Re: Synchronizing slots from primary to standby

On Wed, Aug 30, 2023 at 9:29 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
>
> PFA new patch-set which attempts to fix these:
>

PFA v15 which implements below changes:

1) It parses synchronize_slot_names and standby_slot_names and caches
the list to avoid repeated parsing. This parsing is done at Walsender
startup on primary and slot-sync worker startup on standby and then
during each SIGHUP.

2) Handles slots invaliation:
2.1) If the slot is invalidated on primary, it is now invalidated on
standby as well. Standby gets invalidation info from primary using a
new system function 'pg_get_invalidation_cause(slotname)'.
2.2) if the slot is invalidated on standby alone, it is dropped and
recreated as per synchronize_slot_names in next sync-cycle.

3) The test file 051_slot_sync.pl is removed from patch2 for the
time-being. It was testing whether the logical slot on standby is
conflicted or not once slot on primary is removed by 'Drop
Subscription' and WALs needed by logical slot on standby are flushed
on primary (with hot_standby_feedback=off). But as per current
implementation, we drop the slot on standby as soon as subscription is
dropped on primary. So the testcase no longer solves the purpose for
which it was added. Correct set of test cases will be added going
forward.

4) Address most of the comments by Peter.

Change 1 is in patch01 along with patch02, rest are in patch02 alone.

Thank You Ajin for assisting on the above changes.

Next in the pipeline:
1) Currently it allows specifying logical slots in standby_slot_names.
This should be prohibited.
2) We need to ensure that WAL is replayed on standby before moving the
slot's position to the target location received from the primary.
3) Rest of the comments upthread.

thanks
Shveta