RE: logical replication empty transactions

On Wednesday, July 14, 2021 9:30 PM Ajin Cherian <itsajin(at)gmail(dot)com> wrote:
> I've had to rebase the patch after a recent commit by Amit Kapila of supporting
> two-phase commits in pub-sub [1].
> Also I've modified the patch to also skip replicating empty prepared
> transactions. Do let me know if you have any comments.
Hi

I started to test this patch but will give you some really minor quick feedbacks.

(1) pg_logical_slot_get_binary_changes() params.

Technically, looks better to have proto_version 3 & two_phase option for the function
to test empty prepare ? I felt proto_version 1 doesn't support 2PC.
[1] says "The following messages (Begin Prepare, Prepare, Commit Prepared, Rollback Prepared)
are available since protocol version 3." Then, if the test wants to skip empty *prepares*,
I suggest to update the proto_version and set two_phase 'on'.

+##############################
+# Test empty prepares
+##############################
...
+# peek at the contents of the slot
+$result = $node_publisher->safe_psql(
+ 'postgres', qq(
+ SELECT get_byte(data, 0)
+ FROM pg_logical_slot_get_binary_changes('tap_sub', NULL, NULL,
+ 'proto_version', '1',
+ 'publication_names', 'tap_pub')
+));

(2) The following sentences may start with a lowercase letter.
There are other similar codes for this.

+ elog(DEBUG1, "Skipping replication of an empty transaction");

[1] - https://www.postgresql.org/docs/devel/protocol-logicalrep-message-formats.html

Best Regards,
Takamichi Osumi