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

On Mon, Apr 22, 2024 at 7:21 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> > Please find the attached v35 patch.
>
> The documentation says about both 'active' and 'inactive_since'
> columns of pg_replication_slots say:
>
> ---
> active bool
> True if this slot is currently actively being used
>
> inactive_since timestamptz
> The time since the slot has become inactive. NULL if the slot is
> currently being used. Note that for slots on the standby that are
> being synced from a primary server (whose synced field is true), the
> inactive_since indicates the last synchronization (see Section 47.2.3)
> time.
> ---
>
> When reading the description I thought if 'active' is true,
> 'inactive_since' is NULL, but it doesn't seem to apply for temporary
> slots.

Right.

> Since we don't reset the active_pid field of temporary slots
> when the release, the 'active' is still true in the view but
> 'inactive_since' is not NULL.

Right. inactive_since is reset whenever the temporary slot is acquired
again within the same backend that created the temporary slot.

> Do you think we need to mention it in
> the documentation?

I think that's the reason we dropped "active" from the statement. It
was earlier "NULL if the slot is currently actively being used.". But,
per Bertrand's comment
https://www.postgresql.org/message-id/ZehE2IJcsetSJMHC%40ip-10-97-1-34.eu-west-3.compute.internal
changed it to ""NULL if the slot is currently being used.".

Temporary slots retain the active = true and active_pid = <pid of the
backend that created it> even when the slot is not being used until
the lifetime of the backend process. We haven't tied active or
active_pid flags to inactive_since, doing so now to represent the
temporary slot behaviour for active and active_pid will confuse users
more. As far as the inactive_since of a slot is concerned, it is set
to 0 when the slot is being used (acquired) and set to current
timestamp when the slot is not being used (released).

> As for the timeout-based slot invalidation feature, we could end up
> invalidating the temporary slots even if they are shown as active,
> which could confuse users. Do we want to somehow deal with it?

Yes. As long as the temporary slot is lying unused holding up
resources for more than the specified
replication_slot_inactive_timeout, it is bound to get invalidated.
This keeps behaviour consistent and less-confusing to the users.

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