你真的明白RPC 嗎？一起來探究 RPC 的實質

​ 不論你是科班出身還是半路轉行，這麼優秀的你一定上過小學語文，那麼對擴句和縮句你一定不陌生。縮句就是去除各種修飾提煉出一句話的核心，而不失基本的語義。下面來實現一個簡易的 rpc 程式探究其實質，進而去理解複雜的 rpc 框架。所謂複雜的框架就是在簡單的過程中加入了一些設計裝飾將rpc的功能豐富起來，如 dubbo 的 filter、router、loadblance、叢集容錯、多種 Invoker 、通訊協議等等，這就是一個擴句的過程。 文中福利，附一張劉祕美照

​ RPC是指遠端過程呼叫，也就是說兩臺伺服器A、B，一個應用部署在A伺服器上，想要呼叫B伺服器上應用提供的函式/方法，由於不在一個記憶體空間，不能直接呼叫，需要通過網路去發起一次呼叫請求獲取結果。

​ 無論是市面上主流的 rpc 框架還是小眾的 rpc 框架都實現了上述 rpc的語義。【服務治理型：dubbo、dubbox、motan；多語言型：grpc、thrift、avro、protocol buffers】

打一波廣告：【博主最近在寫一個 java 實現的 rpc 框架 bridge 歡迎關注，考慮Mesh 化】

一、原理

首先用一幅圖來簡單描述一下 rpc 的呼叫過程，從 dubbo 官網拿來的，不算是最簡單的圖，但是也非常簡單了，去掉上面的 Registry 和下面的 Monitor 剩下的就是最簡單的 rpc 呼叫，說白了就是一個網路請求。

過程描述：

1. 啟動服務端provider，並向註冊中心登記一下自己暴露服務的地址和服務詳情
2. 然後啟動消費端consumer， 訂閱註冊中心的內容，也就是訂閱服務，獲取服務的詳情
3. 如果服務有變動，註冊中心會通知消費端去更新訂閱內容，更新服務詳情。
4. 客戶端拿到了服務詳情，通過網路對服務端發起網路請求，獲取結果
5. 監視器可以獲取到服務呼叫詳情和消費詳情，但不限於此

OK，原理就是這麼簡單，接下來根據上面的描述逐步實現。

二、動手實踐

下面基於 springboot 來實現上述的過程。

2.1 構建模組

搭建工程和子模組，工程結構如下：

2.2 實現服務端

看下服務端的內容，貼圖

把介面定義在 api 模組，consumer 和 provider 模組都要引用到，介面HelloService程式碼如下

package com.glmapper.simple.api;

/**
 * service interface
 *
 * @author: Jerry
 */
public interface HelloService {

    /**
     * service function
     *
     * @param name
     * @return
     */
    String hello(String name);
}
複製程式碼

然後在 provider 模組實現介面，用自定註解 @SimpleProvider 標識，先看下註解內容

package com.glmapper.simple.provider.annotation;

/**
 * 自定義服務註解
 *
 * @author Jerry
 */
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
// 標明可被 Spring 掃描
@Component 
public @interface SimpleProvider {

    Class<?> value();
}
複製程式碼

註解使用了@Component標識，所以可被 spring 掃描到，接下來看實現類HelloServiceImpl：

package com.glmapper.simple.provider.service;

/**
 * service implement class
 *
 * @author: Jerry
 */
@SimpleProvider(HelloService.class)
public class HelloServiceImpl implements HelloService {

    /**
     * service function
     *
     * @param name
     * @return
     */
    @Override
    public String hello(String name) {
        return "Hello! " + name;
    }
}
複製程式碼

在定義一個服務配置的類SimpleProviderProperties，方便通過 application.yml 檔案配置，

package com.glmapper.simple.provider.property;

/**
 * provider properties
 *
 * @author: Jerry
 */
public class SimpleProviderProperties {

    /**
     * 暴露服務的埠
     */
    private Integer port;

    public Integer getPort() {
        return port;
    }

    public void setPort(Integer port) {
        this.port = port;
    }
}
複製程式碼

到這裡基礎的類檔案就已經結束了，下面開始服務初始化，入口 ProviderInitializer

package com.glmapper.simple.provider;

/**
 * 啟動並註冊服務
 *
 * @author Jerry
 */
public class ProviderInitializer implements ApplicationContextAware, InitializingBean {

    private static final Logger LOGGER = LoggerFactory.getLogger(ProviderInitializer.class);

    private SimpleProviderProperties providerProperties;

    /**
     * service registry
     */
    private ServiceRegistry serviceRegistry;

    /**
     * store interface and service implement mapping
     */
    private Map<String, Object> handlerMap = new HashMap<>();

    public ProviderInitializer(SimpleProviderProperties providerProperties, ServiceRegistry serviceRegistry) {
        this.providerProperties = providerProperties;
        this.serviceRegistry = serviceRegistry;
    }

