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

[Mantas Mikulėnas]

If you only care about processes on the same system – why not put the
actual socket in /run, as an AF_UNIX socket? That's mostly what /run is for.

On Tue, Jun 15, 2021, 04:18 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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