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

On Fri, Oct 30, 2020 at 2:46 PM Ajin Cherian <itsajin@gmail.com> wrote:
>
> On Thu, Oct 29, 2020 at 11:19 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> >
> > 6.
> > +pg_decode_stream_prepare(LogicalDecodingContext *ctx,
> > + ReorderBufferTXN *txn,
> > + XLogRecPtr prepare_lsn)
> > +{
> > + TestDecodingData *data = ctx->output_plugin_private;
> > +
> > + if (data->skip_empty_xacts && !data->xact_wrote_changes)
> > + return;
> > +
> > + OutputPluginPrepareWrite(ctx, true);
> > +
> > + if (data->include_xids)
> > + appendStringInfo(ctx->out, "preparing streamed transaction TXN %u", txn->xid);
> > + else
> > + appendStringInfo(ctx->out, "preparing streamed transaction");
> >
> > I think we should include 'gid' as well in the above messages.
>
> Updated.
>

gid needs to be included in the case of 'include_xids' as well.

> >
> > 7.
> > @@ -221,12 +235,26 @@ StartupDecodingContext(List *output_plugin_options,
> >   ctx->streaming = (ctx->callbacks.stream_start_cb != NULL) ||
> >   (ctx->callbacks.stream_stop_cb != NULL) ||
> >   (ctx->callbacks.stream_abort_cb != NULL) ||
> > + (ctx->callbacks.stream_prepare_cb != NULL) ||
> >   (ctx->callbacks.stream_commit_cb != NULL) ||
> >   (ctx->callbacks.stream_change_cb != NULL) ||
> >   (ctx->callbacks.stream_message_cb != NULL) ||
> >   (ctx->callbacks.stream_truncate_cb != NULL);
> >
> >   /*
> > + * To support two-phase logical decoding, we require
> > prepare/commit-prepare/abort-prepare
> > + * callbacks. The filter-prepare callback is optional. We however
> > enable two-phase logical
> > + * decoding when at least one of the methods is enabled so that we
> > can easily identify
> > + * missing methods.
> > + *
> > + * We decide it here, but only check it later in the wrappers.
> > + */
> > + ctx->twophase = (ctx->callbacks.prepare_cb != NULL) ||
> > + (ctx->callbacks.commit_prepared_cb != NULL) ||
> > + (ctx->callbacks.rollback_prepared_cb != NULL) ||
> > + (ctx->callbacks.filter_prepare_cb != NULL);
> > +
> >
> > I think stream_prepare_cb should be checked for the 'twophase' flag
> > because we won't use this unless two-phase is enabled. Am I missing
> > something?
>
> Was fixed in v14.
>

But you still have it in the streaming check. I don't think we need
that for the streaming case.

Few other comments on v15-0002-Support-2PC-txn-backend-and-tests:
======================================================================
1. The functions DecodeCommitPrepared and DecodeAbortPrepared have a
lot of code similar to DecodeCommit/Abort. Can we merge these
functions?

2.
DecodeCommitPrepared()
{
..
+ * If filter check present and this needs to be skipped, do a regular commit.
+ */
+ if (ctx->callbacks.filter_prepare_cb &&
+ ReorderBufferPrepareNeedSkip(ctx->reorder, xid, parsed->twophase_gid))
+ {
+ ReorderBufferCommit(ctx->reorder, xid, buf->origptr, buf->endptr,
+ commit_time, origin_id, origin_lsn);
+ }
+ else
+ {
+ ReorderBufferFinishPrepared(ctx->reorder, xid, buf->origptr, buf->endptr,
+ commit_time, origin_id, origin_lsn,
+ parsed->twophase_gid, true);
+ }
+
+}

Can we expand the comment here to say why we need to do ReorderBufferCommit?

3. There are a lot of test cases in this patch which is a good thing
but can we split them into a separate patch for the time being as I
would like to focus on the core logic of the patch first. We can later
see if we need to retain all or part of those tests.

4. Please run pgindent on your patches.

-- 
With Regards,
Amit Kapila.