    @Override
    public void setApplicationContext(ApplicationContext ctx) throws BeansException {
        // 獲取被 SimpleProvider 註解的 Bean
        Map<String, Object> serviceBeanMap = ctx.getBeansWithAnnotation(SimpleProvider.class);
        if (MapUtils.isNotEmpty(serviceBeanMap)) {
            for (Object serviceBean : serviceBeanMap.values()) {
                String interfaceName = serviceBean.getClass().getAnnotation(SimpleProvider.class).value().getName();
                handlerMap.put(interfaceName, serviceBean);
            }
        }
    }

    @Override
    public void afterPropertiesSet() throws Exception {
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            ServerBootstrap bootstrap = new ServerBootstrap();
            ChannelHandler channelHandler = new ChannelInitializer<SocketChannel>() {
                @Override
                public void initChannel(SocketChannel channel) throws Exception {
                    channel.pipeline()
                            .addLast(new SimpleDecoder(SimpleRequest.class))
                            .addLast(new SimpleEncoder(SimpleResponse.class))
                            .addLast(new SimpleHandler(handlerMap));
                }
            };
            bootstrap.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .childHandler(channelHandler)
                    .option(ChannelOption.SO_BACKLOG, 128)
                    .childOption(ChannelOption.SO_KEEPALIVE, true);

            String host = getLocalHost();
            if (null == host) {
                LOGGER.error("can't get service address,because address is null");
                throw new SimpleException("can't get service address,because address is null");
            }
            int port = providerProperties.getPort();
            ChannelFuture future = bootstrap.bind(host, port).sync();
            LOGGER.debug("server started on port {}", port);

            if (serviceRegistry != null) {
                String serverAddress = host + ":" + port;
                serviceRegistry.register(serverAddress);
            }
            future.channel().closeFuture().sync();
        } finally {
            workerGroup.shutdownGracefully();
            bossGroup.shutdownGracefully();
        }
    }

    /**
     * get service host
     *
     * @return
     */
    private String getLocalHost() {
        Enumeration<NetworkInterface> allNetInterfaces;
        try {
            allNetInterfaces = NetworkInterface.getNetworkInterfaces();
        } catch (SocketException e) {
            LOGGER.error("get local address error,cause:", e);
            return null;
        }
        while (allNetInterfaces.hasMoreElements()) {
            NetworkInterface netInterface = allNetInterfaces.nextElement();
            Enumeration<InetAddress> addresses = netInterface.getInetAddresses();
            while (addresses.hasMoreElements()) {
                InetAddress ip = addresses.nextElement();
                if (ip instanceof Inet4Address && !ip.isLoopbackAddress() && !ip.getHostAddress().contains(":")) {
                    return ip.getHostAddress();
                }
            }
        }
        return null;
    }
}
複製程式碼

描述一下這個類做了什麼工作：

• 首先他實現了ApplicationContextAware, InitializingBean這兩個 spring 中介面，根據IOC容器初始化的順序，會依次回撥用介面中的setApplicationContext 和 afterPropertiesSet 方法。
  • setApplicationContext方法中獲取了容器中被@SimpleProvider標註的類，並將服務介面名和服務實現類繫結，存放到handlerMap中，在@SimpleProvider中有一個 value 屬性，是考慮到一個類可以實現多個介面，通過 value 可以指定哪個服務介面，當然也可以定義為陣列，處理多個介面
  • afterPropertiesSet 方法中做了兩件事：
    • 在服務端開啟了一個處理socket請求的執行緒池，監聽和處理服務暴露埠上接受到的請求，指定了一個處理器SimpleHandler
    • 呼叫ServiceRegistry類的registry方法向 zookeeper 註冊服務的地址和埠，這裡沒有用到協議，只註冊了 ip:port

SimpleHandler是一個實現了 netty的SimpleChannelInboundHandler的請求處理器類

package com.glmapper.simple.provider.handler;

/**
 * request handler
 *
 * @author Jerry
 */
public class SimpleHandler extends SimpleChannelInboundHandler<SimpleRequest> {

    private static final Logger LOGGER = LoggerFactory.getLogger(SimpleHandler.class);

    private final Map<String, Object> handlerMap;

    public SimpleHandler(Map<String, Object> handlerMap) {
        this.handlerMap = handlerMap;
    }

    @Override
    public void channelRead0(final ChannelHandlerContext ctx, SimpleRequest request) throws Exception {
        SimpleResponse response = new SimpleResponse();
        response.setRequestId(request.getRequestId());
        try {
            Object result = handle(request);
            response.setResult(result);
        } catch (Throwable t) {
            response.setError(t);
        }
        ctx.writeAndFlush(response).addListener(ChannelFutureListener.CLOSE);
    }

    private Object handle(SimpleRequest request) throws Throwable {
        String className = request.getClassName();
        Object serviceBean = handlerMap.get(className);

