Re: BUG #18897: Logical replication conflict after using pg_createsubscriber under heavy load

Hi Euler, thanks for your reply.

On Wed, Apr 23, 2025 at 11:58 AM Euler Taveira <euler@eulerto.com> wrote:

> On Wed, Apr 16, 2025, at 8:14 PM, PG Bug reporting form wrote:
>
> I'm in the process of converting our databases from pglogical logical
> replication to the native logical replication implementation on PostgreSQL
> 17. One of the bugs we encountered and had to work around with pglogical
> was
> the plugin dropping records while converting to a streaming replica to
> logical via pglogical_create_subscriber (reported
> https://github.com/2ndQuadrant/pglogical/issues/349). I was trying to
> confirm that the native logical replication implementation did not have
> this
> problem, and I've found that it might have a different problem.
>
>
> pg_createsubscriber uses a different approach than pglogical. While
> pglogical
> uses a restore point, pg_createsubscriber uses the LSN from the latest
> replication slot as a replication start point. The restore point approach
> is
> usually suitable to physical replication but might not cover all scenarios
> for
> logical replication (such as when there are in progress transactions).
> Since
> creating a logical replication slot does find a consistent decoding start
> point, it is a natural choice to start the logical replication (that also
> needs
> to find a decoding start point).
>
> I should say that I've been operating under the assumption that
> pg_createsubscriber is designed for use on a replica for a *live* primary
> database, if this isn't correct then someone please let me know.
>
>
> pg_createsubscriber expects a physical replica that is preferably stopped
> before running it.
>

I think pg_createsubscriber actually gives you an error if the replica is
not stopped. I was talking about the primary.


> Your script is not waiting enough time until it applies the backlog.
> Unless,
> you are seeing a different symptom, there is no bug.
>
> You should have used something similar to wait_for_subscription_sync
> routine
> (Cluster.pm) before counting the rows. That's what is used in the
> pg_createsubscriber tests. It guarantees the subscriber has caught up.
>
>
It may be true that the script doesn't wait long enough for all systems,
but when I reproduced the issue on my machine(s) I confirmed that the
logical decoder process was properly stuck on a conflicting primary key,
rather than just catching up.

From the log file

> 2025-04-16 09:17:16.090 AEST [3845786] port=5341 ERROR:  duplicate key
> value violates unique constraint "test_table_pkey"
> 2025-04-16 09:17:16.090 AEST [3845786] port=5341 DETAIL:  Key (f1)=(20700)
> already exists.
> 2025-04-16 09:17:16.090 AEST [3845786] port=5341 CONTEXT:  processing
> remote data for replication origin "pg_24576" during message type "INSERT"
> for replication target relation "public.test_table" in transaction 1581,
> finished at 0/3720058
> 2025-04-16 09:17:16.091 AEST [3816845] port=5341 LOG:  background worker
> "logical replication apply worker" (PID 3845786) exited with exit code 1


  wait_for_subscription_sync sounds like a better solution than what I
have, but you might still be able to reproduce the problem if you increase
the sleep interval on line 198.

I wonder if Shlok could confirm whether they found the conflicting primary
key in their reproduction?

Thanks,
Zane