On 12/11/2012 06:41 AM, Martyn Taylor wrote:
Gentlemen,
We discussed how to model state transitions in the ReST API in the
Tech Cabal and also offline for those who had particularly strong
views. One of the objections brought forward was how to distinguish
between clients (end users or applications) requesting state changes.
For example: A developer decides he wants to terminate his running
instances and system services that are used to notify conductor of
when an instance has changed state. For example db-o-matic wants to
tell conductor that the instance has now started.
After some deliberation offline between myself and sseago and without
objections from the Tech Cabal. We decided that the services and the
end users should access different APIs and we should not add
complexity into the end user API in order to support these system
services.
So this matches what we'd agreed to fairly closely. I just have a minor
comment around the wording. I think the distinction here is not so much
'end users' vs. 'services' but rather 'making state changes with the
intent of making the resource state reflect that change' vs. 'fixing the
state in the conductor database when the state in conductor does not
match the state of the underlying resource' (i.e. the conductor model of
the instance vs. the actual state of the instance in ec2) End users, of
course, would only need access to the former sort of state change.
Services, however, might need access to both. Essentially, one way of
making state changes is the normal state change API that both end users
and services may make use of. The second way is the lower-level
highly-privileged API that essentially only services should have access to.
Scott
With this behind us. We then were able to move forward to make a
decision on how to handle requests from a user to transition between
states. Like in the developer example above.
It was agreed that treating the State on an instance as simply an
attribute on the resource (and not an action) was more inline with the
ReST principal (Since adding actions breaks ReST principal of having
a unified interface - in our case the unified interface is defined by
HTTP).
So, this now brings us to how we should represent these resources. I
think this is something that can and should be decided on list. I have
included below my opinion on how this should be represented, please
add in your own comments.
==
So in my opinion we should model a state machine as an independent
resource. When client wants to change the state of an instance, it
looks at the state machine to figure out what are the possible states
I can move to now. The instance can hold a reference to it's current
state and the state machine.
A simple example here: https://gist.github.com/4257951
Regards
Martyn
