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对于后端连接，我们只关心MySQL的。 从后端连接工厂开始MySQLConnectionFactory.java：
public MySQLConnection make(MySQLDataSource pool, ResponseHandler handler, String schema) throws IOException { //DBHost配置 DBHostConfig dsc = pool.getConfig(); //根据是否为NIO返回SocketChannel或者AIO的AsynchronousSocketChannel NetworkChannel channel = openSocketChannel(MycatServer.getInstance() .isAIO()); //新建MySQLConnection MySQLConnection c = new MySQLConnection(channel, pool.isReadNode()); //根据配置初始化MySQLConnection MycatServer.getInstance().getConfig().setSocketParams(c, false); c.setHost(dsc.getIp()); c.setPort(dsc.getPort()); c.setUser(dsc.getUser()); c.setPassword(dsc.getPassword()); c.setSchema(schema); //目前实际连接还未建立，handler为MySQL连接认证MySQLConnectionAuthenticator c.setHandler(new MySQLConnectionAuthenticator(c, handler)); c.setPool(pool); c.setIdleTimeout(pool.getConfig().getIdleTimeout()); //AIO和NIO连接方式建立实际的MySQL连接 if (channel instanceof AsynchronousSocketChannel) { ((AsynchronousSocketChannel) channel).connect( new InetSocketAddress(dsc.getIp(), dsc.getPort()), c, (CompletionHandler) MycatServer.getInstance() .getConnector()); } else { //通过NIOConnector建立连接 ((NIOConnector) MycatServer.getInstance().getConnector()) .postConnect(c); } return c; }通过NIOConnector建立实际连接的过程与前端连接的建立相似，也是先放在队列中，之后由NIOConnector去建立连接。
public void postConnect(AbstractConnection c) { connectQueue.offer(c); selector.wakeup();}public void run() { final Selector tSelector = this.selector; for (;;) { ++connectCount; try { //查看有无连接就绪 tSelector.select(1000L); connect(tSelector); Set<SelectionKey> keys = tSelector.selectedKeys(); try { for (SelectionKey key : keys) { Object att = key.attachment(); if (att != null && key.isValid() && key.isConnectable()) { finishConnect(key, att); } else { key.cancel(); } } } finally { keys.clear(); } } catch (Exception e) { LOGGER.warn(name, e); } }}private void connect(Selector selector) { AbstractConnection c = null; while ((c = connectQueue.poll()) != null) { try { SocketChannel channel = (SocketChannel) c.getChannel(); //注册OP_CONNECT监听与后端连接是否真正建立 channel.register(selector, SelectionKey.OP_CONNECT, c); //主动连接 channel.connect(new InetSocketAddress(c.host, c.port)); } catch (Exception e) { c.close(e.toString()); } }}private void finishConnect(SelectionKey key, Object att) { BackendAIOConnection c = (BackendAIOConnection) att; try { if (finishConnect(c, (SocketChannel) c.channel)) { //做原生NIO连接是否完成的判断和操作 clearSelectionKey(key); c.setId(ID_GENERATOR.getId()); //绑定特定的NIOProcessor以作idle清理 NIOProcessor processor = MycatServer.getInstance() .nextProcessor(); c.setProcessor(processor); //与特定NIOReactor绑定监听读写 NIOReactor reactor = reactorPool.getNextReactor(); reactor.postRegister(c); } } catch (Exception e) { //如有异常，将key清空 clearSelectionKey(key); c.close(e.toString()); c.onConnectFailed(e); }}private boolean finishConnect(AbstractConnection c, SocketChannel channel) throws IOException { if (channel.isConnectionPending()) { channel.finishConnect(); c.setLocalPort(channel.socket().getLocalPort()); return true; } else { return false; }}private void clearSelectionKey(SelectionKey key) { if (key.isValid()) { key.attach(null); key.cancel(); }}绑定到具体的NIOReactor之后，监听读事件。和之前讲的前端连接建立过程类似。这次是后端MySQL主动发握手包。这时，读事件就绪，NIOReactor中的RW线程会调用对应AbstractConnection（这里是MySQLConnection）的handler的处理方法处理。这里MySQLConnection中的handler参考工厂方法发现是MySQLConnectionAuthenticator。查看handle方法：
