Open Stack

Sounds similar to https://review.openstack.org/#/c/224022/ (which is the 
ironic version of expose state machine transitions over the REST API); 
probably useful to read over the review commentary there and/or talk to 
the ironic folks about that before doing much here (to learn some of the 
pros/cons and such).

Andrew Laski wrote:
>
> On Wed, Jun 1, 2016, at 05:51 AM, Miles Gould wrote:
>> On 31/05/16 21:03, Timofei Durakov wrote:
>>> there is blueprint[1] that was approved during Liberty and resubmitted
>>> to Newton(with spec[2]).
>>> The idea is to define state machines for operations as live-migration,
>>> resize, etc. and to deal with them operation states.
>> +1 to introducing an explicit state machine - IME they make complex
>> logic much easier to reason about. However, think carefully about how
>> you'll make changes to that state machine later. In Ironic, this is an
>> ongoing problem: every time we change the state machine, we have to
>> decide whether to lie to older clients (and if so, what lie to tell
>> them), or whether to present them with the truth (and if so, how badly
>> they'll break). AIUI this would be a much smaller problem if we'd
>> considered this possibility carefully at the beginning.
>
> This is a great point. I think most people have an implicit assumption
> that the state machine will be exposed to end users via the API. I would
> like to avoid that for exactly the reason you've mentioned. Of course
> we'll want to expose something to users but whatever that is should be
> loosely coupled with the internal states that actually drive the system.
>
>
>> Miles
>>
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