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: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: Peter Smith <smithpb2250@gmail.com>, Ajin Cherian <itsajin@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-11-09T11:22:08Z
Lists: pgsql-hackers 
On Mon, Nov 9, 2020 at 1:38 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Mon, Nov 9, 2020 at 3:23 PM Peter Smith <smithpb2250@gmail.com> wrote:
> >
>
> I've looked at the patches and done some tests. Here is my comment and
> question I realized during testing and reviewing.
>
> +static void
> +DecodePrepare(LogicalDecodingContext *ctx, XLogRecordBuffer *buf,
> +             xl_xact_parsed_prepare *parsed)
> +{
> +   XLogRecPtr  origin_lsn = parsed->origin_lsn;
> +   TimestampTz commit_time = parsed->origin_timestamp;
>
>  static void
>  DecodeAbort(LogicalDecodingContext *ctx, XLogRecordBuffer *buf,
> -           xl_xact_parsed_abort *parsed, TransactionId xid)
> +           xl_xact_parsed_abort *parsed, TransactionId xid, bool prepared)
>  {
>     int         i;
> +   XLogRecPtr  origin_lsn = InvalidXLogRecPtr;
> +   TimestampTz commit_time = 0;
> +   XLogRecPtr  origin_id = XLogRecGetOrigin(buf->record);
>
> -   for (i = 0; i < parsed->nsubxacts; i++)
> +   if (parsed->xinfo & XACT_XINFO_HAS_ORIGIN)
>     {
> -       ReorderBufferAbort(ctx->reorder, parsed->subxacts[i],
> -                          buf->record->EndRecPtr);
> +       origin_lsn = parsed->origin_lsn;
> +       commit_time = parsed->origin_timestamp;
>     }
>
> In the above two changes, parsed->origin_timestamp is used as
> commit_time. But in DecodeCommit() we use parsed->xact_time instead.
> Therefore it a transaction didn't have replorigin_session_origin the
> timestamp of logical decoding out generated by test_decoding with
> 'include-timestamp' option is invalid. Is it intentional?
>

I think all three DecodePrepare/DecodeAbort/DecodeCommit should have
same handling for this with the exception that at DecodePrepare time
we can't rely on XACT_XINFO_HAS_ORIGIN but instead we need to check if
origin_timestamp is non-zero then we will overwrite commit_time with
it. Does that make sense to you?

> ---
> +   if (is_commit)
> +       txn->txn_flags |= RBTXN_COMMIT_PREPARED;
> +   else
> +       txn->txn_flags |= RBTXN_ROLLBACK_PREPARED;
> +
> +   if (rbtxn_commit_prepared(txn))
> +       rb->commit_prepared(rb, txn, commit_lsn);
> +   else if (rbtxn_rollback_prepared(txn))
> +       rb->rollback_prepared(rb, txn, commit_lsn);
>
> RBTXN_COMMIT_PREPARED and RBTXN_ROLLBACK_PREPARED are used only here
> and it seems to me that it's not necessarily necessary.
>

+1.

> ---
> +               /*
> +                * If this is COMMIT_PREPARED and the output plugin supports
> +                * two-phase commits then set the prepared flag to true.
> +                */
> +               prepared = ((info == XLOG_XACT_COMMIT_PREPARED) &&
> ctx->twophase) ? true : false;
>
> We can write instead:
>
> prepared = ((info == XLOG_XACT_COMMIT_PREPARED) && ctx->twophase);
>
>
> +               /*
> +                * If this is ABORT_PREPARED and the output plugin supports
> +                * two-phase commits then set the prepared flag to true.
> +                */
> +               prepared = ((info == XLOG_XACT_ABORT_PREPARED) &&
> ctx->twophase) ? true : false;
>
> The same is true here.
>

+1.

> ---
> 'git show --check' of v18-0002 reports some warnings.
>

I have also noticed this. Actually, I have already started making some
changes to these patches apart from what you have reported so I'll
take care of these things as well.

-- 
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