Re: Skipping logical replication transactions on subscriber side

On Tue, Nov 30, 2021 at 6:28 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Mon, Nov 29, 2021 at 11:38 AM vignesh C <vignesh21@gmail.com> wrote:
> >
>
> I have pushed this patch and there is a buildfarm failure for it. See:
> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=sidewinder&dt=2021-11-30%2005%3A05%3A25
>
> Sawada-San has shared his initial analysis on pgsql-committers [1] and
> I am responding here as the fix requires some more discussion.
>
> > Looking at the result the test actually got, we had two error entries
> > for test_tab1 instead of one:
> >
> > #   Failed test 'check the error reported by the apply worker'
> > #   at t/026_worker_stats.pl line 33.
> > #          got: 'tap_sub|INSERT|test_tab1|t
> > # tap_sub||test_tab1|t'
> > #     expected: 'tap_sub|INSERT|test_tab1|t'
> >
> > The possible scenarios are:
> >
> > The table sync worker for test_tab1 failed due to an error unrelated
> > to apply changes:
> >
> > 2021-11-30 06:24:02.137 CET [18990:2] ERROR:  replication origin with
> > OID 2 is already active for PID 23706
> >
> > At this time, the view had one error entry for the table sync worker.
> > After retrying table sync, it succeeded:
> >
> > 2021-11-30 06:24:04.202 CET [28117:2] LOG:  logical replication table
> > synchronization worker for subscription "tap_sub", table "test_tab1"
> > has finished
> >
> > Then after inserting a row on the publisher, the apply worker inserted
> > the row but failed due to violating a unique key violation, which is
> > expected:
> >
> > 2021-11-30 06:24:04.307 CET [4806:2] ERROR:  duplicate key value
> > violates unique constraint "test_tab1_pkey"
> > 2021-11-30 06:24:04.307 CET [4806:3] DETAIL:  Key (a)=(1) already exists.
> > 2021-11-30 06:24:04.307 CET [4806:4] CONTEXT:  processing remote data
> > during "INSERT" for replication target relation "public.test_tab1" in
> > transaction 721 at 2021-11-30 06:24:04.305096+01
> >
> > As a result, we had two error entries for test_tab1: the table sync
> > worker error and the apply worker error. I didn't expect that the
> > table sync worker for test_tab1 failed due to "replication origin with
> > OID 2 is already active for PID 23706” error.
> >
> > Looking at test_subscription_error() in 026_worker_stats.pl, we have
> > two checks; in the first check, we wait for the view to show the error
> > entry for the given relation name and xid. This check was passed since
> > we had the second error (i.g., apply worker error). In the second
> > check, we get error entries from pg_stat_subscription_workers by
> > specifying only the relation name. Therefore, we ended up getting two
> > entries and failed the tests.
> >
> > To fix this issue, I think that in the second check, we can get the
> > error from pg_stat_subscription_workers by specifying the relation
> > name *and* xid like the first check does. I've attached the patch.
> > What do you think?
> >
>
> I think this will fix the reported failure but there is another race
> condition in the test. Isn't it possible that for table test_tab2, we
> get an error "replication origin with OID ..." or some other error
> before copy, in that case also, we will proceed from the second call
> of test_subscription_error() which is not what we expect in the test?

Right.

> Shouldn't we someway check that the error message also starts with
> "duplicate key value violates ..."?

Yeah, I think it's a good idea to make the checks more specific. That
is, probably we can specify the prefix of the error message and
subrelid in addition to the current conditions: relid and xid. That
way, we can check what error was reported by which workers (tablesync
or apply) for which relations. And both check queries in
test_subscription_error() can have the same WHERE clause.

Regards,

--
Masahiko Sawada
EDB:  https://www.enterprisedb.com/