Re: Question about StartLogicalReplication() error path

On Fri, Jun 11, 2021 at 11:52 AM Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
>
> On Fri, 2021-06-11 at 10:13 +0530, Amit Kapila wrote:
> > I think because there is no need to process the WAL that has been
> > confirmed by the client. Do you see any problems with this scheme?
>
> Several:
>
> * Replication setups are complex, and it can be easy to misconfigure
> something or have a bug in some control code. An error is valuable to
> detect the problem closer to the source.
>
> * There are plausible configurations where things could go badly wrong.
> For instance, if you are storing the decoded data in another postgres
> server with syncrhonous_commit=off, and acknowledging LSNs before they
> are durable. A crash of the destination system would be consistent, but
> it would be missing some data earlier than the confirmed_flush_lsn. The
> client would then request the data starting at its stored lsn progress
> value, but the server would skip ahead to the confirmed_flush_lsn;
> silently missing data.
>

AFAIU, currently, in such a case, the subscriber (client) won't
advance the flush location (confirmed_flush_lsn). So, we won't lose
any data.

--
With Regards,
Amit Kapila.