On 04/11/2012 11:02 AM, Eoghan Glynn wrote:
* Inconsistent with other flows. We do not normally update status
fields to perform actions. For example to run a VM, we do not update
it's status to 'activated', we run an action (start).
I think this point is the crux of the matter.
IMO the consistency between activation implemented as direct state
manipulation versus the state change occurring as a side-effect
of an action, would be a deal-breaker.
IIRC one of the reasons we avoided that kind of direct state
manipulation first time round is that it doesn't lend itself to
multi-state transitions, e.g. activation that traversed multiple
intermediate states, say:
quiescent->pending->activating->activated
That example is a bit contrived and unrealistic, but in any case
the principal is that the client-visible state machine for a resource
may not necessarily transition directly from the initial to the terminal
state.
Exactly. The target state is not always indicative of the exact state
transition path. A simpler example is "power off" vs "shutdown". Both
end up with a VM in the "down" state, but the two are actually quite
different.
Also an action as opposed to a direct state manipulation makes it more
natural to express side-effects of the activation, and to implement
in-progress status querying or cancelation.
For those reasons, I'm thinking that using actions consistently accross
the board is better than exposing an action in one case while allowing
a direct state manipulation in another.
So my vote would be for option #2.
One question though (probably to Ori): from the API that you propose, i
assume that disk hotplug is implemented as a two stage process in the
backend?
- add the disk to a VM
- activate it
Because if it's just a one stage process then that would change things.
Then you'd probably want to activate it directly as part of the POST
call that adds the disk.
As an action name i think "hotplug" and "unplug" are slightly more
descriptive of what you actually mean. So my vote would go for those
names instead.
Regards,
Geert
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