数据库路由中间件MyCat - 源代码篇（4）

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2. 前端连接建立与认证

Title:MySql连接建立以及认证过程client->MySql:1.TCP连接请求 
MySql->client:2.接受TCP连接client->MySql:3.TCP连接建立MySql->client:4.握手包HandshakePacketclient->MySql:5.认证包AuthPacketMySql->client:6.如果验证成功，则返回OkPacketclient->MySql:7.默认会发送查询版本信息的包MySql->client:8.返回结果包

2.5 (7~8) 默认会发送查询版本信息的包,返回结果包

MySql客户端在连接建立后，默认会发送查询版本信息的包，这其实就是一个SQL查询请求了。只不过这个请求不用路由到后台某个数据库^_^。连接成功建立后，连接绑定的RW线程会监听上面的读事件。在客户端发送查询版本信息的包之后，会触发RW线程去读取对应连接，过程与之前接收AuthPacket类似： RW类代码片段

//监听到有效读if (key.isValid() && key.isReadable()) {                                 try {                                     //异步读取数据并处理数据
                                     con.asynRead();
                                 } catch (IOException e) {
                                     con.close("program err:" + e.toString());                                     continue;
                                 } catch (Exception e) {
                                     LOGGER.debug("caught err:", e);
                                     con.close("program err:" + e.toString());                                     continue;
                                 }
                             }

之后的读取过程也是调用AbstractConnection的asynRead()方法，进行异步读取。过程就不再赘述，读取到的数据交由FrontendCommandHandler处理。查询版本信息的包（是一种CommandPacket）内容： CommandPacket：

packet length (3)
packet number (1)
command (1)
statement (null terminated string)

FrontendCommandHandler的处理方法：

@Override
    public void handle(byte[] data)
    {        if(source.getLoadDataInfileHandler()!=null&&source.getLoadDataInfileHandler().isStartLoadData())
        {
            MySQLMessage mm = new MySQLMessage(data);            int  packetLength = mm.readUB3();            if(packetLength+4==data.length)
            {
                source.loadDataInfileData(data);
            }            return;
        }        switch (data[4])
        {            case MySQLPacket.COM_INIT_DB:
                commands.doInitDB();
                source.initDB(data);                break;            case MySQLPacket.COM_QUERY:
                commands.doQuery();
                source.query(data);                break;            case MySQLPacket.COM_PING:
                commands.doPing();
                source.ping();                break;            case MySQLPacket.COM_QUIT:
                commands.doQuit();
                source.close("quit cmd");                break;            case MySQLPacket.COM_PROCESS_KILL:
                commands.doKill();
                source.kill(data);                break;            case MySQLPacket.COM_STMT_PREPARE:
                commands.doStmtPrepare();
                source.stmtPrepare(data);                break;            case MySQLPacket.COM_STMT_EXECUTE:
                commands.doStmtExecute();
                source.stmtExecute(data);                break;            case MySQLPacket.COM_STMT_CLOSE:
                commands.doStmtClose();
                source.stmtClose(data);                break;            case MySQLPacket.COM_HEARTBEAT:
                commands.doHeartbeat();
                source.heartbeat(data);                break;            default:
                     commands.doOther();
                     source.writeErrMessage(ErrorCode.ER_UNKNOWN_COM_ERROR,                             "Unknown command");
 
        }
    }

根据CommandPacket的第五字节判断command类型，不同类型有不同的处理。首先querycommand计数加1，之后调用对应FrontendConnection的query(byte[])方法：

public void query(byte[] data) {        if (queryHandler != null) {            // 取得语句|get sql
            MySQLMessage mm = new MySQLMessage(data);            //从第六字节开始读取|read from the 6th byte
            mm.position(5);
            String sql = null;            try {
                sql = mm.readString(charset);
            } catch (UnsupportedEncodingException e) {
                writeErrMessage(ErrorCode.ER_UNKNOWN_CHARACTER_SET, "Unknown charset '" + charset + "'");                return;
            }            if (sql == null || sql.length() == 0) {
                writeErrMessage(ErrorCode.ER_NOT_ALLOWED_COMMAND, "Empty SQL");                return;
            }            // sql = StringUtil.replace(sql, "`", "");
 
            // 移除末尾';'|remove last ';'
            if (sql.endsWith(";")) {
                sql = sql.substring(0, sql.length() - 1);
            }            // 记录SQL|record SQL
            this.setExecuteSql(sql);            // 执行查询
            queryHandler.setReadOnly(privileges.isReadOnly(user));
            queryHandler.query(sql);
        } else {
            writeErrMessage(ErrorCode.ER_UNKNOWN_COM_ERROR, "Query unsupported!");
        }
    }

执行查询，调用对应的FrontendQueryHandler：这里，很明显，是ServerQueryHandler。

public void query(String sql) {
 
        ServerConnection c = this.source;        if (LOGGER.isDebugEnabled()) {
            LOGGER.debug(new StringBuilder().append(c).append(sql).toString());
        }        //
        int rs = ServerParse.parse(sql);        int sqlType = rs & 0xff;        switch (sqlType) {        case ServerParse.EXPLAIN:
            ExplainHandler.handle(sql, c, rs >>> 8);            break;        case ServerParse.EXPLAIN2:
            Explain2Handler.handle(sql, c, rs >>> 8);            break;        case ServerParse.SET:
            SetHandler.handle(sql, c, rs >>> 8);            break;        case ServerParse.SHOW:
            ShowHandler.handle(sql, c, rs >>> 8);            break;        case ServerParse.SELECT:            if(QuarantineHandler.handle(sql, c)){
                SelectHandler.handle(sql, c, rs >>> 8);
            }            break;        case ServerParse.START:
            StartHandler.handle(sql, c, rs >>> 8);            break;        case ServerParse.BEGIN:
            BeginHandler.handle(sql, c);            break;        case ServerParse.SAVEPOINT:
            SavepointHandler.handle(sql, c);            break;        case ServerParse.KILL:
            KillHandler.handle(sql, rs >>> 8, c);            break;        case ServerParse.KILL_QUERY:
            LOGGER.warn(new StringBuilder().append("Unsupported command:").append(sql).toString());
            c.writeErrMessage(ErrorCode.ER_UNKNOWN_COM_ERROR,"Unsupported command");            break;        case ServerParse.USE:
            UseHandler.handle(sql, c, rs >>> 8);            break;        case ServerParse.COMMIT:
            c.commit();            break;        case ServerParse.ROLLBACK:
            c.rollback();            break;        case ServerParse.HELP:
            LOGGER.warn(new StringBuilder().append("Unsupported command:").append(sql).toString());
            c.writeErrMessage(ErrorCode.ER_SYNTAX_ERROR, "Unsupported command");            break;        case ServerParse.MYSQL_CMD_COMMENT:
            c.write(c.writeToBuffer(OkPacket.OK, c.allocate()));            break;        case ServerParse.MYSQL_COMMENT:
            c.write(c.writeToBuffer(OkPacket.OK, c.allocate()));            break;            case ServerParse.LOAD_DATA_INFILE_SQL:
                c.loadDataInfileStart(sql);                break;        default:            if(readOnly){
                LOGGER.warn(new StringBuilder().append("User readonly:").append(sql).toString());
                c.writeErrMessage(ErrorCode.ER_USER_READ_ONLY, "User readonly");                break;
            }            if(QuarantineHandler.handle(sql, c)){
                c.execute(sql, rs & 0xff);
            }
        }
    }

针对每种command，都有不同的handler和处理方式。之后如何处理，就在之后的SQL解析器等章节进行分析。

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