Horton

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

Amit Kapila <amit.kapila16@gmail.com>

View thread View on postgr.es
From: Amit Kapila <amit.kapila16@gmail.com>
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: Ajin Cherian <itsajin@gmail.com>, Peter Smith <smithpb2250@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-11-17T11:15:40Z
Lists: pgsql-hackers 
On Mon, Nov 16, 2020 at 3:20 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Mon, Nov 16, 2020 at 4:25 PM Ajin Cherian <itsajin@gmail.com> wrote:
> >
> > Updated with a new test case
> > (contrib/test_decoding/t/002_twophase-streaming.pl) that tests
> > concurrent aborts during streaming prepare. Had to make a few changes
> > to the test_decoding stream_start callbacks to handle
> > "check-xid-aborted"
> > the same way it was handled in the non stream callbacks. Merged
> > Peter's v20-0006 as well.
> >
>
> Thank you for updating the patch.
>
> I have a question about the timestamp of PREPARE on a subscriber node,
> although this may have already been discussed.
>
> With the current patch, the timestamps of PREPARE are different
> between the publisher and the subscriber but the timestamp of their
> commits are the same. For example,
>
> -- There is 1 prepared transaction on a publisher node.
> =# select * from pg_prepared_xact;
>
>  transaction | gid |           prepared            |  owner   | database
> -------------+-----+-------------------------------+----------+----------
>          510 | h1  | 2020-11-16 16:57:13.438633+09 | masahiko | postgres
> (1 row)
>
> -- This prepared transaction is replicated to a subscriber.
> =# select * from pg_prepared_xact;
>
>  transaction | gid |           prepared            |  owner   | database
> -------------+-----+-------------------------------+----------+----------
>          514 | h1  | 2020-11-16 16:57:13.440593+09 | masahiko | postgres
> (1 row)
>
> These timestamps are different. Let's commit the prepared transaction
> 'h1' on the publisher and check the commit timestamps on both nodes.
>
> -- On the publisher node.
> =# select pg_xact_commit_timestamp('510'::xid);
>
>    pg_xact_commit_timestamp
> -------------------------------
>  2020-11-16 16:57:13.474275+09
> (1 row)
>
> -- Commit prepared is also replicated to the subscriber node.
> =# select pg_xact_commit_timestamp('514'::xid);
>
>    pg_xact_commit_timestamp
> -------------------------------
>  2020-11-16 16:57:13.474275+09
> (1 row)
>
> The commit timestamps are the same. At PREPARE we use the local
> timestamp when PREPARE is executed as 'prepared' time while at COMMIT
> PREPARED we use the origin's commit timestamp as the commit timestamp
> if the commit WAL has.
>

Doesn't this happen only if you set replication origins? Because
otherwise both PrepareTransaction() and
RecordTransactionCommitPrepared() used the current timestamp.

-- 
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