Re: Disallow quorum uncommitted (with synchronous standbys) txns in logical replication subscribers

On Thu, Jan 6, 2022 at 11:24 PM Jeff Davis <pgsql(at)j-davis(dot)com> wrote:

> On Wed, 2022-01-05 at 23:59 -0800, SATYANARAYANA NARLAPURAM wrote:
> > I would like to propose a GUC send_Wal_after_quorum_committed which
> > when set to ON, walsenders corresponds to async standbys and logical
> > replication workers wait until the LSN is quorum committed on the
> > primary before sending it to the standby. This not only simplifies
> > the post failover steps but avoids unnecessary downtime for the async
> > replicas. Thoughts?
>
> Do we need a GUC? Or should we just always require that sync rep is
> satisfied before sending to async replicas?
>

I proposed a GUC to not introduce a behavior change by default. I have no
strong opinion on having a GUC or making the proposed behavior default,
would love to get others' perspectives as well.

>
> It feels like the sync quorum should always be ahead of the async
> replicas. Unless I'm missing a use case, or there is some kind of
> performance gotcha.
>

I couldn't think of a case that can cause serious performance issues but
will run some experiments on this and post the numbers.

>
> Regards,
> Jeff Davis
>
>
>