Re: Unexpected data when subscribing to logical replication slot

Thank you all for your input.

We have solved the problem by -

1. Configuring wal2json to include xids
<https://github.com/eulerto/wal2json/blob/master/README.md>.
2. Updating our enrichment queries to return the xmin
<https://www.postgresql.org/docs/9.1/ddl-system-columns.html>.
3. Retrying our enrichment queries after a brief sleep if xmin < xid.

This seems to be working very well so far.

Daniel McKenzie
Software Developer

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On Thu, May 9, 2024 at 5:45 PM Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com>
wrote:

> On 5/9/24 00:32, Daniel McKenzie wrote:
>
> > We've had this running in live now for years without a hiccup so we are
> > surprised to learn that we have this massive race condition and it just
> > so happens that the hardware is fast enough to process the transaction
> > before the .NET application can react to replication slot changes.
>
> On broad scale I'm going to say that over 'for years' there has been an
> increase in load on the Postgres server as well as the I/0 system of the
> machine it is running on. What you are seeing now is the canary in the
> mine giving you the heads up that more trouble lies ahead as the
> hardware and software is reaching load limits.
>
> On finer scale my guess is that the following is happening when
> synchronous_commit = on:
>
> 1) Postgres session #1 does data change.
>
> 2) This is replicated out and picked up by wal2json, which sees the new
> data.
>
> 3) The Postgres server waits for the confirmation that the WAL record
> has been written out to disk. Upon confirmation it commits on the
> server. This is the part that I am not sure of in relation to wal2json.
>
> 4) Postgres session #2 queries the database for the record. In the case
> where 3) has not completed it sees the old values as the data change in
> session #1 has not committed and therefore the new values are not seen
> by other sessions.
>
> >
> > Daniel McKenzie
> > Software Developer
>
> --
> Adrian Klaver
> adrian(dot)klaver(at)aklaver(dot)com
>
>