Horton

Re: Skipping logical replication transactions on subscriber side

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: "David G. Johnston" <david.g.johnston@gmail.com>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>, "tanghy.fnst@fujitsu.com" <tanghy.fnst@fujitsu.com>, vignesh C <vignesh21@gmail.com>, Greg Nancarrow <gregn4422@gmail.com>, "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>, "osumi.takamichi@fujitsu.com" <osumi.takamichi@fujitsu.com>, Alexey Lesovsky <lesovsky@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2022-01-26T03:54:42Z
Lists: pgsql-hackers 
On Wed, Jan 26, 2022 at 8:55 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Wed, Jan 26, 2022 at 11:51 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > On Wed, Jan 26, 2022 at 11:28 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > >
> > > IIUC, the proposal is to compare the skip_xid with the very
> > > transaction the apply worker received to apply and raise a warning if
> > > it doesn't match with skip_xid and then continue. This seems like a
> > > reasonable idea but can we guarantee that it is always the first
> > > transaction that we want to skip? We seem to guarantee that we won't
> > > get something again once it is written durably/flushed on the
> > > subscriber side. I guess here it can happen that before the errored
> > > transaction, there is some empty xact, or maybe part of the stream
> > > (consider streaming transactions) of some xact, or there could be
> > > other cases as well where the server will send those xacts again.
> >
> > Good point.
> >
> > I guess that in the situation the worker entered an error loop, we can
> > guarantee that the worker fails while applying the first non-empty
> > transaction since starting logical replication. And the transaction is
> > what we’d like to skip. If the transaction that can be applied without
> > an error is resent after a restart, it’s a problem of logical
> > replication. As you pointed out, it's possible that there are some
> > empty transactions before the transaction in question since we don't
> > advance replication origin LSN if the transaction is empty. Also,
> > probably the same is true for a streamed transaction that is rolled
> > back or ROLLBACK-PREPARED transactions. So, we can also skip clearing
> > subskipxid if the transaction is empty? That is, we make sure to clear
> > it after applying the first non-empty transaction. We would need to
> > carefully think about this solution otherwise ALTER SUBSCRIPTION SKIP
> > ends up not working at all in some cases.

I think it is okay to clear after the first successful application of
any transaction. What I was not sure was about the idea of giving
WARNING/ERROR if the first xact to be applied is not the same as
skip_xid.

>
> Probably, we also need to consider the case where the tablesync worker
> entered an error loop and the user wants to skip the transaction? The
> apply worker is also running at the same time but it should not clear
> subskipxid. Similarly, the tablesync worker should not clear
> subskipxid if the apply worker wants to skip the transaction.
>

I think for tablesync workers, the skip_xid set via this mechanism
won't work as we don't have any remote_xid for them, and neither any
XID is reported in the view for them.

-- 
With Regards,
Amit Kapila.

Commits

  1. Test ALIGNOF_DOUBLE==4 compatibility under ALIGNOF_DOUBLE==8.

  2. Reorder subskiplsn in pg_subscription to avoid alignment issues.

  3. Add ALTER SUBSCRIPTION ... SKIP.

  4. Optionally disable subscriptions on error.

  5. Update docs of logical replication for commit 8d74fc96db.

  6. Respect permissions within logical replication.

  7. Fix regression test failure caused by commit 8d74fc96db.

  8. Add a view to show the stats of subscription workers.

  9. Add logical change details to logical replication worker errcontext.

  10. Rename LOGICAL_REP_MSG_STREAM_END to LOGICAL_REP_MSG_STREAM_STOP.

  11. Fix typo in protocol.sgml.

  12. Remove unused argument in apply_handle_commit_internal().

  13. Fix replication of in-progress transactions in tablesync worker.

  14. Reorder pg_sequence columns to avoid alignment issue