[occi-wg] OCCI MC - State Machine Diagram
Sam Johnston
samj at samj.net
Thu May 14 06:24:27 CDT 2009
On Thu, May 14, 2009 at 12:52 PM, Roger Menday
<roger.menday at uk.fujitsu.com>wrote:
>
> On 14 May 2009, at 10:59, Alexis Richardson wrote:
>
> +1 to Sam's "we may need to revisit this point in the name of interop"
>>
>
> I'm not sure if this is *just* an interop thing ...
>
> I thought my suggestions yesterday on how to transition state, error
> reporting, handling 'processing' states, etc ... were reasonable.
>
> Kind of disappointed this morning that I didn't get some feedback from you
> guys ... :(
>
Roger, I was working on OCCI until 5am this morning and while this is by far
the most interesting part of the work it's only half of the problem. The
other half, adoption/marketing, is boring grunt work that keeps us
organisers very much on our toes, preventing us from being responsive at
times.
In particular the absence of consensus around formats has put me in a fairly
awkward position for my scheduled talk at Prague on Tuesday (which was due
yesterday) and is jeopardising previously agreed deadlines that I have been
advertising heavily in my own name. It doesn't help that I don't really
share your concerns about states being a problem and am confident both that
what we have will work and that it will invariably be refined in due course.
Thanks for your understanding - I think you would be surprised to see how
much behind-the-scenes work goes on in constantly driving this kind of
initiative forward, which is why one has to be 100% committed to, and
believe in, the cause.
Sam
>
>
>> At this stage we are shooting for a draft. The draft will let people
>> implement prototypes which will let us debug interop and refine the
>> model.
>>
>>
>> On Thu, May 14, 2009 at 10:55 AM, Sam Johnston <samj at samj.net> wrote:
>>
>>> On Thu, May 14, 2009 at 10:23 AM, Andre Merzky <andre at merzky.net> wrote:
>>>
>>>>
>>>> Quoting [Sam Johnston] (May 13 2009):
>>>>
>>>>>
>>>>> Yes, me, I don't think HATEOAS should be applied in this
>>>>>> context. But I realise/accept that I maybe the only one
>>>>>> with that opinion - thats ok. So I'll say it here one last
>>>>>> time, for the record, and then will shut up: "a static
>>>>>> simple state model allows for very simple clients.
>>>>>> Extensions can be defined via substates, or additional
>>>>>> transitions."
>>>>>>
>>>>>
>>>>> I would counterargue that HATEOAS allows for even simpler clients
>>>>> because they don't have to worry about hardwiring even a simple state
>>>>> model. Using HTTP we can even feed them plain $LANG descriptions of
>>>>> what the transitions and targets are - it doesn't get any easier than
>>>>> that and you don't have to worry about updating clients to implement
>>>>> new goodies.
>>>>>
>>>>
>>>> I don't see that. If I want to write a client tool which
>>>> starts a resource, I want to make sure the resource is in
>>>> RUNNING state when the client reports success. But if that
>>>> client (a) has to infer the available states from a
>>>> registry, it cannot posisbly know which state has the
>>>> semantic meaning of RUNNING attached. Further (b), if the
>>>> client only sees those state transitions it is allowed in
>>>> its current state, how does it know what transition path to
>>>> take to reach that target state? Is it (I am making those
>>>> up obviously):
>>>>
>>>> INITIAL -> create() -> CREATED -> elevate() -> ELEVATED () -> run() ->
>>>> RUNNING
>>>>
>>>> or
>>>>
>>>> INITIAL -> create() -> CREATED -> init() -> INITIALIZED -> run() ->
>>>> RUNNING
>>>>
>>>> Or should the tool simply fail because it cannot see a run()
>>>> transition in its INITIAL state?
>>>>
>>>
>>> The client must at least know how to create a resource and when it has
>>> done
>>> so successfully a "start" actuator will appear, perhaps with a target
>>> state
>>> of "running" (TBD). In that case it knows that if it pulls the "start"
>>> handle eventually the resource should end up "running". Otherwise it
>>> could
>>> know (from the registry) that "start" is the right button to push, but
>>> that's starting to break HATEOAS principles. We have options - it's just
>>> a
>>> matter of finding the right one.
>>>
>>>
>>>> I think HATEOAS works pretty well if a human is in the loop
>>>> who can parse the available transition description, and
>>>> deduce a semantic meaning. I don't think it makes for
>>>> simple tooling, really.
>>>>
>>>
>>> I agree that humans are better at this stuff than computers but I'm
>>> unconvinced this translates to complex tooling.
>>>
>>>
>>>> Then again, I may misunderstand the proposed usage of
>>>> HATEOAS in OCCI. So, can you help me out: what mechanism
>>>> will avoid the confusion from the example above, if a vendor
>>>> can provide init() and elevate() transitions on the fly,
>>>> with no predefined semantics attached? How would my tool
>>>> deduce the transition path it needs to enact?
>>>>
>>>
>>> The semantics for common functions will be in the registry. It's ones
>>> that
>>> are uncommon and impossible to predict like "translate" and "migrate"
>>> that
>>> we're catering for here, and generally there will need to be some kind of
>>> client side support for these.
>>>
>>> As I said below, "we may need to revisit this point in the name of
>>> interop",
>>> and I suggested categories as one possible solution (e.g. a "starting" vs
>>> a
>>> "stopping" transition)... parametrised transition calls are another...
>>> for
>>> example, how do I tell something to start *without* saved state if saved
>>> state is present (ala cold start vs resume)?
>>>
>>> Sam
>>>
>>>
>>>> I don't think anyone knows every possible thing that users are going
>>>>> to
>>>>> want to do with the API (I certainly don't have the confidence to say
>>>>> I
>>>>> do anyway) but we may need to revisit this point in the name of
>>>>> interop... Atom categories would be one way to achieve this (e.g.
>>>>> "Cold
>>>>> Reboot" and "Warm Reboot" might go in the "restart" category).
>>>>> Sam
>>>>>
>>>>> References
>>>>>
>>>>> 1. mailto:andre at merzky.net
>>>>>
>>>>
>>>>
>>>>
>>>> --
>>>> Nothing is ever easy.
>>>>
>>>
>>>
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>>>
>>>
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>
>
>
> Roger Menday (PhD)
> <roger.menday at uk.fujitsu.com>
>
> Senior Researcher, Fujitsu Laboratories of Europe Limited
> Hayes Park Central, Hayes End Road, Hayes, Middlesex, UB4 8FE, U.K.
> Tel: +44 (0) 208 606 4534
>
>
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