Re: [HACKERS] 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 &
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