Dependencies

[Sean E. Russell]

On Sunday 08 October 2006 11:02, you wrote:
> > Well, Cups was just an example.  The Gentoo init scripts are full of
> > examples of services which have conditional requirements.  The idea is
> > that there's a mechanism for saying "if this service is going to be
> > started, then wait for it to start.  Otherwise, just start now."
>
> Could be an interesting problem mapping to an event-driven scenario, how
> do you know that a service is "going to be started later" ?

You've got a good point.  Since Upstart is an indeterminate system, it is 
possible that a service could be available, but never started because the 
correct event (network/started, say) was never generated.

There are three cases I can think of:

	0) A job doesn't exist. This is the simple case -- if there's no job 
in /etc/events.d and another job has it as a conditional dependency, then 
Upstart simply removes the dependency.
	1) A job fails to start (say, network) and generates a job-failed event.  In 
this case, Upstart would "satisfy" the requirement if it is conditional.
	2) A job goes into a perpetual "wait" state.  The only way these could be 
handled is by time-out, but that's a really bad solution.

#2 is the show-stopper, and may be sufficient to prove that Upstart (or any 
event-driven design) can't be made handle conditional dependencies without 
ugly hacks (timeouts).


Finally, would somebody with write access to the main Upstart page please add 
this link to the page? I keep losing it:

	https://wiki.ubuntu.com/UpstartDesignChanges

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