        Class<?> serviceClass = serviceBean.getClass();
        String methodName = request.getMethodName();
        Class<?>[] parameterTypes = request.getParameterTypes();
        Object[] parameters = request.getParameters();

        FastClass serviceFastClass = FastClass.create(serviceClass);
        FastMethod serviceFastMethod = serviceFastClass.getMethod(methodName, parameterTypes);
        return serviceFastMethod.invoke(serviceBean, parameters);
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        LOGGER.error("server caught exception", cause);
        ctx.close();
    }
}
複製程式碼

SimpleHandler基於 netty 的事件驅動模型觸發對應的方法，當收到請求事件會呼叫channelRead0方法，這個方法的作用就是，根據請求引數中的介面名找到對應的實現類呼叫指定的方法，然後把結果返回。

再瞅瞅ServiceRegistry，入口是ProviderInitializer呼叫了ServiceRegistry的 registry方法

package com.glmapper.simple.provider.registry;

/**
 * connect zookeeper to registry service
 *
 * @author Jerry
 */
public class ServiceRegistry {

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

    private ZookeeperProperties zookeeperProperties;

    public ServiceRegistry(ZookeeperProperties zookeeperProperties) {
        this.zookeeperProperties = zookeeperProperties;
    }

    public void register(String data) {
        if (data != null) {
            ZooKeeper zk = ZookeeperUtils.connectServer(zookeeperProperties.getAddress(), zookeeperProperties.getTimeout());
            if (zk != null) {
                addRootNode(zk);
                createNode(zk, data);
            }
        }
    }

    /**
     * add one zookeeper root node
     *
     * @param zk
     */
    private void addRootNode(ZooKeeper zk) {
        try {
            String registryPath = zookeeperProperties.getRootPath();
            Stat s = zk.exists(registryPath, false);
            if (s == null) {
                zk.create(registryPath, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
            }
        } catch (KeeperException | InterruptedException e) {
            LOGGER.error("zookeeper add root node error,cause:", e);
        }
    }

    private void createNode(ZooKeeper zk, String data) {
        try {
            byte[] bytes = data.getBytes(Charset.forName("UTF-8"));
            String dataPath = zookeeperProperties.getRootPath() + zookeeperProperties.getDataPath();
            String path = zk.create(dataPath, bytes, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
            LOGGER.debug("create zookeeper node ({} => {})", path, data);
        } catch (KeeperException | InterruptedException e) {
            LOGGER.error("create zookeeper node error,cause:", e);
        }
    }
}
複製程式碼

ServiceRegistry類做的工作比較簡單，就是把 服務ip:port註冊到 zk 的指定目錄下

• 建立根節點，根節點是個永久節點
• 在根節點下建立臨時的子節點，子節點儲存了服務的 ip:port，服務被掛掉對應的子節點就會被幹掉

2.3 消費端

消費端內容：

消費端的內容比較少，核心就三個類：ServiceDiscovery、ConsumerHandler、ConsumerProxy

先看下ServiceDiscovery內容：

package com.glmapper.simple.consumer.discovery;

/**
 * 服務發現:連線ZK,新增watch事件
 *
 * @author Jerry
 */
public class ServiceDiscovery {

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

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

    private ZookeeperProperties zookeeperProperties;

    public ServiceDiscovery(ZookeeperProperties zookeeperProperties) {
        this.zookeeperProperties = zookeeperProperties;
        String address = zookeeperProperties.getAddress();
        int timeout = zookeeperProperties.getTimeout();
        ZooKeeper zk = ZookeeperUtils.connectServer(address, timeout);
        if (zk != null) {
            watchNode(zk);
        }
    }

    public String discover() {
        String data = null;
        int size = nodes.size();
        if (size > 0) {
            if (size == 1) {
                data = nodes.get(0);
                LOGGER.debug("using only node: {}", data);
            } else {
                data = nodes.get(ThreadLocalRandom.current().nextInt(size));
                LOGGER.debug("using random node: {}", data);
            }
        }
        return data;
    }

    private void watchNode(final ZooKeeper zk) {
        try {
            Watcher childrenNodeChangeWatcher = event -> {
                if (event.getType() == Watcher.Event.EventType.NodeChildrenChanged) {
                    watchNode(zk);
                }
            };
            String rootPath = zookeeperProperties.getRootPath();
            List<String> nodeList = zk.getChildren(rootPath, childrenNodeChangeWatcher);
            List<String> nodes = new ArrayList<>();
            for (String node : nodeList) {
                byte[] bytes = zk.getData(rootPath + "/" + node, false, null);
                nodes.add(new String(bytes, Charset.forName("UTF-8")));
            }
            LOGGER.info("node data: {}", nodes);
            this.nodes = nodes;
        } catch (KeeperException | InterruptedException e) {
            LOGGER.error("節點監控出錯，原因：", e);
        }
    }
}
複製程式碼

