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: Ajin Cherian <itsajin@gmail.com>
Cc: Peter Smith <smithpb2250@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-11-13T12:30:29Z
Lists: pgsql-hackers 
On Wed, Nov 11, 2020 at 4:30 PM Ajin Cherian <itsajin@gmail.com> wrote:
>
> Did some further tests on the problem I saw and I see that it does not
> have anything to do with this patch. I picked code from top of head.
> If I have enough changes in a transaction to initiate streaming, then
> it will also stream the same changes after a commit.
>
> BEGIN;
> INSERT INTO stream_test SELECT repeat('a', 10) || g.i FROM
> generate_series(1,800) g(i);
> SELECT data FROM pg_logical_slot_get_changes('regression_slot',
> NULL,NULL, 'two-phase-commit', '1', 'include-xids', '0',
> 'skip-empty-xacts', '1', 'stream-changes', '1');
> ** see streamed output here **
> END;
> SELECT data FROM pg_logical_slot_get_changes('regression_slot',
> NULL,NULL, 'two-phase-commit', '1', 'include-xids', '0',
> 'skip-empty-xacts', '1', 'stream-changes', '1');
> ** see the same streamed output here **
>
> I think this is because since the transaction has not been committed,
> SnapBuildCommitTxn is not called which is what moves the
> "builder->start_decoding_at", and as a result
> later calls to pg_logical_slot_get_changes will start from the
> previous lsn.
>

No, we always move start_decoding_at after streaming changes. It will
be moved because we have advanced the confirmed_flush location after
streaming all the changes (via LogicalConfirmReceivedLocation()) which
will be used to set 'start_decoding_at' when we create decoding
context (CreateDecodingContext) next time. However, we don't advance
'restart_lsn' due to which it start from the same point and accumulate
all changes for transaction each time. Now, after Commit we get an
extra record which is ahead of 'start_decoding_at' and we try to
decode it, it will get all the changes of the transaction. It might be
that we update the documentation for pg_logical_slot_get_changes() to
indicate the same but I don't think this is a problem.

> I did do a quick test in pgoutput using pub/sub and I
> dont see duplication of data there but I haven't
> verified exactly what happens.
>

Yeah, because we always move ahead for WAL locations in that unless
the subscriber/publisher is restarted in which case it should start
from the required location. But still, we can try to see if there is
any bug.

-- 
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