[systemd-devel] 'tasks' as first-order objects?

[Bill Nottingham]

Lennart Poettering (lennart at poettering.net) said: 
> > upstart jobs have two main types - 'services' and 'tasks'. systemd, while it
> > has many other types, doesn't have a 'task' analogue. So, for anything that
> > has the semantics of a task (at this time/dependency, do this thing and
> > exit), ends up being encoded as a service of type 'finish', often
> > with 'ValidNoProcess=yes'.
> 
> Well, "Type=finish" is supposed to be exactly what Upstart calls
> "tasks". I must admit that the word "finish" for this sucks, so we are
> open to change this to make it more discoverable. However, I am not sure
> "task" is a good choice of names here either, since the low-level stuff
> we work with here already uses "tasks" for something else (i.e. as a
> generalization of processes and threads in the kernel). I wouldn't wan't
> to add confusion here.

Will those that are interacting with the unit files be the same set that
will interact with the 'task' low-level infrastructure?

If not, it might be OK to use the same verbiage.

> So, maybe "Type=transient" is nicer? Sounds a bit scientific but this
> word is not unheard in the Free software world at least (i.e. X uses
> that iirc) and describes pretty accurately what these kind of processes
> are used for, i.e. transient, temporary, short-running processes? Other
> suggestions?

'function'
'oneshot'
'single'
'not-respawning'

(None of these are necessarily good.)

> Two Examples: the audit policy loading is something for Type=finish,
> since the process "audictl" which does this starts and ends during boot
> and does not stay around during system runtime. You probably want to run
> it before you run auditd and as a dependency of it, but it probably
> makes sense to run it again if auditd terminates and you restart it
> later on. So this would be something to set to "Type=finish" and
> "ValidNoProcess=no". And it would *not* show up as "active" in systemctl
> during normal system runtime, because when the processes exited the
> service in systemd went away too.

I'd think you'd just run that as ExecStartPre in your auditd service.
Perhaps that's not the best example?

> So, I guess what I want to say is that processes have different
> semantics, and there is not clear dichotomy available. And because of
> that I'd wrap all this in one unit, and then differentiate them via
> Type= into the different styles. 
> 
> Does that make sense to you?

I think it might still make sense to separate them. You'd have other
benefits by distinguishing them:

- Separate defaults that make sense (Restart=on-success for services,
  Restart=once (or not even a valid option) for tasks)
- Separated reporting, if you wanted (the random example above could
  be considered as 'finished' instead of 'active/exited'.) This
  may not fit fully into how systemd is organized.

> (Oh, and if you have a better suggestion for a name of ValidNoProcess= I
> am all ears too!)

Well, the neat thing about splitting off task and service like this
is that that key goes away.

Bill