One of the reasons for last years Rmgr plugin patch was the idea of a signalling interface between Primary and Standby nodes. Such a feature would help in producing further programmatic tests for Hot Standby.
Proposal is to use up the last rmgr id, slot 7: RM_INFO_ID. Functions would be provided on primary side to send a user-defined binary message into WAL stream, with a single message type XLOG_INFO_MSG. (Superuser only, though can be wrapped and given to trusted users). On Standby side, we would provide a plugin interface to allow the message to be read and handled by user-written code. This would then allow a simple value or notification to be passed to a waiting (polling) connection on standby. If new LISTEN/NOTIFY gets into 8.5 then this would be an obvious fit, but I have no need for message passing, just info passing, so no need to wait for that; something much simpler. We could do this by adding stuff into Streaming Rep patch, but seems best to provide a generic interface that lives in the WAL stream itself, rather than being at a meta level and divorced from the WAL. This would allow the writing of a test suite that can create certain conditions on the primary, then send notification of the event to the standby. Standby would then halt recovery in a precise state while a user runs an appropriate test. -- Simon Riggs www.2ndQuadrant.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers