Re: libpq compression (part 3)

On Wed, May 15, 2024 at 8:38 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> I agree with that goal, but I'm somewhat confused by how your proposal
> achieves it. You had
> libpq_compression=lzma:client_to_server=off;gzip:server_to_client=off,
> so how do we parse that? Is that two completely separate
> specifications, one for lzma and one for gzip, and each of those has
> one option which is set to off? And then they are separated from each
> other by a semicolon? That actually does make sense, and I think it
> may do a better job allowing for compression options than my proposal,
> but it also seems a bit weird, because client_to_server and
> server_to_client are not really compression options at all. They're
> framing when this compression specification applies, rather than what
> it does when it applies. In a way it's a bit like the fact that you
> can prefix a pg_basebackup's --compress option with client- or server-
> to specify where the compression should happen. But we can't quite
> reuse that idea here, because in that case there's no question of
> doing it in both places, whereas here, you might want one thing for
> upstream and another thing for downstream.

Your interpretation is correct, but I don't disagree that it ends up
feeling confusing.

> I'm not a fan of three settings; I could go with two settings, one for
> each direction, and if you want both you have to set both. Or, another
> idea, what if we just separated the two directions with a slash,
> SEND/RECEIVE, and if there's no slash, then it applies to both
> directions. So you could say
> connection_compression='gzip:level=9/lzma' or whatever.
>
> But now I'm wondering whether these options should really be symmetric
> on the client and server sides? Isn't it for the server just to
> specify a list of acceptable algorithms, and the client to set the
> compression options? If both sides are trying to set the compression
> level, for example, who wins?

Compression options really only ever apply to the side doing the
compressing, and at least as I had imagined things each party
(client/server) only used its own level/other compression params.
That leaves me thinking, maybe we really want two independent GUCs,
one for "what algorithms are enabled/negotiable" and one for "how
should I configure my compressors" and then we reduce the dimensions
we are trying to shove into one GUC and each one ends up with a very
clear purpose:
connection_compression=(yes|no|alg1,alg2:server_to_client=alg1,alg2:client_to_server=alg3)
connection_compression_opts=gzip:level=2

--
Jacob Burroughs | Staff Software Engineer
E: jburroughs(at)instructure(dot)com