Re: Allow async standbys wait for sync replication (was: Disallow quorum uncommitted (with synchronous standbys) txns in logical replication subscribers)

On Mon, Feb 28, 2022 at 06:45:51PM +0530, Bharath Rupireddy wrote:
> On Sat, Feb 26, 2022 at 9:37 PM Nathan Bossart <nathandbossart@gmail.com> wrote:
>> For
>> this feature, I think we always need to consider what the primary considers
>> synchronously replicated.  My suggested approach doesn't change that.  I'm
>> saying that instead of spinning in a loop waiting for the WAL to be
>> synchronously replicated, we just immediately send WAL up to the LSN that
>> is presently known to be synchronously replicated.
> 
> As I said above, v1 patch does that i.e. async standbys wait until the
> sync standbys update their flush LSN.
> 
> Flush LSN is this - flushLSN = walsndctl->lsn[SYNC_REP_WAIT_FLUSH];
> which gets updated in SyncRepReleaseWaiters.
> 
> Async standbys with their SendRqstPtr will wait in XLogSendPhysical or
> XLogSendLogical until SendRqstPtr <= flushLSN.

My feedback is specifically about this behavior.  I don't think we should
spin in XLogSend*() waiting for an LSN to be synchronously replicated.  I
think we should just choose the SendRqstPtr based on what is currently
synchronously replicated.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com