[Engine-devel] Hot-plug of disks and nics

[Geert Jansen]

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