Re: Introduce XID age and inactive timeout based replication slot invalidation

On Wed, Mar 6, 2024 at 4:49 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> You might want to consider its interaction with sync slots on standby.
> Say, there is no activity on slots in terms of processing the changes
> for slots. Now, we won't perform sync of such slots on standby showing
> them inactive as per your new criteria where as same slots could still
> be valid on primary as the walsender is still active. This may be more
> of a theoretical point as in running system there will probably be
> some activity but I think this needs some thougths.

I believe the xmin and catalog_xmin of the sync slots on the standby
keep advancing depending on the slots on the primary, no? If yes, the
XID age based invalidation shouldn't be a problem.

I believe there are no walsenders started for the sync slots on the
standbys, right? If yes, the inactive timeout based invalidation also
shouldn't be a problem. Because, the inactive timeouts for a slot are
tracked only for walsenders because they are the ones that typically
hold replication slots for longer durations and for real replication
use. We did a similar thing in a recent commit [1].

Is my understanding right? Do you still see any problems with it?

[1]
commit 7c3fb505b14e86581b6a052075a294c78c91b123
Author: Amit Kapila <akapila(at)postgresql(dot)org>
Date: Tue Nov 21 07:59:53 2023 +0530

Log messages for replication slot acquisition and release.
.........
Note that these messages are emitted only for walsenders but not for
backends. This is because walsenders are the ones that typically hold
replication slots for longer durations, unlike backends which hold them
for executing replication related functions.

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com