Horton

Re: [HACKERS] logical decoding of two-phase transactions

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Peter Smith <smithpb2250@gmail.com>
Cc: Ajin Cherian <itsajin@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-10-22T03:00:13Z
Lists: pgsql-hackers 
On Thu, Oct 22, 2020 at 4:58 AM Peter Smith <smithpb2250@gmail.com> wrote:
>
> On Wed, Oct 21, 2020 at 7:42 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Wed, Oct 21, 2020 at 1:38 PM Peter Smith <smithpb2250@gmail.com> wrote:
> > >
> > > The PG docs for PREPARE TRANSACTION [1] don't say anything about an
> > > empty (zero length) transaction-id.
> > > e.g. PREPARE TRANSACTION '';
> > > [1] https://www.postgresql.org/docs/current/sql-prepare-transaction.html
> > >
> > > ~
> > >
> > > Meanwhile, during testing I found the 2PC prepare hangs when an empty
> > > id is used.
> > >
> >
> > Can you please take an example to explain what you are trying to say?
>
> I was referring to an empty (zero length) transaction ID, not an empty
> transaction.
>

oh, I got it confused with the system generated 32-bit TransactionId.
But now, I got what you were referring to.

> The example was already given as PREPARE TRANSACTION '';
>
>
> Is that something that should be made to work for 2PC pub/sub, or was
> Postgres PREPARE TRANSACTION statement wrong to allow the user to
> specify an empty transaction ID in the first place?
>

I don't see any problem with the empty transaction identifier used in
Prepare Transaction. This is just used as an identifier to uniquely
identify the transaction. If you try to use an empty string ('') more
than once for Prepare Transaction, it will give an error like below:
postgres=*# prepare transaction '';
ERROR:  transaction identifier "" is already in use

So, I think this should work for pub/sub as well. Did you find out the
reason of hang?

-- 
With Regards,
Amit Kapila.

Commits

  1. Add prepare API support for streaming transactions in logical replication.

  2. Doc: minor improvements for logical replication protocol documentation.

  3. Fix test failure in 021_twophase.pl.

  4. Refactor to make common functions in proto.c and worker.c.

  5. Unify PostgresNode's new() and get_new_node() methods

  6. Fix potential buffer overruns in proto.c.

  7. Add support for prepared transactions to built-in logical replication.

  8. Refactor function parse_subscription_options.

  9. Allow enabling two-phase option via replication protocol.

  10. Don't use Asserts to check for violations of replication protocol.

  11. Improve psql tab completion for options of subcriptions and publications

  12. Rearrange logrep worker's snapshot handling some more.

  13. doc: Update information of new messages for logical replication.

  14. ALTER SUBSCRIPTION ... ADD/DROP PUBLICATION

  15. Allow pgoutput to send logical decoding messages.

  16. Refactor function parse_output_parameters.

  17. Avoid repeated decoding of prepared transactions after a restart.

  18. Fix an oversight in ReorderBufferFinishPrepared.

  19. Allow multiple xacts during table sync in logical replication.

  20. Fix replication of in-progress transactions in tablesync worker.

  21. Fix 'skip-empty-xacts' option in test_decoding for streaming mode.

  22. Use Enum for top level logical replication message types.

  23. Add support for streaming to built-in logical replication.

  24. Fix the logical streaming test.

  25. Implement streaming mode in ReorderBuffer.

  26. Extend the logical decoding output plugin API with stream methods.

  27. Store 2PC GID in commit/abort WAL recs for logical decoding