這個類的入口是構造器，作用是獲取 zk 的地址，然後獲取 zk 上的節點資訊，這裡沒有實現服務訂閱，也就是說如果 zk 上原本有兩個服務，掛掉一個，客戶端不會剔除掛掉的服務資訊，導致呼叫失敗。

然後是ConsumerProxy，它是一個代理工廠：

package com.glmapper.simple.consumer.proxy;

/**
 * ConsumerProxy
 *
 * @author Jerry
 */
public class ConsumerProxy {

    private ServiceDiscovery serviceDiscovery;

    public ConsumerProxy(ServiceDiscovery serviceDiscovery) {
        this.serviceDiscovery = serviceDiscovery;
    }

    @SuppressWarnings("unchecked")
    public <T> T create(Class<?> interfaceClass) {
        return (T) Proxy.newProxyInstance(interfaceClass.getClassLoader(),
                new Class<?>[]{interfaceClass},
                new SimpleInvocationHandler());
    }

    private class SimpleInvocationHandler implements InvocationHandler {
        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            SimpleRequest request = buildRequest(method, args);
            String serverAddress = getServerAddress();
            String[] array = serverAddress.split(":");
            String host = array[0];
            int port = Integer.parseInt(array[1]);
            ConsumerHandler consumerHandler = new ConsumerHandler(host, port);
            SimpleResponse response = consumerHandler.send(request);
            if (response.getError() != null) {
                throw new SimpleException("service invoker error,cause:", response.getError());
            } else {
                return response.getResult();
            }
        }

        private SimpleRequest buildRequest(Method method, Object[] args) {
            SimpleRequest request = new SimpleRequest();
            request.setRequestId(UUID.randomUUID().toString());
            request.setClassName(method.getDeclaringClass().getName());
            request.setMethodName(method.getName());
            request.setParameterTypes(method.getParameterTypes());
            request.setParameters(args);
            return request;
        }

        private String getServerAddress() {
            String serverAddress = null;
            if (serviceDiscovery != null) {
                serverAddress = serviceDiscovery.discover();
            }
            if (null == serverAddress) {
                throw new SimpleException("no server address available");
            }
            return serverAddress;
        }
    }
}
複製程式碼

這裡有個內部類SimpleInvocationHandler是生產代理的核心，方法的核心是在 SimpleInvocationHandler.invoke()中是呼叫這兩行程式碼

ConsumerHandler consumerHandler = new ConsumerHandler(host, port);
SimpleResponse response = consumerHandler.send(request);
複製程式碼

發起網路請求，下面看下ConsumerHandler類

package com.glmapper.simple.consumer.handler;

/**
 * RPC真正呼叫客戶端
 *
 * @author Jerry
 */
public class ConsumerHandler extends SimpleChannelInboundHandler<SimpleResponse> {

    private static final Logger LOGGER = LoggerFactory.getLogger(ConsumerHandler.class);

    private int port;

    private String host;

    private SimpleResponse response;

    private CountDownLatch latch = new CountDownLatch(1);

    public ConsumerHandler(String host, int port) {
        this.host = host;
        this.port = port;
    }

    @Override
    public void channelRead0(ChannelHandlerContext ctx, SimpleResponse response) throws Exception {
        this.response = response;
        latch.countDown();
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        LOGGER.error("client caught exception", cause);
        ctx.close();
    }

    public SimpleResponse send(SimpleRequest request) throws Exception {
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            Bootstrap bootstrap = new Bootstrap();
            ChannelInitializer<SocketChannel> channelHandler = new ChannelInitializer<SocketChannel>() {
                @Override
                public void initChannel(SocketChannel channel) throws Exception {
                    channel.pipeline()
                            // 將 RPC 請求進行編碼（為了傳送請求）
                            .addLast(new SimpleEncoder(SimpleRequest.class))
                            // 將 RPC 響應進行解碼（為了處理響應）
                            .addLast(new SimpleDecoder(SimpleResponse.class))
                            // 使用 RpcClient 傳送 RPC 請求
                            .addLast(ConsumerHandler.this);
                }
            };
            bootstrap.group(group).channel(NioSocketChannel.class)
                    .handler(channelHandler)
                    .option(ChannelOption.SO_KEEPALIVE, true);

            ChannelFuture future = bootstrap.connect(host, port).sync();
            future.channel().writeAndFlush(request).sync();
            latch.await();
            if (response != null) {
                future.channel().closeFuture().sync();
            }
            return response;
        } finally {
            group.shutdownGracefully();
        }
    }
}
複製程式碼

這個類和服務端的 ProviderHandler 的程式碼差不多，也是netty通訊類

附一下 GitHub 地址 simple-rpc