[systemd-devel] What is the recommended way of announcing a TCP port?

[John Ioannidis]

On Mon, Jun 14, 2021 at 9:29 PM Kevin P. Fleming <kevin at km6g.us> wrote:

> You might consider having systemd itself create the listening sockets
> and then pass them into the service; if you did that, then systemd
> would already know the port number that was allocated for the socket.
>
>
I can't; a .socket unit gets activated, and subsequently starts its
corresponding service, when the first connection is made. The primary
purpose of the service is not to serve data over that socket; the socket is
there for scraping metrics.

Thanks though,

/ji

> On Mon, Jun 14, 2021 at 9:17 PM John Ioannidis <systemd-devel at tla.org>
> wrote:
> >
> > I have an instanced service that gets started and stopped by another
> service: alice.service runs the equivalent of systemsctl start
> alice at foo.service, systemctl start alice at bar.service, systemctl stop
> alice at cat.service, and so on.
> > Each of the instanced services runs a little http service so its status
> can be monitored, metrics scraped, etc. The tcp port on which that service
> runs is just whatever the kernel allocated. I want to export that port
> number so other processes can find it and use it, for example, by doing the
> equivalent of systemctl list-units | grep alice@ so they find which
> instances are actually running, and then going about finding the
> corresponding ports.
> >
> > I can think of a number of ad hoc ways:
> >
> > * they can write the port number in a file like /run/alice/foo.port,
> /run/alice/bar.port, and whoever is interested can go read those files, in
> the same way that we use .pid files.
> > * They can use systemd-notify to export it as "Status"
> > * Using a service discovery mechanism would be an overkill, especially
> since whatever is actually talking to those ports is on the same host as
> the services themselves, but that's also a possibility.
> >
> > Is there a systemd-native way of accomplishing this? It would be nice if
> it were possible to have user-defined properties that could be set with
> systemctl set-property, but that is not the case.
> >
> > Thanks
> >
> > /ji
> > _______________________________________________
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> > systemd-devel at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/systemd-devel
>
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