Horton

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

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Ajin Cherian <itsajin@gmail.com>, Peter Smith <smithpb2250@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-11-18T02:23:56Z
Lists: pgsql-hackers 
On Tue, Nov 17, 2020 at 9:05 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Tue, Nov 17, 2020 at 5:02 PM Ajin Cherian <itsajin@gmail.com> wrote:
> >
> > On Tue, Nov 17, 2020 at 10:14 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > >
> > > Doesn't this happen only if you set replication origins? Because
> > > otherwise both PrepareTransaction() and
> > > RecordTransactionCommitPrepared() used the current timestamp.
> > >
> >
> > I was also checking this, even if you set replicating origins, the
> > preparedTransaction will reflect the local prepare time in
> > pg_prepared_xacts. pg_prepared_xacts fetches this information
> > from GlobalTransaction data which does not store the origin_timestamp;
> > it only stores the prepared_at which is the local timestamp.
> >
>
> Sure, but my question was does this difference in behavior happens
> without replication origins in any way? The reason is that if it
> occurs only with replication origins, I don't think we need to bother
> about the same because that feature is not properly implemented and
> not used as-is. See the discussion [1] [2]. OTOH, if this behavior can
> happen without replication origins then we might want to consider
> changing it.

Logical replication workers always have replication origins, right? Is
that what you meant 'with replication origins'?

IIUC logical replication workers always set the origin's commit
timestamp as the commit timestamp of the replicated transaction. OTOH,
the timestamp of PREPARE, ‘prepare’ of pg_prepared_xacts, always uses
the local timestamp even if the caller of PrepareTransaction() sets
replorigin_session_origin_timestamp. In terms of user-visible
timestamps of transaction operations, I think users might expect these
timestamps are matched between the origin and its subscribers. But the
pg_xact_commit_timestamp() is a function of the commit timestamp
feature whereas ‘prepare’ is a pure timestamp when the transaction is
prepared. So I’m not sure these timestamps really need to be matched,
though.

Regards,

-- 
Masahiko Sawada
EnterpriseDB:  https://www.enterprisedb.com/

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