Hadoop中客户端和服务器端的方法调用过程
1、Java动态代理实例
Java 动态代理一个简单的demo：（用以对比Hadoop中的动态代理）
Hello接口：
public interface Hello { void sayHello(String to); void print(String p); }
Hello接口的实现类：
与代理类（HelloImpl类）相关联的InvocationHandler对象
最后测试代码如下：
2、Hadoop中的动态代理
2.1、客户端方法调用过程
IPC客户端的处理比动态代理实例稍微复杂：代理对象上的调用被InvocationHandler捕获后，请求被打包并通过IPC连接发送到服务器上，客户端等待并在服务器的处理应答到达后，生成并返回调用结果。IPC上的调用是个同步操作，即，线程会一直等待调用结束，才会开始后续处理；而网络的处理时异步的，请求发送后，不需要等待应答。客户端通过java的wait()/notify()机制简单地解决了异步网络处理和同步IPC调用的差异。
Hadoop对外提供查询文件状态的接口，如下：
public interface IPCQueryStatus extends VersionedProtocol { IPCFileStatus getFileStatus(String filename); }
客户端通过如下代码调用：
IPCQueryStatus query = (IPCQueryStatus) RPC.getProxy(IPCQueryStatus.class, IPCQueryServer.IPC_VER, addr, new Configuration()); IPCFileStatus status = query.getFileStatus("\tmp\testIPC");
2.1.1、Client端动态代理实现
在RPC的getProxy代码如下：
public static VersionedProtocol getProxy( Class<? extends VersionedProtocol> protocol, long clientVersion, InetSocketAddress addr, UserGroupInformation ticket, Configuration conf, SocketFactory factory, int rpcTimeout) throws IOException { ...... VersionedProtocol proxy = (VersionedProtocol) Proxy.newProxyInstance( protocol.getClassLoader(), new Class[] { protocol }, new Invoker(protocol, addr, ticket, conf, factory, rpcTimeout)); ...... return proxy; ...... }
需要制定一个InvocationHandler，对于所有的调用请求，这个InvocationHandler都是Invoke，如下：
private static class Invoker implements InvocationHandler { private Client.ConnectionId remoteId;// 用来标示一个connection，用以复用 private Client client;//最重要的成员变量，RPC客户端 private boolean isClosed = false; public Invoker(Class<? extends VersionedProtocol> protocol, InetSocketAddress address, UserGroupInformation ticket, Configuration conf, SocketFactory factory, int rpcTimeout) throws IOException { this.remoteId = Client.ConnectionId.getConnectionId(address, protocol, ticket, rpcTimeout, conf); this.client = CLIENTS.getClient(conf, factory);//★ } ...... public Object invoke(Object proxy, Method method, Object[] args) ...... ObjectWritable value = (ObjectWritable) client.call(new Invocation(method, args), remoteId); ...... return value.get(); } }
在上面的代码中，client负责发送IPC请求，并获取结果，类似最上面demo中LogHandler中的dele。
2.1.2、Client通过Connection发送IPC请求并获取结果
如下为client.call方法调用Connection.sendParam发送IPC请求：
public Writable call(Writable param, ConnectionId remoteId) throws InterruptedException, IOException { Call call = new Call(param); Connection connection = getConnection(remoteId, call); connection.sendParam(call); // send the parameter ... synchronized (call) { while (!call.done) { try { call.wait(); // wait for the result } catch (InterruptedException ie) { ... } } ... if (call.error != null) { ... throw call.error; ... } else { return call.value; } } }
connection.sendParam后，会再调用receiveMessage来获取返回结果。如下：
private class Connection extends Thread { ...... public void run() { ...... while (waitForWork()) {//wait here for work - read or close connection receiveResponse(); } ...... } ...... private void receiveResponse() { ...... touch(); try { int id = in.readInt(); // try to read an id ...... Call call = calls.get(id); int state = in.readInt(); // read call status if (state == Status.SUCCESS.state) { Writable value = ReflectionUtils.newInstance(valueClass, conf); value.readFields(in); // read value call.setValue(value); calls.remove(id); } else if (state == Status.ERROR.state) { call.setException(new RemoteException(WritableUtils.readString(in), WritableUtils.readString(in))); calls.remove(id); } else if (state == Status.FATAL.state) { // Close the connection markClosed(new RemoteException(WritableUtils.readString(in), WritableUtils.readString(in))); } } catch (IOException e) { markClosed(e); } } }
connection会调用call的setValue或者setException，两个方法都会调用callComplete方法，来调用notify通知进程IPC调用已结束
protected synchronized void callComplete() { this.done = true; notify(); // notify caller } public synchronized void setException(IOException error) { this.error = error; callComplete(); } public synchronized void setValue(Writable value) { this.value = value; callComplete(); }
2.2、服务器端方法调用过程
服务端由Listener接收。
2.2.1、Listener接收IPC请求的工作过程
Listener主要运行NIO选择器循环，并在Listener.doRead()方法中读取数据，Connection.readAndProcess()中恢复数据帧，然后调用processData().
