[systemd-devel] Service activation

[Mantas Mikulėnas]

On Sun, Feb 13, 2022 at 1:09 PM Wols Lists <antlists at youngman.org.uk> wrote:

> On 13/02/2022 09:54, Mantas Mikulėnas wrote:
> > On Sun, Feb 13, 2022 at 2:03 AM Wol <antlists at youngman.org.uk
> > <mailto:antlists at youngman.org.uk>> wrote:
> >
> >     More fun getting things to work ... :-)
> >
> >     So I've got a service, scarletdme.service, which fires up my db
> backend
> >     for running interactively. However, I also need a socket service for
> >     remote connections.
> >
> >     I've got the xinetd files, but if I'm running systemd, I want to use
> >     systemd :-)
> >
> >     So I've written scarletdme.socket, and scarletdme at .service, but the
> >     more
> >     I read, the more I don't understand ...
> >
> >     Do I enable scarletdme.socket the same as anything else eg "systemctl
> >     enable scarletdme.socket"? How does it know the difference between
> >     scarletdme.service and scarletdme at .service? I get the impression I
> need
> >     to put something in the .socket file to make it use scarletdme@
> rather
> >     than scarletdme?
> >
> >
> > If it's a 'nowait' socket (which is "[Socket] Accept=yes" in systemd
> > terms), then it will use the templated @.service, starting a new
> > instance for each "accepted" socket (i.e. instance per connection). See
> > oidentd.socket for comparison.
> >
> > Otherwise (by default) it uses the non-templated service and directly
> > gives it the "listening" socket, letting the service itself handle
> accept().
> >
> ??? Sorry, that's double dutch to me.
>
> Are you telling me that just copying the files into /lib/systemd/system
> will enable them? That seems weird to me because it doesn't do it for
> normal services afaik. (Or shouldn't I be copying it direct into
> /lib/systemd/system? I just don't know ...)
>

No, I was not talking about any of that. You asked how systemd knows the
difference between dme.service and dme at .service.

Sockets (and other units) are "started" and/or "enabled" the same way
services are. If you want a service to launch its process right now, you
`systemctl start` it. Likewise, if you want a .socket to *start listening*
right now, you `systemctl start` it, and if you want a .timer to start
scheduling right now, you `systemctl start` it.

You use `systemctl enable` if you want a unit to be automatically started *at
boot time *– or whatever else its [Install] section says. The "enable"
command sets up dependency symlinks from the [Install] section, where you
have "WantedBy=sockets.target" or similar. (If there's no [Install] section
yet, then `systemctl enable` will do nothing.)

Now whether dme.socket uses dme.service vs dme at .service has absolutely no
relationship to any of the above.

-- 
Mantas Mikulėnas
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