Horton

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

Ajin Cherian <itsajin@gmail.com>

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From: Ajin Cherian <itsajin@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Peter Smith <smithpb2250@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-12-01T02:25:47Z
Lists: pgsql-hackers 
On Tue, Dec 1, 2020 at 12:46 AM Amit Kapila <amit.kapila16@gmail.com> wrote:

> So what caused it to skip due to start_decoding_at? Because the commit
> where the snapshot became consistent is after Prepare. Does it happen
> due to the below code in SnapBuildFindSnapshot() where we bump
> start_decoding_at.
>
> {
> ...
> if (running->oldestRunningXid == running->nextXid)
> {
> if (builder->start_decoding_at == InvalidXLogRecPtr ||
> builder->start_decoding_at <= lsn)
> /* can decode everything after this */
> builder->start_decoding_at = lsn + 1;

I think the reason is that in the function
DecodingContextFindStartpoint(), the code
loops till it finds the consistent snapshot. Then once consistent
snapshot is found, it sets
slot->data.confirmed_flush = ctx->reader->EndRecPtr; This will be used
as the start_decoding_at when the slot is
restarted for decoding.


> Sure, but you can see in your example above it got skipped due to
> start_decoding_at not due to DecodingContextReady. So, the problem as
> mentioned by me previously was how we distinguish those cases because
> it can skip due to start_decoding_at during restart as well when we
> would have already sent the prepare to the subscriber.

The distinguishing factor is that at restart, the Prepare does satisfy
DecodingContextReady (because the snapshot is consistent then).
In both cases, the prepare is prior to start_decoding_at, but when the
prepare is before a consistent point,
it does not satisfy DecodingContextReady. Which is why I suggested
using the check DecodingContextReady  to mark the prepare as 'Not
decoded".

regards,
Ajin Cherian
Fujitsu Australia

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