"Not necessarily. There has been discussion of adding a new mode which will delay the commit on the primary until it is visible on a synchronous standby, but I don't recall where that left off. " Joshua: THis essentially contradicts your statement to me. On Wed, Jul 29, 2015 at 5:10 PM, Kevin Grittner <kgrittn@xxxxxxxxx> wrote: > Ravi Krishna <sravikrishna3@xxxxxxxxx> wrote: > >> As per this: >> >> http://www.postgresql.org/docs/current/static/warm-standby.html#SYNCHRONOUS-REPLICATION >> >> "When requesting synchronous replication, each commit of a write >> transaction will wait until confirmation is received that the commit >> has been written to the transaction log on disk of both the primary >> and standby server." >> >> Does it mean that, on the standby, when PG writes the transaction log >> on the disk, it also updates the data buffers to make the transaction >> visible for all sessions. > > No, it means that if the primary is hit by a meteor and you promote > the standby, the data will not have been lost. The time between > the successful return of the commit on the primary and the time at > which the change becomes visible on the standby is normally quite > small; you may have trouble running into a case where you notice > it, but it can happen. > >> Eg: >> On the primary >> A big transaction committed >> Now if I issue a select on the primary looking for the transaction I >> committed above, I will get what I want. >> Will I get the same result if instead of primary I issue the select on >> the standby. > > Not necessarily. There has been discussion of adding a new mode > which will delay the commit on the primary until it is visible on a > synchronous standby, but I don't recall where that left off. One > of the issues is that with the current guarantee you need multiple > replicas to prevent a failure of a standby from stalling the > primary indefinitely, and you don't have an easy way to know > *which* replica succeeded in persisting the transaction without > doing a lot of work. > > -- > Kevin Grittner > EDB: http://www.enterprisedb.com > The Enterprise PostgreSQL Company -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
