Hadoop建立IPC连接和数据读写

建立IPC连接

IPC Client通过调用getConnection获取IPC连接，具体流程图如下：

服务器端的IPC连接代码分散在Listener和Server.Connection中。

Listener.run() 实现了NIO中的选择器循环。如下代码：

//Listener构造函数
public Listener() throws IOException {
      address = new InetSocketAddress(bindAddress, port);
      // Create a new server socket and set to non blocking mode
      acceptChannel = ServerSocketChannel.open();
      acceptChannel.configureBlocking(false);

      // Bind the server socket to the local host and port
      bind(acceptChannel.socket(), address, backlogLength);
      port = acceptChannel.socket().getLocalPort(); //Could be an ephemeral port
      // create a selector;
      selector= Selector.open();
      readers = new Reader[readThreads];
      readPool = Executors.newFixedThreadPool(readThreads);
      for (int i = 0; i < readThreads; i++) {
        Selector readSelector = Selector.open();
        Reader reader = new Reader(readSelector);
        readers[i] = reader;
        readPool.execute(reader);
      }

Listener.run()开启选择器循环,并处理Accept请求,如下:

//Listener运行函数
public void run() {
      LOG.info(getName() + ": starting");
      SERVER.set(Server.this);
      while (running) {
        SelectionKey key = null;
        try {
          selector.select();
          Iterator<SelectionKey> iter = selector.selectedKeys().iterator();
          while (iter.hasNext()) {
            key = iter.next();
            iter.remove();
            try {
              if (key.isValid()) {
                if (key.isAcceptable())
                  doAccept(key);
              }
            } catch (IOException e) {
            }
            key = null;
          }
        } catch (OutOfMemoryError e) {
          // we can run out of memory if we have too many threads
          // log the event and sleep for a minute and give
          // some thread(s) a chance to finish
          LOG.warn("Out of Memory in server select", e);
          closeCurrentConnection(key, e);
          cleanupConnections(true);
          try { Thread.sleep(60000); } catch (Exception ie) {}
        } catch (Exception e) {
          closeCurrentConnection(key, e);
        }
        cleanupConnections(false);
      }
      LOG.info("Stopping " + this.getName());

      synchronized (this) {
        try {
          acceptChannel.close();
          selector.close();
        } catch (IOException e) { }

        selector= null;
        acceptChannel= null;

        // clean up all connections
        while (!connectionList.isEmpty()) {
          closeConnection(connectionList.remove(0));
        }
      }
    }

doAccept()中通过server.accpet获取SocketChannel,并获取一个Reader对象,该对象包含一个Selector:readerSelector,通过reader.registerChannel,将SocketChannel注册到readerSelector下.并新建connection对象。

//Do_Accept
void doAccept(SelectionKey key) throws IOException,  OutOfMemoryError {
      Connection c = null;
      ServerSocketChannel server = (ServerSocketChannel) key.channel();
      SocketChannel channel;
      while ((channel = server.accept()) != null) {
        channel.configureBlocking(false);
        channel.socket().setTcpNoDelay(tcpNoDelay);
        Reader reader = getReader();
        try {
          reader.startAdd();
          SelectionKey readKey = reader.registerChannel(channel);
          c = new Connection(readKey, channel, System.currentTimeMillis());
          readKey.attach(c);
          synchronized (connectionList) {
            connectionList.add(numConnections, c);
            numConnections++;
          }
          if (LOG.isDebugEnabled())
            LOG.debug("Server connection from " + c.toString() +
                "; # active connections: " + numConnections +
                "; # queued calls: " + callQueue.size());
        } finally {
          reader.finishAdd();
        }

      }
    }

public synchronized SelectionKey registerChannel(SocketChannel channel)
                                                          throws IOException {
          return channel.register(readSelector, SelectionKey.OP_READ);
      }

Listener拥有一个ExecutorService线程池用来运行Reader对象，Reader对象采用轮叫调度（Round Robin Scheduling算法就是以轮叫的方式依次将请求调度不同的服务器），如下：

Reader getReader() {
      currentReader = (currentReader + 1) % readers.length;
      return readers[currentReader];
 }

Reader对象的startAdd方法和finishAdd方法之间来注册channel和新建conncetion。

以下是Reader对象的run方法，可以看出，当Reader对象调用startAdd方法后，线程从上一次的select阻塞中返回，并循环等待，直到finishAdd方法调用后，adding参数由true转false，跳出循环，再次进入select中。

public void run() {
        LOG.info("Starting SocketReader");
        synchronized (this) {
          while (running) {
            SelectionKey key = null;
            try {
              readSelector.select();
              while (adding) {
                this.wait(1000);
              }              

              Iterator<SelectionKey> iter = readSelector.selectedKeys().iterator();
              while (iter.hasNext()) {
                key = iter.next();
                iter.remove();
                if (key.isValid()) {
                  if (key.isReadable()) {
                    doRead(key);
                  }
                }
                key = null;
              }
            } catch (InterruptedException e) {
              if (running) {                      // unexpected -- log it
                LOG.info(getName() + " caught: " +
                         StringUtils.stringifyException(e));
              }
            } catch (IOException ex) {
              LOG.error("Error in Reader", ex);
            }
          }
        }
      }

