Re: Add new protocol message to change GUCs for usage with future protocol-only GUCs

On Wed, 5 Jun 2024 at 17:12, Robert Haas <robertmhaas@gmail.com> wrote:
> Well, hang on. The discussion on that thread suggests that this is
> only going to break connections to servers that don't have
> NegotiateProtocolVersion.

Yes, correct.

> What
> I really don't want is for v18 to break connections to v17 servers.
> That would be exponentially more disruptive.

Totally agreed, and that's easily achievable because of the
NegotiateProtocolVersion message. We're all good on v18 to v17
connections and protocol changes, as long as we make any new behaviour
depend on either a protocol parameter or a protocol version bump.

> I do take your point that poolers haven't added support for
> NegotiateProtocolVersion yet, but I bet that will change pretty
> quickly once there's a benefit to doing so.

I think at minimum we'll need a mechanism for people to connect using
a StartupMessage that doesn't break existing poolers. If users have no
way to connect at all to their semi-recently installed connection
pooler with the newest libpq, then I expect we'll have many unhappy
users. I think it's debatable whether the compatible StartupMessage
should be opt-in or opt-out. I'd personally at minimum want to wait
one PG release before using the incompatible StartupMessage by
default, just to give pooler installs some time to catch up.

> I think it's a lot easier
> for people to install a new pooler version than a new server version,
> because the server has the data in it. Also, it's not like they
> haven't had time.

I agree that it's a lot easier to upgrade poolers than it is to
upgrade PG, but still people are generally hesitant to upgrade stuff
in their infrastructure. And I totally agree that poolers have had the
time to implement NegotiateProtocolVersion support, but I'm pretty
sure many users will assign blame to libpq anyway.

> But the real question here is whether we think the longer cancel key
> is really important enough to justify the partial compatibility break.
> I'm not deathly opposed to it, but I think it's debatable and I'm sure
> some people are going to be unhappy.

I think if it's an opt-in compatibility break, users won't care how
important it is. It's either important enough to opt-in to this
compatibility break for them, or it's not and nothing changes for
them.

My feeling is that we're unlikely to find a feature that's worth
breaking compatibility (with poolers and pre-9.3 PGs) by default on
its own. But after adding a few new protocol features, at some point
together these features become worth this break. Especially, because
by then 9.3 will be even older and poolers will have started to
support NegotiateProtocolVersion (due to people complaining that they
cannot connect with the new opt-in libpq break-compatibility flag).
But if we're going to wait until we have the one super important
protocol feature, then I don't see us moving forward.

To summarize: I'd like to make some relatively small opt-in change(s)
in PG18 that breaks compatibility (with poolers and pre-9.3 PGs). So
that when we have an important enough reason to break compatibility by
default, that break will be much less painful to do.