什麼是 MDL？（例如“MDL 語義”或“MDL 鎖”）可能與 Galera 相關

我發現一個人在 Galera/XtraDB 錯誤報告中談論MDL 語義。

什麼是 MDL？術語 MDL 也在 Galera 日誌MDL conflict中。請詳細說明。

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:

它代表“元數據鎖定”查看 MySQL https://dev.mysql.com/doc/dev/mysql-server/8.0.0/mdl_8h.html的來源

這是一種通過避免並發訪問相同數據來保持 ACID 數據庫的方法。這個想法是，當事務中的查詢使用該表時，表的元數據被“鎖定”。這樣做，將不允許另一個事務對這些鎖定的表執行 DDL 查詢。

例如，如果您在一個客戶端應用程序中執行START TRANSACTION然後SELECT * FROM t，則表的元數據將t被鎖定。沒有其他客戶端能夠執行此操作，ALTER TABLE t ...因為它會在第一個客戶端事務仍然打開時弄亂表結構。

MySQL 文件中的更多資訊https://dev.mysql.com/doc/refman/5.5/en/metadata-locking.html

引用自：https://dba.stackexchange.com/questions/210839