Re: logical replication - possible remaining problem

Hi,

On 07/06/17 22:49, Erik Rijkers wrote:
> I am not sure whether what I found here amounts to a bug, I might be
> doing something dumb.
>
> During the last few months I did tests by running pgbench over logical
> replication. Earlier emails have details.
>
> The basic form of that now works well (and the fix has been comitted)
> but as I looked over my testing program I noticed one change I made to
> it, already many weeks ago:
>
> In the cleanup during startup (pre-flight check you might say) and also
> before the end, instead of
>
> echo "delete from pg_subscription;" | psql -qXp $port2 -- (1)
>
> I changed that (as I say, many weeks ago) to:
>
> echo "delete from pg_subscription;
> delete from pg_subscription_rel;
> delete from pg_replication_origin; " | psql -qXp $port2 -- (2)
>
> This occurs (2x) inside the bash function clean_pubsub(), in main test
> script pgbench_detail2.sh
>
> This change was an effort to ensure to arrive at a 'clean' start (and
> end-) state which would always be the same.
>
> All my more recent testing (and that of Mark, I have to assume) was thus
> done with (2).
>
> Now, looking at the script again I am thinking that it would be
> reasonable to expect that after issuing
> delete from pg_subscription;
>
> the other 2 tables are /also/ cleaned, automatically, as a consequence.
> (Is this reasonable? this is really the main question of this email).
>

Hmm, they are not cleaned automatically, deleting from system catalogs
manually like this never propagates to related tables, we don't use FKs
there.

> So I removed the latter two delete statements again, and ran the tests
> again with the form in (1)
>
> I have established that (after a number of successful cycles) the test
> stops succeeding with in the replica log repetitions of:
>
> 2017-06-07 22:10:29.057 CEST [2421] LOG: logical replication apply
> worker for subscription "sub1" has started
> 2017-06-07 22:10:29.057 CEST [2421] ERROR: could not find free
> replication state slot for replication origin with OID 11
> 2017-06-07 22:10:29.057 CEST [2421] HINT: Increase
> max_replication_slots and try again.
> 2017-06-07 22:10:29.058 CEST [2061] LOG: worker process: logical
> replication worker for subscription 29235 (PID 2421) exited with exit
> code 1
>
> when I manually 'clean up' by doing:
> delete from pg_replication_origin;
>

Yeah because you consumed all the origins (I am still not huge fan of
how that limit works, but that's separate discussion).

> then, and only then, does the session finish and succeed ('replica ok').
>
> So to me it looks as if there is an omission of
> pg_replication_origin-cleanup when pg_description is deleted.
>

There is no omission, origin is not supposed to be deleted automatically
unless you use DROP SUBSCRIPTION.

--
Petr Jelinek http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services