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

[John Ioannidis]

On Tue, Jun 15, 2021 at 1:45 AM Mantas Mikulėnas <grawity at gmail.com> wrote:

> 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.
>

I thought of that, but Prometheus cannot scrape AF_UNIX addresses.

Thanks though,

/ji


> 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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>
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