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: Masahiko Sawada <sawada.mshk@gmail.com>, Peter Smith <smithpb2250@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-11-13T10:45:01Z
Lists: pgsql-hackers 
On Thu, Nov 12, 2020 at 2:28 PM Ajin Cherian <itsajin@gmail.com> wrote:
>
> On Wed, Nov 11, 2020 at 12:35 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>  I have rearranged the code in DecodeCommit/Abort/Prepare so
> > that it does only the required things (like in DecodeCommit was still
> > processing subtxns even when it has to just perform FinishPrepared,
> > also the stats were not updated properly which I have fixed.) and
> > added/edited the comments. Apart from 0001 and 0002, I have not
> > changed anything in the remaining patches.
>
> One small comment on the patch:
>
> - DecodeCommit(ctx, buf, &parsed, xid);
> + /*
> + * If we have already decoded this transaction data then
> + * DecodeCommit doesn't need to decode it again. This is
> + * possible iff output plugin supports two-phase commits and
> + * doesn't skip the transaction at prepare time.
> + */
> + if (info == XLOG_XACT_COMMIT_PREPARED && ctx->twophase)
> + {
> + already_decoded = !(ctx->callbacks.filter_prepare_cb &&
> + ReorderBufferPrepareNeedSkip(ctx->reorder, xid, parsed.twophase_gid));
> + }
> +
>
> Just a small nitpick but the way already_decoded is assigned here is a
> bit misleading. It appears that the callbacks determine if the
> transaction is already decoded when in
> reality the callbacks only decide if the transaction should skip two
> phase commits. I think it's better to either move it to the if
> condition or if that is too long then have one more variable
> skip_twophase.
>
> if (info ==  XLOG_XACT_COMMIT_PREPARED && ctx->twophase &&
>                      !(ctx->callbacks.filter_prepare_cb &&
> ReorderBufferPrepareNeedSkip(ctx->reorder, xid, parsed.twophase_gid)))
> already_decoded = true;
>
> OR
> bool skip_twophase = false;
> skip_twophase =  !(ctx->callbacks.filter_prepare_cb &&
> ReorderBufferPrepareNeedSkip(ctx->reorder, xid, parsed.twophase_gid));
> if (info ==  XLOG_XACT_COMMIT_PREPARED && ctx->twophase && skip_twophase)
> already_decoded = true;
>

Hmm, introducing an additional boolean variable for this doesn't seem
like a good idea neither the other alternative suggested by you. How
about if we change the comment to make it clear. How about: "If output
plugin supports two-phase commits and doesn't skip the transaction at
prepare time then we don't need to decode the transaction data at
commit prepared time as it would have already been decoded at prepare
time."?

-- 
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