Dependencies

[Scott James Remnant]

On Sun, 2006-10-08 at 11:43 -0400, Sean E. Russell wrote:

> 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).
> 
Can you provide any actual use cases for #2?  I've had a think and can't
actually think of any.

> 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
> 
Have done so.

Scott
-- 
Have you ever, ever felt like this?
Had strange things happen?  Are you going round the twist?
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