Re: protocol-level wait-for-LSN

On Wed, 30 Oct 2024 at 19:04, Jesper Pedersen
<jesper(dot)pedersen(at)comcast(dot)net> wrote:
> Having LSN would be nice, but to break all existing implementations, no.
> Having to specify with startup parameters how a core message format
> looks like sounds like a bad idea to me,

It would really help if you would explain why you think it's a bad
idea to use a startup parameter for that, instead of simply stating
that you think it needs a major protocol version bump.

The point of enabling it through a startup parameter (aka protocol
option) is exactly so it will not break any existing implementations.
If clients request the protocol option (which as the name suggests is
optional), then they are expected to be able to parse it. If they
don't, then they will get the old message format. So no existing
implementation will be broken. If some middleware/proxy gets a request
for a startup option it does not support it can advertise that to the
client using the NegotiateProtocolVersion message. Allowing the client
to continue in a mode where the option is not enabled.

So, not bumping the major protocol version and enabling this feature
through a protocol option actually causes less breakage in practice.

Also regarding the wishlist. I think it's much more likely for any of
those to happen in a minor version bump and/or protocol option than it
is that we'll bump the major protocol version.

P.S. Like I said in another email on this thread: I think for this
specific case I'd also prefer a separate new message, because that
makes it easier to filter that message out when received by PgBouncer.
But I'd still like to understand your viewpoint better on this,
because adding fields to existing message types is definitely one of
the types of changes that I personally think would be fine for some
protocol changes.