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

On Thu, May 16, 2024 at 1:39 PM Jacob Burroughs
<jburroughs@instructure.com> wrote:
> As currently implemented [1], the client sends the server the list of
> all compression algorithms it is willing to accept, and the server can
> use one of them.  If the server knows what `_pq_.compression` means
> but doesn't actually support any compression, it will both send the
> client its empty list of supported algorithms and just never send any
> compressed messages, and everyone involved will be (relatively) happy.
> There is a libpq function that a client can use to check what
> compression is in use if a client *really* doesn't want to continue
> with the conversation without compression, but 99% of the time I can't
> see why a client wouldn't prefer to continue using a connection with
> whatever compression the server supports (or even none) without more
> explicit negotiation.  (Unlike TLS, where automagically picking
> between using and not using TLS has strange security implications and
> effects, compression is a convenience feature for everyone involved.)

This all seems sensible to me.

> As the protocol layer is currently designed [1], it explicitly makes
> it very easy to change/restart compression streams, specifically for
> this use case (and in particular for the general connection pooler use
> case).  Compressed data is already framed in a `CompressedData`
> message, and that message has a header byte that corresponds to an
> enum value for which algorithm is currently in use.  Any time the
> compression stream was restarted by the sender, the first
> `CompressedData` message will set that byte, and then the client will
> restart its decompression stream with the chosen algorithm from that
> point.  For `CompressedData` messages that continue using the
> already-established stream, the byte is simply set to 0.  (This is
> also how the "each side sends a list" form of negotiation is able to
> work without additional round trips, as the `CompressedData` framing
> itself communicates which compression algorithm has been selected.)

OK, so you made it so that compressed data is fully self-identifying.
Hence, there's no need to worry if something gets changed: the
receiver, if properly implemented, can't help but notice. The only
downside that I can see to this design is that you only have one byte
to identify the compression algorithm, but that doesn't actually seem
like a real problem at all, because I expect the number of supported
compression algorithms to grow very slowly. I think it would take
centuries, possibly millenia, before we started to get short of
identifiers. So, cool.

But, in your system, how does the client ask the server to switch to a
different compression algorithm, or to restart the compression stream?

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