Hadoop学习八：Hadoop-Hdfs RPC源码 Client

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一.Client类图

 

二.详细描述

1.  ConnectionId：This class holds the address and the user ticket. The client connections to servers are uniquely identified by 。一个connection由一个ConnectionId唯一标识；所以要重写ConnectionId的equals和hashcode方法。
2. ConnectionHeader：The IPC connection header sent by the client to the server on connection establishment.
3. Connection：继承Thread。代表client到server的一个连接。我在文中将Connection对象称为“连接”。

     /** Thread that reads responses and notifies callers.  Each connection owns a
      * socket connected to a remote address.  Calls are multiplexed through this
      * socket: responses may be delivered out of order. */
     private class Connection extends Thread {
   	 //一个连接的基本信息
       private InetSocketAddress server;             // server ip:port，注意是服务端
       private ConnectionHeader header;              // connection header
       private final ConnectionId remoteId;                // connection id
       private Socket socket = null;                 // connected socket
       private DataInputStream in;
       private DataOutputStream out;
       
       //所有的调用
       private Hashtable calls = new Hashtable();
       
       //1.构造函数只初始化一些基本信息
       public Connection(ConnectionId remoteId) throws IOException {
         this.remoteId = remoteId;
         this.server = remoteId.getAddress();
         header = new ConnectionHeader(protocol == null ? null : protocol.getName(), ticket, authMethod);
         this.setName("IPC Client (" + socketFactory.hashCode() +") connection to " +
             remoteId.getAddress().toString() +" from " + ((ticket==null)?"an unknown user":ticket.getUserName()));
         this.setDaemon(true);
       }
   
       //2.与IPC server建立socket连接
       private synchronized void setupConnection() throws IOException {
         while (true) {
           try {
             this.socket = socketFactory.createSocket();
         }
       }
       /** Connect to the server and set up the I/O streams. It then sends
        * a header to the server and starts
        * the connection thread that waits for responses.
        */
       private synchronized void setupIOstreams() throws InterruptedException {
         try {
           while (true) {
             //2.与IPC server建立socket连接
             setupConnection();
             //3.创建流
             InputStream inStream = NetUtils.getInputStream(socket);
             OutputStream outStream = NetUtils.getOutputStream(socket);
             //4.发送RPC报头
             writeRpcHeader(outStream);
             this.in = new DataInputStream(new BufferedInputStream (new PingInputStream(inStream)));
             this.out = new DataOutputStream(new BufferedOutputStream(outStream));
             //5.发送connection header到server
             writeHeader();
             //6.启动自己(线程)，接受response
             start();
             return;
           }
         } catch (Throwable t) {
           if (t instanceof IOException) {
             markClosed((IOException)t);
           } else {
             markClosed(new IOException("Couldn't set up IO streams", t));
           }
           close();
         }
       }
   
       //4.发送RPC报头
       private void writeRpcHeader(OutputStream outStream) throws IOException {
         DataOutputStream out = new DataOutputStream(new BufferedOutputStream(outStream));
         //public static final ByteBuffer HEADER = ByteBuffer.wrap("hrpc".getBytes());
         //public static final byte CURRENT_VERSION = 4;
         out.write(Server.HEADER.array());
         out.write(Server.CURRENT_VERSION);
         authMethod.write(out);
         out.flush();
       }
       
       //5.发送connection header到server
       private void writeHeader() throws IOException {
         DataOutputBuffer buf = new DataOutputBuffer();
         header.write(buf);
         
         int bufLen = buf.getLength();
         out.writeInt(bufLen);
         out.write(buf.getData(), 0, bufLen);
       }
       
       //6.启动自己(线程)，等待接受response，接受完后关闭此连接
       public void run() {
         while (waitForWork()) {//Return true if it is time to read a response; false otherwise.
           receiveResponse();
         }
         //关闭此连接
         close();
       }
   
       //7.发送请求：长度+内容
       public void sendParam(Call call) {
         DataOutputBuffer d=null;
           synchronized (this.out) {
             d = new DataOutputBuffer();
             d.writeInt(call.id);
             call.param.write(d);
             byte[] data = d.getData();
             int dataLength = d.getLength();
             out.writeInt(dataLength);      //first put the data length
             out.write(data, 0, dataLength);//write the data
             out.flush();
           }
       }  
   
       //6.接受response，把结果赋值给Call对象
       private void receiveResponse() {
           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);
           } 
       }
       
   }

4. Call对象：RPC是基于反射的，每次方法调用都对应一个Call对象，我在文中将Call对象称为“调用”。

       private class Call {
           int id;                                       // call id，唯一标示一个Call
           Writable param;                        // parameter，创建Call对象时赋值
           Writable value;                          // value, Connecion线程接受到server的response后赋值给value
           boolean done;                           // true when call is done
   
           protected Call(Writable param) {
             this.param = param;
             synchronized (Client.this) {
               this.id = counter++;
             }
           }
       }

5. ParallelCall：继承Call，还是一个Call对象，只是这些Call对象共享一个ParallelResults。

           private class ParallelCall extends Call {
               private ParallelResults results; //多个Call共享一个ParallelResults
               private int index;
               
               public ParallelCall(Writable param, ParallelResults results, int index) {
                 super(param);
                 this.results = results;
                 this.index = index;
               }
   
               /** Deliver result to result collector. */
               protected void callComplete() {
                 results.callComplete(this);
               }
             }

    
6. ParallelResults：一组Call对象的返回结果。

   private static class ParallelResults {
               private Writable[] values;
               private int size;   //一共有多少个Call要返回
               private int count;  //实际已经返回几个
   
               public ParallelResults(int size) {
                 this.values = new Writable[size];
                 this.size = size;
               }
   
               /** Collect a result. */
               public synchronized void callComplete(ParallelCall call) {
                 values[call.index] = call.value;            // store the value
                 count++;                                    // count it
                 if (count == size)                          // if all values are in
                   notify();                                 // then notify waiting caller
               }
             } 

7. Client：IPC client端。调用client的call方法，传入Writable作为参数，返回一个Writable作为结果。

   /** A client for an IPC service.  IPC calls take a single {@link Writable} as a
    * parameter, and return a {@link Writable} as their value.  A service runs on
    * a port and is defined by a parameter class and a value class.
    * 
    * @see Server
    */
   public class Client {
     //缓存client到server的所有连接
     private Hashtable connections = new Hashtable();
   
     private Class valueClass;   //Call对象value的类型
     private int counter;                            //创建一个Call的时候，用counter++作为Call的id
     final private Configuration conf;
   
     private SocketFactory socketFactory;           //服务器端的ip+port创建的socketFactory
     
     //1.初始化Client
     public Client(Class valueClass, Configuration conf, 
         SocketFactory factory) {
       this.valueClass = valueClass;
       this.conf = conf;
       this.socketFactory = factory; //初始化client时传入
     }
   
     //2.用client发送一个请求
     public Writable call(Writable param, ConnectionId remoteId)  
                          throws InterruptedException, IOException {
   	  //创建Call对象
       Call call = new Call(param);
       //创建Connection对象
       Connection connection = getConnection(remoteId, call);
       //发送请求，参考Connection类代码7
       connection.sendParam(call);                 
       ...
       return call.value;
     }
   
     //2.用client一次发送多个请求
     public Writable[] call(Writable[] params, InetSocketAddress[] addresses,
         Class protocol, UserGroupInformation ticket, Configuration conf)
         throws IOException, InterruptedException {
   	 //创建一个结果集
       ParallelResults results = new ParallelResults(params.length);
       synchronized (results) {
         for (int i = 0; i < params.length; i++) {
       	 //创建每个Call
           ParallelCall call = new ParallelCall(params[i], results, i);
           try {
            //创建每个Connection对象，不同的Call存放到不同的连接上(Each parameter is sent to the corresponding address.)。
             ConnectionId remoteId = ConnectionId.getConnectionId(addresses[i],
                 protocol, ticket, 0, conf);
             Connection connection = getConnection(remoteId, call);
             //发送请求,参考Connection类代码7
             connection.sendParam(call);            
           } catch (IOException e) {
           }
         }
         while (results.count != results.size) {
           try {
             results.wait();                    // wait for all results
           } catch (InterruptedException e) {}
         }
         //放回所有结果
         return results.values;
       }
     }
   
     /** Get a connection from the pool, or create a new one and add it to the
      * pool.  Connections to a given ConnectionId are reused. */
     //获得一个连接，首先从缓存中去；取不到，创建一个，并放到缓存中。
      private Connection getConnection(ConnectionId remoteId, Call call) throws IOException, InterruptedException {
       Connection connection;
       do {
         synchronized (connections) {
           connection = connections.get(remoteId);
           if (connection == null) {
            //参考Connection类代码1
             connection = new Connection(remoteId);
             connections.put(remoteId, connection);
           }
         }
       } while (!connection.addCall(call));//往创建的连接里加入call
       //参考Connection类代码23456
       connection.setupIOstreams();
       return connection;
     }
   
   }