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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