Re: Any objections to implementing LogicalDecodeMessageCB for pgoutput?

On Fri, Nov 6, 2020 at 7:05 AM Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
wrote:

> +/*
> + * Write MESSAGE to stream
> + */
> +void
> +logicalrep_write_message(StringInfo out, ReorderBufferTXN *txn,
> XLogRecPtr lsn,
> + bool transactional, const char *prefix, Size sz,
> + const char *message)
> +{
> + uint8 flags = 0;
> +
> + pq_sendbyte(out, LOGICAL_REP_MSG_MESSAGE);
> +
>
> Similar to the UPDATE/DELETE/INSERT records decoded when streaming is being
> used, we need to add transaction id for transactional messages. May be we
> add
> that even in case of non-streaming case and use it to decide whether it's a
> transactional message or not. That might save us a byte when we are adding
> a
> transaction id.
>

My preference is to add in the xid when streaming is enabled. (1) It is a
more consistent implementation with the other message types, and (2) it
saves 3 bytes when streaming is disabled. I've attached an updated patch.
It is not a strong preference, though, if you suggest a different approach.

> + /* encode and send message flags */
> + if (transactional)
> + flags |= MESSAGE_TRANSACTIONAL;
> +
> + pq_sendint8(out, flags);
>
> Is 8 bits enough considering future improvements? What if we need to use
> more
> than 8 bit flags?
>

8 possible flags already sounds like a lot, here, so I suspect that a byte
will be sufficient for the foreseeable future. If we needed to go beyond
that, it'd be a protocol version bump. (Well, it might first warrant
reflection as to why we had so many flags...)

> @@ -1936,6 +1936,9 @@ apply_dispatch(StringInfo s)
> apply_handle_origin(s);
> return;
>
> + case LOGICAL_REP_MSG_MESSAGE:
>
> Should we add the logical message to the WAL downstream so that it flows
> further down to a cascaded logical replica. Should that be controlled
> by an option?
>

Hmm, I can't think of a use case for this, but perhaps someone could. Do
you, or does anyone, have something in mind? I think we provide a lot of
value with logical messages in pgoutput without supporting consumption from
a downstream replica, so perhaps this is better considered separately.

If we want this, I think we would add a "messages" option on the
subscription. If present, the subscriber will receive messages and pass
them to any downstream subscribers. I started working on this and it does
expand the change's footprint. As is, a developer would consume messages by
connecting to a pgoutput slot on the message's origin. (e.g. via Debezium
or a custom client) The subscription and logical worker infrastructure
don't know about messages, but they would need to in order to support
consuming an origin's messages on a downstream logical replica. In
any case, I'll keep working on it so we can see what it looks like.

Cheers,
Dave