Re: Middleware Messages for FE/BE

One more set of "standard middleware messages" clients/middleware
could turn on could be reporting LSNs
* various local LSNs for progress of WAL persisting
* reporting replication state of some or all replicas

-----
Hannu Krosing
Google Cloud - We have a long list of planned contributions and we are hiring.
Contact me if interested.

On Thu, Aug 19, 2021 at 11:33 AM Hannu Krosing <hannuk(at)google(dot)com> wrote:
>
> Need for this does come up quite often so I very much support this.
>
> In addition to keeping a registry there likely need to be some other
> "generally agreed" rules as well, like
> * it being safe to ignore any and all of the middleware messages (at
> least with no degradation from the state of not having them)
> * and maybe even a standard way to turn them on and off.
>
> On/Off switch could be of course done using flags for each
> individual use case, but it would be good to agree conventions.
>
> Another thing to agree would be a set of standard messages, like "I am
> overloaded, consider moving some load away" or "Planning to switch
> over to replica x.x.x.x, please follow"
>
> -----
> Hannu Krosing
> Google Cloud - We have a long list of planned contributions and we are hiring.
> Contact me if interested.
>
>
> On Thu, Aug 19, 2021 at 10:33 AM Simon Riggs
> <simon(dot)riggs(at)enterprisedb(dot)com> wrote:
> >
> > The current FE/BE protocol assumes that the client is talking to the
> > server directly. If middleware wants to do something like load
> > balancing, the only current option is to inspect the incoming commands
> > and take action from that. That approach performs poorly and has
> > proven difficult to maintain, limiting the functionality in Postgres
> > ecosystem middleware.
> >
> > It would be useful to have a way to speak to middleware within a
> > session or prior to each command. There are ways to frig this and
> > obviously it can always be done out-of-core, but there is a clear
> > requirement for various client tools to agree a standard way for them
> > to send messages to middleware rather than the database server. If we
> > get PostgreSQL Project approval for this, then authors of client and
> > middleware tools will know how to interoperate and can begin adding
> > features to take advantage of this, allowing the Postgres ecosystem to
> > improve and extend its middleware.
> >
> > Byte1('M')
> > Identifies the message as a middleware message. (Or perhaps use 'U'
> > for User Message?).
> >
> > Int32
> > Length of message contents in bytes, including self.
> >
> > Int64
> > Routing/class identifier, allowing middleware to quickly ignore whole
> > classes of message if not appropriate. We would run some kind of
> > allocation scheme to ensure unique meaning, probably via the Postgres
> > Wiki. The first 2 billion values would be reserved for allocation by
> > the PostgreSQL Project itself, values beyond that open for external
> > allocation.
> >
> > Byten
> > The message itself, where n is the remaining bytes in the message.
> >
> > The message is intended for middleware only. The server always ignores
> > these messages, with an optional debug facility that can be enabled to
> > allow printing NOTICEs to allow testing.
> >
> > I will supply a patch to any agreed message format, together with a
> > new libpq call to utilise this.
> >
> > In summary: the patch is easy, the point is we need agreement to allow
> > and encourage interoperation between clients and middleware.
> >
> > Thoughts?
> >
> > --
> > Simon Riggs http://www.EnterpriseDB.com/
> >
> >