Re: Function to get invalidation cause of a replication slot.

Hi,

On 12/20/23 7:01 AM, shveta malik wrote:
> Hello hackers,
>
> Attached is a patch which attempts to implement a new system function
> pg_get_slot_invalidation_cause('slot_name') to get invalidation cause
> of a replication slot.

Thanks! +1 for the idea to display this information through an SQL API.

I wonder if we could add a new field to pg_replication_slots instead
of creating a new function.

> One of the use case scenarios for this function is another ongoing
> thread "Synchronizing slots from primary to standby" at [1]. This
> function is needed there to replicate invalidation-cause of a logical
> replication slot from the primary server to the hot standby. But this
> is an independent requirement in itself and thus makes sense to have
> it implemented separately.

Agree.

My thoughts about it:

1 ===

"Currently, users do not have a way to know the invalidation cause"

I'm not sure about it, I think one could see the reasons in the log file.

2 ===

"This function returns NULL if the replication slot is not found"

Why not returning an error instead? (like pg_drop_replication_slot() does for example)

FWIW, we'd not need to cover this case if the description would be added to pg_replication_slots.

3 ===

+ <para>
+ <literal>3</literal> = wal_level insufficient for the slot
+ </para>

wal_level insufficient on the primary maybe?

4 ===

+ * Returns ReplicationSlotInvalidationCause enum value for valid slot_name;

I think it would be more user friendly to return a description instead of an enum value.

5 ===

doc/src/sgml/func.sgml | 33 +++++++++++++++++++++++++++++++++
src/backend/replication/slotfuncs.c | 27 +++++++++++++++++++++++++++
src/include/catalog/pg_proc.dat | 4 ++++

Worth to add some coverage in 019_replslot_limit.pl and 035_standby_logical_decoding.pl?

Regards,

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