[systemd-devel] Requires and After

[Olaf van der Spek]

On Wed, Jan 2, 2019 at 4:22 AM James Feeney <james at nurealm.net> wrote:
> systemd has two different classes of "dependencies": 1) "activation" dependencies, and 2) "ordering" dependencies.
>
> An activation dependency does not, a priori, have to obey any rules about ordering.  There are not, automatically, any promises or guarantees about in what order service units, for instance, might be queued for execution, based upon a Requires= dependency.
>
> "Ordering" is an independent characteristic from "Activation".  "Activation" only promises to enqueue a unit, and then, only if the unit is some kind of unit that can be "executed", such as a timer or service unit.  In contrast, for instance, systemd is only a "passive observer" of a device unit.  "enqueuing" a device unit for "activation" would make no sense in this context.  A *service* unit that *creates* a device unit could be enqueued for activation, but not the device unit itself.
>
> If "A Requires B", and you don't like that "A" *might* get enqueued, or get executed, before "B", then add an "ordering" dependency.  "Ordering dependencies", then, create guarantees about unit activation *ordering*.

What good is an activation dependency without an ordering dependency?
Activation by itself guarantees basically nothing.

>> . I think a unit in Requires should imply that unit in After too, otherwise the requirement isn't really met. Is there a use case for Requires but not After?

> Are you sure that you were not wondering about "Requisite=", instead of "Requires="?

Yes, I am, though Requisite looks interesting too.

> Because, as far as I know, "Requisite=" is completely broken in systemd.

How is it broken?


-- 
Olaf