[systemd-devel] Still confused with socket activation

[Simon McVittie]

On Thu, 04 Feb 2021 at 13:07:33 +0100, Reindl Harald wrote:
> "Requires=a.service" combined with "Before=a.service" is contradictory -
> don't you get that?

It means what it says: whenever my service is enabled, a.service must
also be enabled, but my service has to start first (and stop last).

For instance, imagine this scenario:

* my service sets something up that will trigger an action later:
  perhaps it creates a configuration fragment in a.conf.d

* any number of other services might be doing the same as my service

* whenever at least one service has done that, when they have all finished
  doing their setup, we need to start a.service, which will take those
  triggers (e.g. contents of a.conf.d) and "commit" whatever is needed
  for all of them

* if multiple services have set up a triggered action, we only want to
  run a.service once, and it will act on all the triggers as a batch

Then my service should have Requires=a.service (because if a.service
never runs, the "commit" action will never happen and my service will
not do what I wanted); and it should have Before=a.service (because the
triggers need to be in place before a.service processes them).

dpkg and RPM triggers are not (currently!) systemd services, but they
work a lot like this. They're typically used to regenerate caches.

Another reason to combine Requires with Before is that Before is really
a short name for "start before and stop after", and After is really a
short name for "start after and stop before". If you're dealing with
actions taken during system shutdown or session logout, the stop action
might be the one that does the real work, making it more likely that
Before dependencies are the important ones.

    smcv