void Listener.doRead(SelectionKey key) throws InterruptedException { int count = 0; Connection c = (Connection)key.attachment(); ... count = c.readAndProcess(); ... } public int Connection.readAndProcess() throws IOException, InterruptedException { ...... processOneRpc(data.array()); ...... } private void Connection.processOneRpc(byte[] buf) throws IOException, InterruptedException { if (headerRead) { processData(buf); } else { processHeader(buf); ...... } } private void Connection.processData(byte[] buf) throws IOException, InterruptedException { DataInputStream dis = new DataInputStream(new ByteArrayInputStream(buf)); int id = dis.readInt(); // try to read an id ...... Writable param = ReflectionUtils.newInstance(paramClass, conf);//★??paramClass在哪儿设置的★在RPC.Server中，paramClass是Invocation，IPC调用传递的都是Invocation param.readFields(dis); Call call = new Call(id, param, this); callQueue.put(call); // queue the call; maybe blocked here }
ProcessData反序列化调用参数，构造服务器端的Call对象。然后放入callQueue队列中。callQueue阻塞队列定义于Server类中，是Listener和Handler的边界。（生产者Listener消费者Handler）。
2.2.2、Handler处理IPC请求的工作过程
Handler主要工作都在run方法中完成。主循环中，每循环一次处理一个请求（通过调用Server的抽象方法call来完成）。
public void run() { ...... SERVER.set(Server.this); ByteArrayOutputStream buf = new ByteArrayOutputStream(INITIAL_RESP_BUF_SIZE); while (running) { final Call call = callQueue.take(); // 获取一个IPC调用 ...... String errorClass = null; String error = null; Writable value = null; CurCall.set(call); ...... value = call(call.connection.protocol, call.param, call.timestamp);//实际代码用到jaas，这里简化 ...... CurCall.set(null); synchronized (call.connection.responseQueue) { ...... setupResponse(buf, call, (error == null) ? Status.SUCCESS : Status.ERROR, value, errorClass, error); ... responder.doRespond(call);//★? } } }
Server.call调用后返回一个writable对象--value，然后通过调用setupResponse将结果序列化到call的Response成员变量中。
private void setupResponse(ByteArrayOutputStream response, Call call, Status status, Writable rv, String errorClass, String error) throws IOException { response.reset(); DataOutputStream out = new DataOutputStream(response); out.writeInt(call.id); // write call id out.writeInt(status.state); // write status if (status == Status.SUCCESS) { rv.write(out); } else { WritableUtils.writeString(out, errorClass); WritableUtils.writeString(out, error); } ...... call.setResponse(ByteBuffer.wrap(response.toByteArray())); }
Server.call抽象方法的具体实现在RPC.Server中。代码如下：
private Object instance; ...... public Writable call(Class<?> protocol, Writable param, long receivedTime) throws IOException { Invocation call = (Invocation)param; Method method = protocol.getMethod(call.getMethodName(), call.getParameterClasses()); method.setAccessible(true); Object value = method.invoke(instance, call.getParameters()); return new ObjectWritable(method.getReturnType(), value); }
Handler所在线程是共享资源，当有一个IPC请求处理完后，即调用Response的doResponse返回结果，而不亲自返回，原因有二：
1. 对共享资源的占用时间越短越好；
2. IPC返回受网络通信时间影响，可能会占用很长时间。
2.2.3、Response的工作过程
doResponse的代码很简单，将Call放入IPC连接的应答队列中，如果应答队列为1，立即调用processResponse发放向客户端发送结果，（队列为1，表明此IPC连接比较空闲，直接发送，避免从Handler线程到Response线程的切换开销）
void doRespond(Call call) throws IOException { synchronized (call.connection.responseQueue) { call.connection.responseQueue.addLast(call); if (call.connection.responseQueue.size() == 1) { processResponse(call.connection.responseQueue, true); } } }
Response有一个类似于Listener的NIO选择器，用来处理当队列不为1时的发送。只是Listener关注OP_READ和OP_ACCEPT事件，而Response关注OP_WRITE事件。代码如下：
public void run() { while (running) { waitPending(); // 等待通道登记 writeSelector.select(PURGE_INTERVAL); // 等待通道可写 Iterator<SelectionKey> iter = writeSelector.selectedKeys().iterator(); while (iter.hasNext()) { SelectionKey key = iter.next(); iter.remove(); try { if (key.isValid() && key.isWritable()) { doAsyncWrite(key);//输出远程IPC调用结果 } } catch (IOException e) { } } ...... } } private void doAsyncWrite(SelectionKey key) throws IOException { Call call = (Call)key.attachment(); ...... synchronized(call.connection.responseQueue) { if (processResponse(call.connection.responseQueue, false)) {//调用输出 try { key.interestOps(0);//processResponse返回true，表示无等待数据，清楚兴趣操作集 } catch (CancelledKeyException e) { ...... } } } } private boolean processResponse(LinkedList<Call> responseQueue, boolean inHandler) throws IOException { ...... synchronized (responseQueue) { ...... int numBytes = channelWrite(channel, call.response); done = true; // error. no more data for this channel. closeConnection(call.connection); } return done; }
processResponse关键点：
1. 可被Handler调用（当应答队列为1），参数inHandler为true，也可被Response调用，参数inHandler为false，表示队列为1或更多。
2. 返回true，表示通道上无需要发送的数据。
2.3总结
IPC Client端，发送Client.Call(new Invocation(method,args), remoteId)
--封装过程：Call.Id , Invocation---(查看Client.Connection.sendParam)
IPC Server端，接收Server.Call(Id, Invocation, Connction)---封装过程：Call.Id，Invocation--(查看Server.Connction.processData)