[systemd-devel] Examples to distinguish Before=/After= and Wants=/Requires=/BindsTo=

[Manuel Wagesreither]

Am Mi, 15. Sep 2021, um 19:23, schrieb Andrei Borzenkov:
> On 15.09.2021 18:15, Manuel Wagesreither wrote:
> > Hello all,
> > 
> > I'm onboarding some collegues who don't have much experience with systemd. One thing I would like to focus on is the difference between Before=/After= and Wants=/Requires=/BindsTo in systemd units.
> > 
> > I think it would get immediately clear if could provide them an example where we want one but not the other. Unfortunately I've got problems coming up with such an example. In my use cases, whenever I needed an After= I needed an Wants= as well.
> > 
> > Can you come up with something good?
> > 
> 
> No. B After A means - select start job for B for execution after start
> job for A completes. This directive is only meaningful if you guarantee
> that both start jobs are present in the queue. And the only way to
> ensure it is to use Wants (or Requires as variant).
> 
> If service B really needs service A to be started before it itself can
> be started, you must use After and Wants, otherwise you are open to race
> conditions. And if service B does not care if service A is started, then
> you do not need After in the first place.
> 
> Of course one can try to ensure that all start jobs are present in queue
> by some external means. Like making all units to be WantedBy some
> super-unit which is called ... default.target (surprise). But that does
> not change underlying requirement, just restricts working case to
> starting one single unit. Presumably on system boot :)
> 
I understand. Still, I'm looking for giving the people I teach an example why it made sense to implement it this way. Something else than just saying "that's just how it is, just accept it".