什么是MDL？(例如“MDL语义”或“MDL锁”)(可能与Galera相关

Question

我在Galera/XtraDB报告中发现了一个正在谈论MDL语义的家伙。

什么是MDL？MDL这个术语在Galera中也记录了MDL conflict。请详细解释。

2018-06-14 17:07:10 140112699321088 [Note] WSREP: cluster conflict due to certification failure for threads:
2018-06-14 17:07:10 140112699321088 [Note] WSREP: Victim thread:
2018-06-14 17:07:10 140112690834176 [Note] WSREP: cluster conflict due to high priority abort for threads:
2018-06-14 17:07:10 140112690834176 [Note] WSREP: Winning thread:
2018-06-14 17:07:10 140112690834176 [Note] WSREP: Victim thread:
2018-06-14 17:07:10 140112690834176 [Note] WSREP: MDL conflict db=APC_MYSQLMON_DB table=TABLE1 ticket=4 solved by abort
2018-06-14 17:07:10 140112695683840 [Note] WSREP: MDL conflict db=APC_MYSQLMON_DB table=TABLE1 ticket=8 solved by abort
2018-06-14 17:07:10 140112695683840 [Note] WSREP: cluster conflict due to certification failure for threads:
2018-06-14 17:07:10 140112695683840 [Note] WSREP: Victim thread:
2018-06-14 17:26:47 140112698108672 [Note] WSREP: cluster conflict due to certification failure for threads:
2018-06-14 17:26:47 140112698108672 [Note] WSREP: Victim thread:


2018-06-14 17:34:48 140087340817152 [Note] WSREP: cluster conflict due to certification failure for threads:
2018-06-14 17:34:48 140087340817152 [Note] WSREP: Victim thread:
2018-06-14 17:36:48 140076554697472 [Note] WSREP: cluster conflict due to high priority abort for threads:
2018-06-14 17:36:48 140076554697472 [Note] WSREP: Winning thread:
2018-06-14 17:36:48 140076554697472 [Note] WSREP: Victim thread:
2018-06-14 17:36:48 140076554697472 [Note] WSREP: MDL conflict db=APC_MYSQLMON_DB table=TABLE1 ticket=4 solved by abort
2018-06-14 17:36:48 140087340817152 [Note] WSREP: MDL conflict db=APC_MYSQLMON_DB table=TABLE1 ticket=8 solved by abort
2018-06-14 17:36:48 140087340817152 [Note] WSREP: cluster conflict due to certification failure for threads:
2018-06-14 17:36:48 140087340817152 [Note] WSREP: Victim thread:


2018-06-14 17:37:50 139917573339904 [Note] WSREP: cluster conflict due to certification failure for threads:
2018-06-14 17:37:50 139917573339904 [Note] WSREP: Victim thread:

2018-06-14 17:05:12 139927950939904 [Warning] WSREP: Failed to report last committed 1941199495, -4 (Interrupted system call)
2018-06-14 17:09:41 139900840323840 [Note] WSREP: cluster conflict due to high priority abort for threads:
2018-06-14 17:09:41 139900840323840 [Note] WSREP: Winning thread:
2018-06-14 17:09:41 139900840323840 [Note] WSREP: Victim thread:

Answer 1

它代表"MetaData锁定“，查看MySQL https://dev.mysql.com/doc/dev/mysql-server/8.0.0/mdl_8h.html的来源。

这是一种通过避免并发访问相同数据来保持ACID数据库的方法。其思想是，当事务中的查询使用此表时，表的元数据将被“锁定”。这样做，将不允许另一个事务对这些锁定的表执行DDL查询。

例如，如果您在一个客户端应用程序中执行START TRANSACTION然后执行SELECT * FROM t，那么表t的元数据将被锁定。没有其他客户端能够执行ALTER TABLE t ...，因为它会在第一个客户端事务仍然打开时破坏表结构。

MySQL doc https://dev.mysql.com/doc/refman/5.5/en/metadata-locking.html中有更多信息