Re: Lag clarification with Sync Replication

On Fri, 2020-05-22 at 12:48 -0400, Rui DeSousa wrote:
> > On May 22, 2020, at 12:36 AM, Raj kumar <rajkumar820999(at)gmail(dot)com> wrote:
> >
> > . But, ideally for sync replication, lag should be always 0 right.
>
> Incorrect. Synchronous replication means that a commit will not return until it has been safely written to disk on the primary and the replica. That means the transaction is written to WAL file on
> both primary and replica. On the primary, the transaction is also visible to transactions with a later xmin. On the replica, the transaction has been recorded in the WAL; it still needs to get
> applied to the database for it to become visible to read transactions.

If you set "synchronous_commit = remote_apply", the commit will only return
when the change has been replayed on the synchronous standby server.

Yours,
Laurenz Albe
--
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