Re: Synchronizing slots from primary to standby

On Sun, Oct 31, 2021 at 7:08 PM Peter Eisentraut
<peter(dot)eisentraut(at)enterprisedb(dot)com> wrote:
>
> I want to reactivate $subject. I took Petr Jelinek's patch from [0],
> rebased it, added a bit of testing. It basically works, but as
> mentioned in [0], there are various issues to work out.

Thank you for working on this feature!

>
> The idea is that the standby runs a background worker to periodically
> fetch replication slot information from the primary. On failover, a
> logical subscriber would then ideally find up-to-date replication slots
> on the new publisher and can just continue normally.
>
> I have also found that this breaks some seemingly unrelated tests in
> the recovery test suite. I have disabled these here. I'm not sure if
> the patch actually breaks anything or if these are just differences in
> timing or implementation dependencies.

I haven’t looked at the patch deeply but regarding 007_sync_rep.pl,
the tests seem to fail since the tests rely on the order of the wal
sender array on the shared memory. Since a background worker for
synchronizing replication slots periodically connects to the walsender
on the primary and disconnects, it breaks the assumption of the order.
Regarding 010_logical_decoding_timelines.pl, I guess that the patch
breaks the test because the background worker for synchronizing slots
on the replica periodically advances the replica's slot. I think we
need to have a way to disable the slot synchronization or to specify
the slot name to sync with the primary. I'm not sure we already
discussed this topic but I think we need it at least for testing
purposes.

Regards,

--
Masahiko Sawada
EDB: https://www.enterprisedb.com/