Re: repeated decoding of prepared transactions

On Tue, Feb 9, 2021 at 11:29 AM Ashutosh Bapat
<ashutosh.bapat.oss@gmail.com> wrote:
>
> On Tue, Feb 9, 2021 at 8:32 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Mon, Feb 8, 2021 at 8:36 PM Markus Wanner
> > <markus.wanner@enterprisedb.com> wrote:
> > >
> > > Hello Amit,
> > >
> > > thanks for your very quick response.
> > >
> > > On 08.02.21 11:13, Amit Kapila wrote:
> > > > /*
> > > >   * It is possible that this transaction is not decoded at prepare time
> > > >   * either because by that time we didn't have a consistent snapshot or it
> > > >   * was decoded earlier but we have restarted. We can't distinguish between
> > > >   * those two cases so we send the prepare in both the cases and let
> > > >   * downstream decide whether to process or skip it. We don't need to
> > > >   * decode the xact for aborts if it is not done already.
> > > >   */
> > >
> > > The way I read the surrounding code, the only case a 2PC transaction
> > > does not get decoded a prepare time is if the transaction is empty.  Or
> > > are you aware of any other situation that might currently happen?
> > >
> >
> > We also skip decoding at prepare time if we haven't reached a
> > consistent snapshot by that time. See below code in DecodePrepare().
> > DecodePrepare()
> > {
> > ..
> > /* We can't start streaming unless a consistent state is reached. */
> > if (SnapBuildCurrentState(builder) < SNAPBUILD_CONSISTENT)
> > {
> > ReorderBufferSkipPrepare(ctx->reorder, xid);
> > return;
> > }
> > ..
> > }
>
> Can you please provide steps which can lead to this situation?
>

Ajin has already shared the example with you.

> If
> there is an earlier discussion which has example scenarios, please
> point us to the relevant thread.
>

It started in the email [1] and from there you can read later emails
to know more about this.

> If we are not sending PREPARED transactions that's fine,
>

Hmm, I am not sure if that is fine because if the output plugin sets
the two-phase-commit option, it would expect all prepared xacts to
arrive not some only some of them.

> but sending
> the same prepared transaction as many times as the WAL sender is
> restarted between sending prepare and commit prepared is a waste of
> network bandwidth.
>

I think similar happens without any of the work done in PG-14 as well
if we restart the apply worker before the commit completes on the
subscriber. After the restart, we will send the start_decoding_at
point based on some previous commit which will make publisher send the
entire transaction again. I don't think restart of WAL sender or WAL
receiver is such a common thing. It can only happen due to some bug in
code or user wishes to stop the nodes or some crash happened.

[1] - https://www.postgresql.org/message-id/CAA4eK1%2Bd3gzCyzsYjt1m6sfGf_C_uFmo9JK%3D3Wafp6yR8Mg8uQ%40mail.gmail.com

-- 
With Regards,
Amit Kapila.