Track in pg_replication_slots the reason why slots conflict?

Michael Paquier <michael@paquier.xyz>

From: Michael Paquier <michael@paquier.xyz>

To: Postgres hackers <pgsql-hackers@lists.postgresql.org>

Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Andres Freund <andres@anarazel.de>

Date: 2023-12-21T00:21:04Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

Track conflict_reason in pg_replication_slots.
- 007693f2a3ac 17.0 landed

Hi all,
(Bertrand and Andres in CC.)

While listening at Bertrand's talk about logical decoding on standbys
last week at Prague, I got surprised by the fact that we do not
reflect in the catalogs the reason why a conflict happened for a slot.
There are three of them depending on ReplicationSlotInvalidationCause:
- WAL removed.
- Invalid horizon.
- Insufficient WAL level.

This idea has been hinted around here on the original thread that led
to be87200efd93:
https://www.postgresql.org/message-id/d7547f2c-a0c3-6aad-b631-b7ed5efaf298@gmail.com
However v44 has picked up the idea of a boolean:
https://www.postgresql.org/message-id/bdc49e0b-cd39-bcd3-e391-b0ad6e48b5cf@gmail.com

ReplicationSlotCtl holds this information, so couldn't it be useful
for monitoring purposes to know why a slot got invalidated and add a
column to pg_get_replication_slots()?  This could just be an extra
text conflicting_reason, defaulting to NULL when there's nothing to
see.

Thoughts?
--
Michael