data
     */
    private void createNode(ZooKeeper zk, String data) {
        try {
            byte[] bytes = data.getBytes();
            String path = zk.create(Constants.ZK_DATA_PATH, bytes, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
            logger.debug("create zookeeper node ({} => {})", path, data);
        } catch (KeeperException | InterruptedException e) {
            logger.error("", e);
        }
    }
}

有了服务注册，在 Netty 服务端启动之后，将 Netty 服务端的 ip 和 port 注册到 ZooKeeper。

        EventLoopGroup boss = new NioEventLoopGroup(1);
        EventLoopGroup worker = new NioEventLoopGroup();
        ServerBootstrap bootstrap = new ServerBootstrap();
        bootstrap.group(boss, worker)
                .channel(NioServerSocketChannel.class)
                .option(ChannelOption.SO_BACKLOG, 128)
                .option(ChannelOption.TCP_NODELAY, true)
                .childOption(ChannelOption.SO_KEEPALIVE, true)
                .childHandler(new ChannelInitializer() {
                    @Override
                    protected void initChannel(Channel channel) throws Exception {
                        ChannelPipeline p = channel.pipeline();
                        p.addLast(new MessageEncoder());
                        p.addLast(new MessageDecoder());
                        p.addLast(new PushServerHandler());
                    }
                });

        ChannelFuture future = bootstrap.bind(host,port).sync();
        if (future.isSuccess()) {
            logger.info("server start...");
        }

        if (serviceRegistry != null) {
            serviceRegistry.register(host + ":" + port);
        }

3.2 服务发现

这里我们采用的是客户端的服务发现，即服务发现机制由客户端实现。

客户端在和服务端建立连接之前，通过查询注册中心的方式来获取服务端的地址。如果存在有多个 Netty 服务端的话，可以做服务的负载均衡。在我们的项目中只采用了简单的随机法进行负载。

public class ServiceDiscovery {

    private static final Logger logger = LoggerFactory.getLogger(ServiceDiscovery.class);

    private CountDownLatch latch = new CountDownLatch(1);

    private volatile List<String> serviceAddressList = new ArrayList<>();

    private String registryAddress; // 注册中心的地址

    public ServiceDiscovery(String registryAddress) {
        this.registryAddress = registryAddress;

        ZooKeeper zk = connectServer();
        if (zk != null) {
            watchNode(zk);
        }
    }

    /**
     * 通过服务发现，获取服务提供方的地址
     * @return
     */
    public String discover() {
        String data = null;
        int size = serviceAddressList.size();
        if (size > 0) {
            if (size == 1) {  //只有一个服务提供方
                data = serviceAddressList.get(0);
                logger.info("unique service address : {}", data);
            } else {          //使用随机分配法。简单的负载均衡法
                data = serviceAddressList.get(ThreadLocalRandom.current().nextInt(size));
                logger.info("choose an address : {}", data);
            }
        }
        return data;
    }

    /**
     * 连接 zookeeper
     * @return
     */
    private ZooKeeper connectServer() {

        ZooKeeper zk = null;
        try {
            zk = new ZooKeeper(registryAddress, Constants.ZK_SESSION_TIMEOUT, new Watcher() {
                @Override
                public void process(WatchedEvent event) {
                    if (event.getState() == Watcher.Event.KeeperState.SyncConnected) {
                        latch.countDown();
                    }
                }
            });
            latch.await();
        } catch (IOException | InterruptedException e) {
            logger.error("", e);
        }
        return zk;
    }

    /**
     * 获取服务地址列表
     * @param zk
     */
    private void watchNode(final ZooKeeper zk) {

        try {
            //获取子节点列表
            List<String> nodeList = zk.getChildren(Constants.ZK_REGISTRY_PATH, new Watcher() {
                @Override
                public void process(WatchedEvent event) {
                    if (event.getType() == Event.EventType.NodeChildrenChanged) {
                        //发生子节点变化时再次调用此方法更新服务地址
                        watchNode(zk);
                    }
                }
            });
            List<String> dataList = new ArrayList<>();
            for (String node : nodeList) {
                byte[] bytes = zk.getData(Constants.ZK_REGISTRY_PATH + "/" + node, false, null);
                dataList.add(new String(bytes));
            }
            logger.debug("node data: {}", dataList);
            this.serviceAddressList = dataList;
        } catch (KeeperException | InterruptedException e) {
            logger.error("", e);
        }
    }
}

Netty 客户端启动之后，通过服务发现获取 Netty 服务端的 ip 和 port。

    /**
     * 支持通过服务发现来获取 Socket 服务端的 host、port
     * @param discoveryAddress
     * @throws InterruptedException
     */
    public PushClientBootstrap(String discoveryAddress) throws InterruptedException {

        serviceDiscovery = new ServiceDiscovery(discoveryAddress);
        serverAddress = serviceDiscovery.discover();

        if (serverAddress!=null) {
            String[] array = serverAddress.split(":");
            if (array!=null && array.length==2) {

                String host = array[0];
                int port = Integer.parseInt(array[1]);

                start(host,port);
            }
        }
    }

四. 总结

服务注册和发现一直是分布式的核心组件。本文介绍了借助 ZooKeeper 做注册中心，如何实现一个简单的服务注册和发现。其实，注册中心的选择有很多，例如 Etcd、Eureka 等等。选择符合我们业务需求的才是最重要的。

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