MySQL的server_uuid获取之uuid()函数和uuid_short()函数

1. mysql> select uuid(),uuid();
2. +--------------------------------------+--------------------------------------+
3. | uuid() | uuid() |
4. +--------------------------------------+--------------------------------------+
5. | 50120c25-d75c-11e7-9484-000c29c9278a | 50120c53-d75c-11e7-9484-000c29c9278a |
6. +--------------------------------------+--------------------------------------+
7. 1 row in set (0.00 sec)

1. String *Item_func_uuid::val_str(String *str)
2. {
3. ...
4. mac\rand get
5. ...
6. uint32 time_low= (uint32) (tv & 0xFFFFFFFF);
7. uint16 time_mid= (uint16) ((tv >> 32) & 0xFFFF);
8. uint16 time_hi_and_version= (uint16) ((tv >> 48) | UUID_VERSION);
9. s[8]=s[13]='-';
10. tohex(s, time_low, 8);
11. tohex(s+9, time_mid, 4);
12. tohex(s+14, time_hi_and_version, 4);
13. strmov(s+18, clock_seq_and_node_str);
14. ...
15. }

time_low、time_mid、time_high_and_version转成16进制后分别对应第1 2 3段。
这个时间是从1582-10-15 00:00:00.00到当前时间的100ns值。（实际上系统只能取到精确us，再乘以10）。所以你短时间连续执行的话，比较可能只有第一个值在改，实际上1 2 3都可能会改变。第4段是你启动这个MySQL后第一次执行select uuid()时的随机数，每次重启会改变。第5段是mac值转过来的，同一个机器多实例的一般相同。如果mac值获取不到，则是一个随机值。所以这个值可以认为是每次执行都不相同。并且不同实例之间也只有极微小概率重复。

2、uuid_short()函数

1. mysql> select uuid_short(),uuid_short();
2. +--------------------+--------------------+
3. | uuid_short() | uuid_short() |
4. +--------------------+--------------------+
5. | 818004335832072194 | 818004335832072195 |
6. +--------------------+--------------------+
7. 1 row in set (0.00 sec)

1. main->init_common_variables()->
2. server_start_time= flush_status_time= my_time(0);
3. //获取server启动时的时间戳

1. ulonglong uuid_value;
2. main->init_common_variables->item_init->
3. void uuid_short_init()
4. {
5. uuid_value= ((((ulonglong) server_id) << 56) +
6. (((ulonglong) server_start_time) << 24));
7. }

1. longlong Item_func_uuid_short::val_int()
2. {
3. ulonglong val;
4. mysql_mutex_lock(&LOCK_uuid_generator);
5. val= uuid_value++;
6. mysql_mutex_unlock(&LOCK_uuid_generator);
7. return (longlong) val;
8. }

1. main->init_server_auto_options：处理server_uuid
2. my_load_defaults(fname, groups, &argc, &argv, NULL)//fname:auto.cnf
3. handle_options(&argc, &argv, auto_options, mysqld_get_one_option);
4. if(uuid){
5. strcpy(server_uuid, uuid);
6. }else{
7. flush= TRUE;
8. generate_server_uuid();
9. }
10. if (flush)
11. DBUG_RETURN(flush_auto_options(fname));
12. //O_CREAT|O_RDWR形式open，即如果没有auto.cnf文件，
13. //则创建一个并将生成的server_uuid写入文件并flush

1. generate_server_uuid：
2. func_uuid= new (thd->mem_root) Item_func_uuid();
3. func_uuid->fixed= 1;
4. func_uuid->val_str(&uuid);
5. strncpy(server_uuid, uuid.c_ptr(), UUID_LENGTH);