Re: Skipping logical replication transactions on subscriber side

On Sat, May 29, 2021 at 8:27 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Thu, May 27, 2021 at 7:04 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Thu, May 27, 2021 at 1:46 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > >
> > > 1. the worker records the XID and commit LSN of the failed transaction
> > > to a catalog.
> > >
> >
> > When will you record this info? I am not sure if we can try to update
> > this when an error has occurred. We can think of using try..catch in
> > apply worker and then record it in catch on error but would that be
> > advisable? One random thought that occurred to me is to that apply
> > worker notifies such information to the launcher (or maybe another
> > process) which will log this information.
>
> Yeah, I was concerned about that too and had the same idea. The
> information still could not be written if the server crashes before
> the launcher writes it. But I think it's an acceptable.
>

True, because even if the launcher restarts, the apply worker will
error out again and resend the information. I guess we can have an
error queue where apply workers can add their information and the
launcher will then process those.  If we do that, then we need to
probably define what we want to do if the queue gets full, either
apply worker nudge launcher and wait or it can just throw an error and
continue. If you have any better ideas to share this information then
we can consider those as well.

> >
> > > 2. the user specifies how to resolve that conflict transaction
> > > (currently only 'skip' is supported) and writes to the catalog.
> > > 3. the worker does the resolution method according to the catalog. If
> > > the worker didn't start to apply those changes, it can skip the entire
> > > transaction. If did, it rollbacks the transaction and ignores the
> > > remaining.
> > >
> > > The worker needs neither to reset information of the last failed
> > > transaction nor to mark the conflicted transaction as resolved. The
> > > worker will ignore that information when checking the catalog if the
> > > commit LSN is passed.
> > >
> >
> > So won't this require us to check the required info in the catalog
> > before applying each transaction? If so, that might be overhead, maybe
> > we can build some cache of the highest commitLSN that can be consulted
> > rather than the catalog table.
>
> I think workers can cache that information when starts and invalidates
> and reload the cache when the catalog gets updated.  Specifying to
> skip XID will update the catalog, invalidating the cache.
>
> > I think we need to think about when to
> > remove rows for which conflict has been resolved as we can't let that
> > information grow infinitely.
>
> I guess we can update catalog tuples in place when another conflict
> happens next time. The catalog tuple should be fixed size. The
> already-resolved conflict will have the commit LSN older than its
> replication origin's LSN.
>

Okay, but I have a slight concern that we will keep xid in the system
which might have been no longer valid. So, we will keep this info
about subscribers around till one performs drop subscription,
hopefully, that doesn't lead to too many rows. This will be okay as
per the current design but say tomorrow we decide to parallelize the
apply for a subscription then there could be multiple errors
corresponding to a subscription and in that case, such a design might
appear quite limiting. One possibility could be that when the launcher
is periodically checking for new error messages, it can clean up the
conflicts catalog as well, or maybe autovacuum does this periodically
as it does for stats (via pgstat_vacuum_stat).

-- 
With Regards,
Amit Kapila.