Horton

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

Masahiko Sawada <sawada.mshk@gmail.com>

View thread View on postgr.es
From: Masahiko Sawada <sawada.mshk@gmail.com>
To: Peter Smith <smithpb2250@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Ajin Cherian <itsajin@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-11-09T08:07:37Z
Lists: pgsql-hackers 
On Mon, Nov 9, 2020 at 3:23 PM Peter Smith <smithpb2250@gmail.com> wrote:
>
> > > 4.
> > > +static void
> > > +apply_handle_prepare_txn(LogicalRepPrepareData * prepare_data)
> > > +{
> > > + Assert(prepare_data->prepare_lsn == remote_final_lsn);
> > > +
> > > + /* The synchronization worker runs in single transaction. */
> > > + if (IsTransactionState() && !am_tablesync_worker())
> > > + {
> > > + /* End the earlier transaction and start a new one */
> > > + BeginTransactionBlock();
> > > + CommitTransactionCommand();
> > > + StartTransactionCommand();
> > >
> > > There is no explanation as to why you want to end the previous
> > > transaction and start a new one. Even if we have to do so, we first
> > > need to call BeginTransactionBlock before CommitTransactionCommand.
>
> Done
>
> ---
>
> Also...
>
> pgindent has been run for all patches now.
>
> The latest of all six patches are again reunited with a common v18
> version number.
>

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?

---
+   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.

---
+               /*
+                * 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.

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

Regards,

-- 
Masahiko Sawada
EnterpriseDB:  https://www.enterprisedb.com/

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