Re: [HACKERS] logical decoding of two-phase transactions - Mailing list pgsql-hackers

On Thu, Dec 17, 2020 at 6:19 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Wed, Dec 16, 2020 at 2:54 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Wed, Dec 16, 2020 at 1:04 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > >
> >
> > > Also, I guess we can improve the description of
> > > ’two_phase’ option of CREATE SUBSCRIPTION in the doc by adding the
> > > fact that when this option is not enabled the transaction prepared on
> > > the publisher is decoded as a normal transaction:
> > >
> >
> > Sounds reasonable.
> >
>
> Fixed in the attached.
>
> > > ------
> > > +   if (LookupGXact(begin_data.gid))
> > > +   {
> > > +       /*
> > > +        * If this gid has already been prepared then we dont want to apply
> > > +        * this txn again. This can happen after restart where upstream can
> > > +        * send the prepared transaction again. See
> > > +        * ReorderBufferFinishPrepared. Don't update remote_final_lsn.
> > > +        */
> > > +       skip_prepared_txn = true;
> > > +       return;
> > > +   }
> > >
> > > When PREPARE arrives at the subscriber node but there is the prepared
> > > transaction with the same transaction identifier, the apply worker
> > > skips the whole transaction. So if the users prepared a transaction
> > > with the same identifier on the subscriber, the prepared transaction
> > > that came from the publisher would be ignored without any messages. On
> > > the other hand, if applying other operations such as HEAP_INSERT
> > > conflicts (such as when violating the unique constraint) the apply
> > > worker raises an ERROR and stops logical replication until the
> > > conflict is resolved. IIUC since we can know that the prepared
> > > transaction came from the same publisher again by checking origin_lsn
> > > in TwoPhaseFileHeader I guess we can skip the PREPARE message only
> > > when the existing prepared transaction has the same LSN and the same
> > > identifier. To be exact, it’s still possible that the subscriber gets
> > > two PREPARE messages having the same LSN and name from two different
> > > publishers but it’s unlikely happen in practice.
> > >
> >
> > The idea sounds reasonable. I'll try and see if this works.
> >
>
> I went ahead and used both origin_lsn and origin_timestamp to avoid
> the possibility of a match of prepared xact from two different nodes.
> We can handle this at begin_prepare and prepare time but we don't have
> prepare_lsn and prepare_timestamp at rollback_prepared time, so what
> do about that? As of now, I am using just GID at rollback_prepare time
> and that would have been sufficient if we always receive prepare
> before rollback because at prepare time we would have checked
> origin_lsn and origin_timestamp. But it is possible that we get
> rollback prepared without prepare in case if prepare happened before
> consistent_snapshot is reached and rollback happens after that.
>

Note that it is not easy to detect this case, otherwise, we would have
avoided sending rollback_prepared. See comments in
ReorderBufferFinishPrepared in patch
v32-0002-Allow-decoding-at-prepare-time-in-ReorderBuffer.

--
With Regards,
Amit Kapila.