Re: logical replication empty transactions

On Fri, Jul 23, 2021 at 3:39 PM Ajin Cherian <itsajin@gmail.com> wrote:
>

Let's first split the patch for prepared and non-prepared cases as
that will help to focus on each of them separately. BTW, why haven't
you considered implementing point 1b as explained by Andres in his
email [1]? I think we can send a keepalive message in case of
synchronous replication when we skip an empty transaction, otherwise,
it might delay in responding to transactions synchronous_commit mode.
I think in the tests done in the thread, it might not have been shown
because we are already sending keepalives too frequently. But what if
someone disables wal_sender_timeout or kept it to a very large value?
See WalSndKeepaliveIfNecessary. The other thing you might want to look
at is if the reason for frequent keepalives is the same as described
in the email [2].

Few other miscellaneous comments:
1.
static void
 pgoutput_commit_prepared_txn(LogicalDecodingContext *ctx,
ReorderBufferTXN *txn,
- XLogRecPtr commit_lsn)
+ XLogRecPtr commit_lsn, XLogRecPtr prepare_end_lsn,
+ TimestampTz prepare_time)
 {
+ PGOutputTxnData    *txndata = (PGOutputTxnData *) txn->output_plugin_private;
+
  OutputPluginUpdateProgress(ctx);

+ /*
+ * If the BEGIN PREPARE was not yet sent, then it means there were no
+ * relevant changes encountered, so we can skip the COMMIT PREPARED
+ * message too.
+ */
+ if (txndata)
+ {
+ bool skip = !txndata->sent_begin_txn;
+ pfree(txndata);
+ txn->output_plugin_private = NULL;

How is this supposed to work after the restart when prepared is sent
before the restart and we are just sending commit_prepared after
restart? Won't this lead to sending commit_prepared even when the
corresponding prepare is not sent? Can we think of a better way to
deal with this?

2.
@@ -222,8 +224,10 @@ logicalrep_write_commit_prepared(StringInfo out,
ReorderBufferTXN *txn,
  pq_sendbyte(out, flags);

  /* send fields */
+ pq_sendint64(out, prepare_end_lsn);
  pq_sendint64(out, commit_lsn);
  pq_sendint64(out, txn->end_lsn);
+ pq_sendint64(out, prepare_time);

Doesn't this means a change of protocol and how is it suppose to work
when say publisher is 15 and subscriber from 14 which I think works
without such a change?


[1] - https://www.postgresql.org/message-id/20200309183018.tzkzwu635sd366ej%40alap3.anarazel.de
[2] - https://www.postgresql.org/message-id/CALtH27cip5uQNJb4uHjLXtx1R52ELqXVfcP9fhHr%3DAvFo1dtqw%40mail.gmail.com

-- 
With Regards,
Amit Kapila.