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

[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 :)