wikiadmin@deployment-db03[deploymentwiki]> SELECT @@global.read_only; +--------------------+ | @@global.read_only | +--------------------+ | 0 | +--------------------+ 1 row in set (0.00 sec) wikiadmin@deployment-db04[deploymentwiki]> SELECT @@global.read_only; +--------------------+ | @@global.read_only | +--------------------+ | 0 | +--------------------+ 1 row in set (0.00 sec)
deployment-db04 is the replica, with deployment-db03 as its master, but it's not read-only. Because of a bug in MediaWiki (see T183242: DB handles obtained with DB_REPLICA should not allow writes), writes were being done directly to the replica, which caused inconsistencies and broke replication.
It should not have been possible for this replica server to be writable. Out of paranoia, I checked in production, and none of the replica DBs in production are writable.