/** * MySQL 4.1版本之前是MySQL323加密,MySQL 4.1和之后的版本都是MySQLSHA1加密，在MySQL5.5的版本之后可以客户端插件式加密(这个MyCat实现) * @see @http://dev.mysql.com/doc/internals/en/determining-authentication-method.html */@Overridepublic void handle(byte[] data) { try { switch (data[4]) { //如果是OkPacket，检查是否认证成功 case OkPacket.FIELD_COUNT: HandshakePacket packet = source.getHandshake(); if (packet == null) { //如果为null，证明链接第一次建立，处理 processHandShakePacket(data); // 发送认证数据包 source.authenticate(); break; } // 如果packet不为null，处理认证结果 //首先将连接设为已验证并将handler改为MySQLConnectionHandler source.setHandler(new MySQLConnectionHandler(source)); source.setAuthenticated(true); //判断是否用了压缩协议 boolean clientCompress = Capabilities.CLIENT_COMPRESS==(Capabilities.CLIENT_COMPRESS & packet.serverCapabilities); boolean usingCompress= MycatServer.getInstance().getConfig().getSystem().getUseCompression()==1 ; if(clientCompress&&usingCompress) { source.setSupportCompress(true); } //设置ResponseHandler if (listener != null) { listener.connectionAcquired(source); } break; //如果为ErrorPacket，则认证失败 case ErrorPacket.FIELD_COUNT: ErrorPacket err = new ErrorPacket(); err.read(data); String errMsg = new String(err.message); LOGGER.warn("can‘t connect to mysql server ,errmsg:"+errMsg+" "+source); //source.close(errMsg); throw new ConnectionException(err.errno, errMsg); //如果是EOFPacket，则为MySQL 4.1版本，是MySQL323加密 case EOFPacket.FIELD_COUNT: auth323(data[3]); break; default: packet = source.getHandshake(); if (packet == null) { processHandShakePacket(data); // 发送认证数据包 source.authenticate(); break; } else { throw new RuntimeException("Unknown Packet!"); } } } catch (RuntimeException e) { if (listener != null) { listener.connectionError(e, source); return; } throw e; }}在连接建立并认证后，MySQLConnectionHandler来处理这个连接的请求和相应。 MySQL服务端响应客户端查询请求的流程如下：可以分为三个阶段： （第一阶段）客户端发送查询请求包COM_QUERY （command query packet），如果有结果集返回，且结果集不为空，则返回FieldCount（列数量）包；如果结果集为空，则返回OKPacket；如果命令有错，则返回ERRPacket；如果是Load file data命令，则返回LOCAL_INFILE_Request。 （第二阶段）如果有结果集返回，则先返回列集合，所有列返回完了之后，会返回EOFPacket；如果过程中出现错误，则返回错误包。 （第三阶段）之后返回行记录，返回全部行记录之后，返回EOFPacket。如果有错误，回错误包。 MyCat实现源代码如下：
protected void handleData(byte[] data) { switch (resultStatus) { //第一阶段 case RESULT_STATUS_INIT: switch (data[4]) { //返回OKPacket case OkPacket.FIELD_COUNT: handleOkPacket(data); break; //返回错误包 case ErrorPacket.FIELD_COUNT: handleErrorPacket(data); break; //返回Load Data进一步操作 case RequestFilePacket.FIELD_COUNT: handleRequestPacket(data); break; //返回结果集列数量 default: //记录列数量并进入第二阶段 resultStatus = RESULT_STATUS_HEADER; header = data; fields = new ArrayList<byte[]>((int) ByteUtil.readLength(data, 4)); } break; //第二阶段 case RESULT_STATUS_HEADER: switch (data[4]) { //返回错误包 case ErrorPacket.FIELD_COUNT: resultStatus = RESULT_STATUS_INIT; handleErrorPacket(data); break; //返回EOF，证明列集合返回完毕，进入第三阶段 case EOFPacket.FIELD_COUNT: resultStatus = RESULT_STATUS_FIELD_EOF; handleFieldEofPacket(data); break; //返回的是列集合，记录 default: fields.add(data); } break; //第三阶段 case RESULT_STATUS_FIELD_EOF: switch (data[4]) { //返回错误包 case ErrorPacket.FIELD_COUNT: resultStatus = RESULT_STATUS_INIT; handleErrorPacket(data); break; //返回EOF，证明结果集返回完毕，回到第一阶段等待下一个请求的响应 case EOFPacket.FIELD_COUNT: resultStatus = RESULT_STATUS_INIT; handleRowEofPacket(data); break; //返回结果集包 default: handleRowPacket(data); } break; default: throw new RuntimeException("unknown status!"); }}更多网易技术、产品、运营经验分享请点击。
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