Re: Multiple startup messages over the same connection

On Mon, Jan 22, 2024 at 11:43 PM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:

> On 22/01/2024 21:58, Vladimir Churyukin wrote:
> > A question about protocol design - would it be possible to extend the
> > protocol, so it can handle multiple startup / authentication messages
> > over a single connection? Are there any serious obstacles? (possible
> > issues with re-initialization of backends, I guess?)
> > If that is possible, it could improve one important edge case - where
> > you have to talk to multiple databases on a single host currently, you
> > need to open a separate connection to each of them. In some cases
> > (multitenancy for example), you may have thousands of databases on a
> > host, which leads to inefficient connection utilization on clients (on
> > the db side too). A lot of other RDBMSes don't have this limitation.
>
> The protocol and the startup message are the least of your problems.
> Yeah, it would be nice if you could switch between databases, but the
> assumption that one backend operates on one database is pretty deeply
> ingrained in the code.
>
> --
> Heikki Linnakangas
> Neon (https://neon.tech)
>
>
Sorry to revive this old thread, just want to check on one thing:
Let's say we keep one database per backend rule, I understand at this point
it would be really hard to change.
What if on a new startup message we just signal the postmaster about it, so
it takes over the socket and spawns a new backend.
After that we terminate the old one. How does it sound like in terms of
implementation complexity?
I guess the process of passing control from child processes to the parent
could be a bit tricky for that one, but doable?
Is there anything I'm missing that can be a no-go for this?
The end goal is to minimize a large overhead for clients having to deal
with a large number of connections on multi-tenant systems (say, one client
deals with thousands of databases on the same database server).

-Vladimir Churyukin