[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;
}
}