MySQL auto.cnf 文件

1. auto.cnf文件
MySQL数据目录中通常存在一个名为auto.cnf文件,存储了server-uuid的值,如下所示:

[auto]
server-uuid=f2d0efd6-6ab7-11e8-8fdd-fa163eda7360

MySQL启动时,会自动从data_dir/auto.cnf 文件中获取server-uuid值,并将这个值存储在全局变量server_uuid中。如果这个值或者这个文件不存在,那么将会生成一个新的uuid值,并将这个值保存在auto.cnf文件中。server-uuid与server-id一样,用于标识MySQL实例在集群中的唯一性,这两个参数在主从复制中具有重要作用,默认情况下,如果主、从库的server-uuid或者server-id的值一样,将会导致主从复制报错中断。

在主库中执行show slave hosts; 能够看到主、从库的server-id和从库的server-uuid,如下所示:

mysql> show slave hosts;
+-----------+------+------+-----------+--------------------------------------+
| Server_id | Host | Port | Master_id | Slave_UUID                           |
+-----------+------+------+-----------+--------------------------------------+
| 170238777 |      | 3306 | 170238776 | 5a0280d4-6404-11e8-840e-fa163eab3dcf |
| 170238778 |      | 3306 | 170238776 | 4db07c8a-6f01-11e8-bef6-fa163e767b9a |
+-----------+------+------+-----------+--------------------------------------+
2 rows in set (0.00 sec)

在从库中执行show slave status\G,能够看到主库的server-id和server-uuid,如下所示:

                           ......
             Master_Server_Id: 170238776
                  Master_UUID: f2d0efd6-6ab7-11e8-8fdd-fa163eda7360
             Master_Info_File: mysql.slave_master_info
                    SQL_Delay: 0
          SQL_Remaining_Delay: NULL
      Slave_SQL_Running_State:
           Master_Retry_Count: 86400
                           ......

2. 主、从库server-uuid值一样,导致主从复制异常中断

MySQL在初始化的时候,会去数据目录里的auto.cnf文件读取server-uuid参数,如果没有auto.cnf文件或者没有读到server-uuid参数,通过执行generate_server_uuid 函数生成一个新的uuid值,并通过flush_auto_options函数将server-uuid值写入到auto.cnf文件中。具体的函数调用及源码文件如下:

main                       //main.cc
mysqld_main                //mysqld.cc
init_server_auto_options   //mysqld.cc
generate_server_uuid       //mysqld.cc
flush_auto_options         //mysqld.cc

主从复制时,如果从库发现它所复制的主库的uuid 与 它自己的uuid一样,默认情况下从库的I/O线程会直接报错退出。MySQL提供了额外参数–replicate-same-server-id,用于改变这一默认行为。如果设置replicate-same-server-id为1,即使主从库uuid一样,也能正常复制,但是会产生意想不到的结果,实际使用中也很少会这么使用。相关代码实现位于rpl_slave.cc文件的get_master_uuid函数中,具体源码实现如下:

main                //main.cc
mysqld_main         //mysqld.cc
mysqld_socket_acceptor->connection_event_loop    //mysqld.cc
Connection_handler_manager::process_new_connection(Channel_info* channel_info)  //connection_handler_manager.cc
bool Per_thread_connection_handler::add_connection(Channel_info* channel_info)  //connection_handler_per_thread.cc
handle_connection        //connection_handler_per_thread.cc
do_command               //sql_parse.cc
dispatch_command         //sql_parse.cc
mysql_parse              //sql_parse.cc
mysql_execute_command    //sql_parse.cc
start_slave_cmd          //rpl_slave.cc
start_slave              //rpl_slave.cc
start_slave_threads      //rpl_slave.cc
handle_slave_io          //rpl_slave.cc
get_master_uuid          //rpl_slave.cc
//rpl_slave.cc
//static int get_master_uuid(MYSQL *mysql, Master_info *mi)

char query_buf[]= "SELECT @@GLOBAL.SERVER_UUID";
...
if (!mysql_real_query(mysql, STRING_WITH_LEN(query_buf)) &&
      (master_res= mysql_store_result(mysql)) &&
      (master_row= mysql_fetch_row(master_res)))
  {
    if (!strcmp(::server_uuid, master_row[0]) &&
        !mi->rli->replicate_same_server_id)
    {
      errmsg= "The slave I/O thread stops because master and slave have  equal "
              "MySQL server UUIDs; these UUIDs must be different for "
              "replication to work.";
      mi->report(ERROR_LEVEL, ER_SLAVE_FATAL_ERROR,  ER(ER_SLAVE_FATAL_ERROR),
                 errmsg);
      // Fatal error
      ret= 1;
    }

3. 主、从库server-id值一样,同样导致主从复制异常中断

如果主从复制时,从库发现它所复制的主库的server_id 与 它自己的server_id一样,默认情况下从库的I/O线程也会直接报错退出。通过设置–replicate-same-server-id 参数为1,可改变这一默认行为。代码实现如下:

下面是执行start slave;命令时的函数调用关系。

main                  //main.cc
mysqld_main           //mysqld.cc
mysqld_socket_acceptor->connection_event_loop    //mysqld.cc
Connection_handler_manager::process_new_connection(Channel_info* channel_info)    //connection_handler_manager.cc
bool Per_thread_connection_handler::add_connection(Channel_info* channel_info)    //connection_handler_per_thread.cc
handle_connection                   //connection_handler_per_thread.cc
do_command                          //sql_parse.cc
dispatch_command                    //sql_parse.cc
mysql_parse                         //sql_parse.cc
mysql_execute_command               //sql_parse.cc
start_slave_cmd                     //rpl_slave.cc
start_slave                         //rpl_slave.cc
start_slave_threads                 //rpl_slave.cc
handle_slave_io                     //rpl_slave.cc
get_master_version_and_clock        //rpl_slave.cc

下面是MySQL从库启动时,如果skip-slave-start为0,从库自动启动复制线程的函数调用关系。

main                                  //main.cc
mysqld_main                     //mysqld.cc
init_slave                            //rpl_slave.cc
start_slave_threads            //rpl_slave.cc
handle_slave_io                 //rpl_slave.cc
get_master_version_and_clock    //rpl_slave.cc

最终都是通过调用get_master_version_and_clock函数,来判断主、从库server-id是否相同。

// rpl_slave.cc
// static int get_master_version_and_clock(MYSQL* mysql, Master_info* mi)

if (!mysql_real_query(mysql, STRING_WITH_LEN("SELECT @@GLOBAL.SERVER_ID")) &&
      (master_res= mysql_store_result(mysql)) &&
      (master_row= mysql_fetch_row(master_res)))
  {
    if ((::server_id == (mi->master_id= strtoul(master_row[0], 0, 10))) &&
        !mi->rli->replicate_same_server_id)
    {
      errmsg= "The slave I/O thread stops because master and slave have equal \
MySQL server ids; these ids must be different for replication to work (or \
the --replicate-same-server-id option must be used on slave but this does \
not always make sense; please check the manual before using it).";
      err_code= ER_SLAVE_FATAL_ERROR;
      sprintf(err_buff, ER(err_code), errmsg);
      goto err;
    }
  }

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