Complex Event Config

[Scott James Remnant]

Several of the recent threads on this mailing list can be answered with
the phrase "complex event config", I'd like to start a discussion proper
about what we want to achieve and solve some of the current problems.

Background and the current specification can be found here:

https://blueprints.beta.launchpad.net/upstart/+spec/complex-event-config

Note that this has been effectively rejected since it does not solve the
problems as effectively as we might like.

So let us define the problem.


Currently upstart only offers the simplest mechanism of specifying the
events that can change a job's goal to start or stop; a list of events.
If any one of the events listed occurs, a new instance is spawned, the
job goal is changed and the event is not deemed handled until the job
reaches a rest state.

This already tells us the important things about how events and jobs
interact, that the complex-event-config specification must address.

 - when are new instances spawned?
   (currently "when any start event listed is received by the master
    job")

 - when is the job goal changed?
   (currently "when any event listed is received")

 - when is the event deemed handled?
   (currently "when the job reaches a rest state")


Note that the problem of an event being deemed handled does not
necessarily require that the event information be lost; the fact that
events are discarded once handled is a bug, since it prevents a
respawning job from knowing which event started it.

So we should also add:

 - when is the event information discarded?
   (currently "when the event is deemed handled", soon to be "when the
    job goal is changed")


So given the above interactions that must be addressed, what problems
would we like to solve?

I started collecting them here:

	http://upstart.ubuntu.com/wiki/ComplexEventConfig/UseCases

Please feel free to expand on this, and discuss how we'd address the
above interactions for each use case.

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