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

On Fri, May 17, 2024 at 1:26 PM Jacob Burroughs
<jburroughs(at)instructure(dot)com> wrote:
> I was leaving the details around triggering that for this conversation
> and in that patch just designing the messages in a way that would
> allow adding the reconfiguration/restarting to be easily added in a
> backwards-compatible way in a future patch. I would imagine that an
> explicit `ParameterSet` call that sets `_pq_.connection_compression`
> (or whatever the implementation details turn out to be) would also
> trigger a compressor restart, and when restarted it would pick an
> algorithm/configuration based on the new value of the parameter rather
> than the one used at connection establishment.

Hmm, OK, interesting. I suppose that I thought we were going to handle
problems like this by just adding bespoke messages for each case (e.g.
CompressionSet). I wasn't thinking it would be practical to make a
message whose remit was as general as what it seems like Jelte wants
to do with ParameterSet, because I'm not sure everything can be
handled that simply. It's not an exact analogy, but when you want to
stop the server, it's not enough to say that you want to change from
the Running state to the Stopped state. You have to specify which type
of shutdown should be used to make the transition. You could even have
more complicated cases, where one side says "prepare to do X" and the
other side eventually says "OK, I'm prepared" and the first side says
"great, now activate X" and the other side eventually says "OK, it's
activate, please confirm that you've also activated it from your side"
and the first side eventually says "OK, I confirm that". I think the
fear that we're going to run into cases where such complex handshaking
is necessary is a major reason why I'm afraid of Jelte's ideas about
ParameterSet: it seems much more opinionated to me than he seems to
think it is.

To the extent that I can wrap my head around what Jelte is proposing,
and all signs point to that extent being quite limited, I suppose I
was thinking of something like his option (2). That is, I assumed that
a client would request all the optional features that said client
might wish to use at connection startup time. But I can see how that
assumption might be somewhat problematic in a connection-pooling
environment. Still, at least to me, it seems better than trying to
rely on GUC_REPORT. My opinion is (1) GUC_REPORT isn't a particularly
well-designed mechanism so I dislike trying to double down on it and
(2) trying to mix these protocol-level parameters and the
transactional GUCs we have together in a single mechanism seems
potentially problematic and (3) I'm still not particularly happy about
the idea of making protocol parameters into GUCs in the first place.
Perhaps these are all minority positions, but I can't tell you what
everyone thinks, only what I think.

--
Robert Haas
EDB: http://www.enterprisedb.com