Hadoop学习十：Hadoop-Hdfs RPC源码 RPC

一.RPC类图

二.详细描述

Server：继承org.apache.hadoop.ipc.Server(Hadoop学习九：Hadoop-hdfs RPC源码 Server)。我们称之为RPC Server。

  /** An RPC Server. */
  public static class Server extends org.apache.hadoop.ipc.Server {
    //创建一个RPC server
    //注意调用父类IP.Server的构造函数时传入的就是Invocation.class
    public Server(Object instance, Configuration conf, String bindAddress,  int port,
                  int numHandlers, boolean verbose, 
                  SecretManager<? extends TokenIdentifier> secretManager) 
        throws IOException {
      super(bindAddress, port, Invocation.class, numHandlers, conf,
          classNameBase(instance.getClass().getName()), secretManager);
    }

    //重写ipc.Server的call方法
    public Writable call(Class<?> protocol, Writable param, long receivedTime){
        Invocation call = (Invocation)param;
        if (verbose) log("Call: " + call);

        Method method = protocol.getMethod(call.getMethodName(),
        method.setAccessible(true);
        long startTime = System.currentTimeMillis();
        //真正执行远程命令就体现在这里：client的方法最终在server上被执行，就是所谓的rpc
        Object value = method.invoke(instance, call.getParameters());

        return new ObjectWritable(method.getReturnType(), value);
    }
  }

Invocation：我要在远程server上运行一条命令，也就是一个方法，我们把这个方法的方法名，参数，类型封装成一个Invocation对象。

   //每次方法调用都会实例化一个Invocation对象
   //将方法的方法名，参数类型，值封装成一个Invocation对象
  private static class Invocation implements Writable, Configurable {
    private String methodName;
    private Class[] parameterClasses;
    private Object[] parameters;
    private Configuration conf;

    public Invocation(Method method, Object[] parameters) {
      this.methodName = method.getName();
      this.parameterClasses = method.getParameterTypes();
      this.parameters = parameters;
    }
  }

Invoker：继承InvocationHandler，重写了invoke方法，invoke方法里面就是IPC Client向IPC Server发送Invocation。

  //java反射
  private static class Invoker implements InvocationHandler {
    private Client.ConnectionId remoteId;
    private Client client;
    private boolean isClosed = false;
    //只有内部RPC.getProx时调用
    private Invoker(Class<? extends VersionedProtocol> protocol,
        InetSocketAddress address, UserGroupInformation ticket,
        Configuration conf, SocketFactory factory,
        int rpcTimeout, RetryPolicy connectionRetryPolicy) throws IOException {
      this.remoteId = Client.ConnectionId.getConnectionId(address, protocol,
          ticket, rpcTimeout, connectionRetryPolicy, conf);
      this.client = CLIENTS.getClient(conf, factory);
    }

    //重写invoke方法
    //每次方法调用都将封装成Invocation对象，被发往server端
    //返回server端运行此方法的结果
    public Object invoke(Object proxy, Method method, Object[] args)
      throws Throwable {
      ObjectWritable value = (ObjectWritable)
        client.call(new Invocation(method, args), remoteId);//client向server发生消息
      if (logDebug) {
        long callTime = System.currentTimeMillis() - startTime;
        LOG.debug("Call: " + method.getName() + " " + callTime);
      }
      return value.get();
    }
  }

ClientCache：缓存IPC Client对象。

RPC：前两章学习了IPC Client和IPC Server，RPC就是综合了这两者以及java反射机制，为我们提供了许多静态方法，我们只需要调用RPC.*，即可获得代理类，而不需要关心它们是怎么完成的。

public class RPC {
  private RPC() {}                                  // no public ctor
  
  //相比getProxy，waitForProxy肯定能获得一个代理类VersionedProtocol
  static VersionedProtocol waitForProxy(Class<? extends VersionedProtocol> protocol,
                                               long clientVersion,
                                               InetSocketAddress addr,
                                               Configuration conf,
                                               int rpcTimeout,
                                               long connTimeout)
                                               throws IOException { 
        return getProxy(protocol, clientVersion, addr, conf, rpcTimeout);
        catch(ConnectException se) {  // namenode has not been started
        LOG.info("Server at " + addr + " not available yet, Zzzzz..."); //你是在卖萌
    }
  }

  //获得代理类，剩下的只需用代理类执行命令就行了
  public static VersionedProtocol getProxy(
      Class<? extends VersionedProtocol> protocol,
      long clientVersion, InetSocketAddress addr, UserGroupInformation ticket,
      Configuration conf, SocketFactory factory, int rpcTimeout,
      RetryPolicy connectionRetryPolicy) throws IOException {

    final Invoker invoker = new Invoker(protocol, addr, ticket, conf, factory,
        rpcTimeout, connectionRetryPolicy);
    //反射获得代理类
    VersionedProtocol proxy = (VersionedProtocol)Proxy.newProxyInstance(
        protocol.getClassLoader(), new Class[]{protocol}, invoker);
    long serverVersion = proxy.getProtocolVersion(protocol.getName(), clientVersion);
    //协议检查
    if (serverVersion == clientVersion) {
      return proxy;
    }
  }

  //每次发送一组方法调用到指定server
  //并没有经过反射机制
  //而是直接发送方法名，参数类型，值到指定server；并获取server执行这些方法的结果集
  //如果你确实有一组远程命令要执行，并且知道这些命令的的方法名，参数类型，值，并为每个命令指定执行server，那你可以调用此方法
  public static Object[] call(Method method, Object[][] params,
                              InetSocketAddress[] addrs, 
                              UserGroupInformation ticket, Configuration conf)
    throws IOException, InterruptedException {
    //实例化Invocation
    Invocation[] invocations = new Invocation[params.length];
    for (int i = 0; i < params.length; i++)
      invocations[i] = new Invocation(method, params[i]);
    Client client = CLIENTS.getClient(conf);
    try {//发生Invocation
    Writable[] wrappedValues = 
      client.call(invocations, addrs, method.getDeclaringClass(), ticket, conf);
    
    if (method.getReturnType() == Void.TYPE) {
      return null;
    }

    Object[] values =
      (Object[])Array.newInstance(method.getReturnType(), wrappedValues.length);
    for (int i = 0; i < values.length; i++)
      if (wrappedValues[i] != null)
        values[i] = ((ObjectWritable)wrappedValues[i]).get();
    
    return values;
    } finally {
      CLIENTS.stopClient(client);
    }
  }
 
  //获得RPC Server
  public static Server getServer(final Object instance, final String bindAddress, final int port,
                                 final int numHandlers,
                                 final boolean verbose, Configuration conf,
                                 SecretManager<? extends TokenIdentifier> secretManager) 
    throws IOException {
    return new Server(instance, conf, bindAddress, port, numHandlers, verbose, secretManager);
  }
  
}