On 1 September 2015 at 20:25, Thomas Munro <thomas.mu...@enterprisedb.com> wrote: > Hi > > Do you think it's reasonable to want to COMMIT a particular transaction on a > master node, and then immediately run a read-only query on a hot standby > node that is guaranteed to see that transaction? >
well, that is important to make load balancing completely safe (not returning old data when is important to get the latest). Having said that, i have never seen a case where the apply lag postgres has really matters or where the cause of the apply lag (I/O) doesn't get worst if we try to apply immediatly. Other solutions use a cache on top to apply in-memory at the cost of getting inconsistent in a failure. > A friend of mine who works with a different RDBMS technology that can do > that asked me how to achieve this with Postgres, and I suggested waiting for > the standby's pg_last_xlog_replay_location() to be >= the master's > pg_current_xlog_location() after COMMIT, which might involve some looping > and sleeping. > > As a quick weekend learning exercise/hack I recently went looking into how > we could support $SUBJECT. I discovered we already report the apply > progress back to the master, and the synchronous waiting facility seemed to > be all ready to support this. In fact it seemed a little too easy so > something tells me it must be wrong! But anyway, please see the attached > toy POC patch which does that. > i haven't seen the patch, but probably is as easy as you see it... IIRC, Simon proposed a patch for this a few years ago and this was actually contempleted from the beggining in the design of SR. I guess there were good reasons the patch didn't get applied, i found this thread and in this one Simon suggest is not the first time he submitted that option so it should be other threads too: http://www.postgresql.org/message-id/aanlktinxoymwowbsjxmnpjhjh_yan9vfmnmhnjdme...@mail.gmail.com -- Jaime Casanova www.2ndQuadrant.com Professional PostgreSQL: Soporte 24x7 y capacitación -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