当有新的数据到来时，调用Listener的doRead方法进行读取数据。

doRead方法主要调用了Server.Connection的readAndProcess方法来读取客户端发送过来的数据并处理。

readAndProcess方法，先读取IPC连接魔数和协议版本号分别在缓冲区dataLengthBuffer和versionBuffer读入。

if (dataLengthBuffer.remaining() > 0) {
          count = channelRead(channel, dataLengthBuffer); // dataLengthBuffer长度为4byte，第一次循环读取IPC连接魔数hrpc
            //非第一次循环读取数据data
          if (count < 0 || dataLengthBuffer.remaining() > 0)
            return count;
        }

        if (!rpcHeaderRead) {
          //Every connection is expected to send the header.
          if (rpcHeaderBuffer == null) {
            rpcHeaderBuffer = ByteBuffer.allocate(2);
          }
          count = channelRead(channel, rpcHeaderBuffer); //rpcHeaderBuffer长度为2byte，读取协议版本号和authMehod
          if (count < 0 || rpcHeaderBuffer.remaining() > 0) {
            return count;
          }
          int version = rpcHeaderBuffer.get(0);//协议版本号
          byte[] method = new byte[] {rpcHeaderBuffer.get(1)};//authMehod
          authMethod = AuthMethod.read(new DataInputStream(
              new ByteArrayInputStream(method)));
          dataLengthBuffer.flip();
          if (!HEADER.equals(dataLengthBuffer) || version != CURRENT_VERSION) {
            //Warning is ok since this is not supposed to happen.
            LOG.warn("Incorrect header or version mismatch from " +
                     hostAddress + ":" + remotePort +
                     " got version " + version +
                     " expected version " + CURRENT_VERSION);
            return -1;
          }
          dataLengthBuffer.clear();
          ……//sasl处理部分，可以忽略         

          rpcHeaderBuffer = null;
          rpcHeaderRead = true;
          continue;
        }

在接下类的数据处理部分，readAndProcess主要调用了saslReadAndProcess方法，进而调用processOneRpc方法，进而处理processHeader方法和processData方法。processHeader方法如下：

private void processHeader(byte[] buf) throws IOException {
      DataInputStream in =
        new DataInputStream(new ByteArrayInputStream(buf));
      header.readFields(in);
      try {
        String protocolClassName = header.getProtocol();
        if (protocolClassName != null) {
          protocol = getProtocolClass(header.getProtocol(), conf);
          // getProtocolClass只判断Server是否实现了ConnectionHeader中要求的IPC接口
          //如果实现，返回Class，否则抛异常。
        }
      } catch (ClassNotFoundException cnfe) {
        throw new IOException("Unknown protocol: " + header.getProtocol());
      }
      ......//获取用户信息
    }

数据分帧和读取

Tcp通信基于字节流，没有消息边界的概念，常用的分帧方法如下：

1. 定长消息：通信双方发送的消息长度为固定的，接收者只需要将数据读入到相应的缓冲区即可。

2. 基于界定符：消息结束由唯一标示（特殊字符序列）指出，这个特殊字符序列不能在传输的消息数据中出现，接收者简单扫描输入信息查找界定符。

3. 显式长度：具体消息前面附加固定大小的字段，用来指示消息包含多少字节。

IPC客户端发送请求采用“显式长度”方法，长度是int类型。如下是服务器端接收数据的相关代码：

if (data == null) {
          dataLengthBuffer.flip();
          dataLength = dataLengthBuffer.getInt();//读取数据长度
          //dataLengthBuffer长度为4byte，正好一个int长度。

          if (dataLength == Client.PING_CALL_ID) {//心跳消息
            if(!useWrap) { //covers the !useSasl too
              dataLengthBuffer.clear();
              return 0;  //ping message
            }
          }
          if (dataLength < 0) {
            LOG.warn("Unexpected data length " + dataLength + "!! from " +
                getHostAddress());
          }
          data = ByteBuffer.allocate(dataLength);
          //根据长度，为读取数据分配缓冲区。
        }

        count = channelRead(channel, data);

        if (data.remaining() == 0) {//读到一个完整的消息
          dataLengthBuffer.clear();
          data.flip();
          if (skipInitialSaslHandshake) {
            data = null;
            skipInitialSaslHandshake = false;
            continue;
          }
          boolean isHeaderRead = headerRead;
          if (useSasl) {
            saslReadAndProcess(data.array());
          } else {
            processOneRpc(data.array());//处理消息
          }
          data = null;
          if (!isHeaderRead) {
            continue;
          }
        }

而IPC服务器端到客户端用的是“定长消息”的变种，即利用前面介绍的序列化机制Writable发送响应。双方无需界定符和长度信息，其本质是一种“定长消息”。服务器端往客户端写数据的代码见Server.setupResponse，客户端读数据的代码在Connection.receiveResponse。