[occi-wg] OCCI MC - State Machine Diagram
Andre Merzky
andre at merzky.net
Wed May 13 05:09:34 CDT 2009
Hi all,
I would like to follow up on the state model thread, even if
people seem to have mostly agreed upon the version
documented in the wiki (http://forge.ogf.org/short/occi-wg/states)
I am really unhappy with a number of things:
- too many states
- too many state transitions with no actions (i.e.
methods) attached
- unclear entry and exit points
Please allow me to elaborate
(1) Too many states:
All the XXX-ING states (STARTING, SUSPENDING, RESUMING,
STOPPING) seem to have no real benefit, apart from
informing the client that a state transition is in
progress. State transitions, however, should, IMHO, not be
part of a state diagram, unless specific actions can be
performed during that transition.
For example the STOPPING state: there are no actions
attached to that state, the client can do nothing but wait
until the state reached STOPPED.
So, why not going to STOPPED immediately, and adding the
transition process to the semantics of the STOPPED state?
(e.g., "resources can still be utilized iff they are
required to store or retrieve the VM state").
(2) Too many internal state transitions
That is related to (1) of course: the diagram has 11
internal state transitions (no actions attached), and 5
external transitions (actions attached)....
Well, that is not exactly incorrect or anything, but it is
confusing matters. A state model should help to clarify
what actions cause what state change.
(3) unclear entry and exit points
There is no state where the client can be sure that no
remote resources are utilized anymore - i.e., an
'Destroyed' state is missing. At some point, a client
needs to have the ability to free remote resources (well,
unless one has garbage collection, but we don't want to go
there, right?)
STOPPED is not really the same - a STOPPED state can be
moved to STARTING/ACTIVE via start(), so the backend cannot
be allowed to free the resources.
I do understand that the XXX-ING states are useful for
informative purposes, for example, to allow a GUI to inform
the user that a start() request has been received, and the
backend is spinning up the machine. Those types of
information can, however, easily provided by additional
attributes, or sub-states.
Attached is an alternate state diagram. It's main purpose
is to illustrate that, by limiting the number of states and
internal state transitions, one can derive a much simplier
(and IMHO cleaner) state diagram.
Cheers, Andre.
PS.: Sorry for not using graffle. Sources are in xfig, so I
don't attach them - don't want to get laughed at for using
such ancient technology ;-)
Quoting [Edmonds, AndrewX] (Apr 20 2009):
>
> Updated...
>
> -----Original Message-----
>
> I agree - this is fine with me.
>
> On Mon, 2009-04-20 at 14:02 +0100, Edmonds, AndrewX wrote:
> > Sounds like a good comprise J
> >
> >
> >
> > From: Sam Johnston [mailto:samj at samj.net]
> > Sent: 20 April 2009 13:52
> > To: Edmonds, AndrewX
> > Cc: Thijs.Metsch at Sun.COM; occi-wg at ogf.org; Molino, VictorX M
> > Subject: Re: [occi-wg] OCCI MC - State Machine Diagram
> >
> >
> >
> >
> > I'd tend to side with Thijs on this one - having a complete state
> > machine is a lot less important when the implementation can tell its
> > clients what the next states are (and the humans what they mean in
> > plain $LANGUAGE). That said, if you want to add a dotted loop back to
> > the start then that might help the perfectionists sleep easy.
> >
> > Sam
> >
> > On Mon, Apr 20, 2009 at 2:47 PM, Edmonds, AndrewX
> > <andrewx.edmonds at intel.com> wrote:
> >
> > Then conceptually, how do we recover from an error state if we've
> > exited? Also if we want to support error state recovery, should the
> > state model reflect this?
> >
> >
> > -----Original Message-----
> > From: Thijs.Metsch at Sun.COM [mailto:Thijs.Metsch at Sun.COM]
> > Sent: 20 April 2009 13:42
> > To: Sam Johnston
> > Cc: Edmonds, AndrewX; occi-wg at ogf.org; Molino, VictorX M
> > Subject: Re: [occi-wg] OCCI MC - State Machine Diagram
> >
> > As I stated before maybe we an add another exit-point instead of a
> > real
> > state.
> >
> > Cheers,
> >
> > -Thijs
> >
> >
> > On Mon, 2009-04-20 at 14:39 +0200, Sam Johnston wrote:
> > > On Mon, Apr 20, 2009 at 2:29 PM, Edmonds, AndrewX
> > > <andrewx.edmonds at intel.com> wrote:
> > > I updated the state model (attached) to include an error
> > > state. Couldn???t see how to delete attachments so maybe
> > someone
> > > here has better luck? I also began to add identifiers to
> > each
> > > process transition but the diagram started to become
> > cluttered
> > > so left them out.
> > >
> > >
> > > I would suggest that it's better to keep this clean (though I'm
> > > impressed that you got as far as you did!). You can reach an error
> > > state from anywhere - if a machine is found to be corrupt even while
> > > stopped then a STOPPED->ERROR transition makes sense for example.
> > >
> > > Sam
> > >
> > > From: occi-wg-bounces at ogf.org
> > [mailto:occi-wg-bounces at ogf.org]
> > > On Behalf Of Sam Johnston
> > > Sent: 20 April 2009 11:56
> > > To: Tino Vazquez
> > > Cc: occi-wg at ogf.org; Molino, VictorX M; Thijs Metsch
> > >
> > >
> > > Subject: Re: [occi-wg] OCCI MC - State Machine Diagram
> > >
> > >
> > > Hi Tino,
> > >
> > >
> > >
> > >
> > > A lot of this is covered in the state registry (which I'll
> > > copy below for you) but comments inline nonetheless.
> > >
> > > On Mon, Apr 20, 2009 at 12:35 PM, Tino Vazquez
> > > <tinova at fdi.ucm.es> wrote:
> > >
> > > Howdy everyone,
> > >
> > > Excellent thread. My three cents:
> > >
> > > 1) I think we should define clearly the semantics of the
> > > states. for
> > > instance, what is the difference between STOPPED and
> > > SUSPENDED? Is it
> > > that with SUSPENDED the state is saved and not with STOPPED?
> > >
> > >
> > > Yes, it's exactly that. From the state registry: STOPPED =
> > > "The resource is inactive and has no saved state" and
> > > SUSPENDED="The resource is inactive and has saved state"
> > >
> > >
> > >
> > > 2) I really think we need an entry state like
> > > "PENDING" or "DEFINED".
> > > It will help in implementation relying on a
> > > best-effort scheduler to
> > > match VMs and hosts, like EC2 does. This will be the
> > > state where
> > > machines will wait for a host to be available to run
> > > on. Also, I don't
> > > really think that a machine entering its life cycle
> > in
> > > SUSPENDED state
> > > is a good idea.
> > >
> > >
> > > I tend to agree, but I'd like the terminology to be
> > completely
> > > unambiguous... something like "NEW" [for this AS].
> > >
> > >
> > >
> > > 3) +1 to the "CRASHED", "ERROR" or "FAILED" state.
> > >
> > >
> > > ABORT[ING|ED] = "The resource encountered an error and is
> > > aborting/has aborted".
> > >
> > >
> > > What do you think?
> > >
> > >
> > > Some of these transitions take a while so some way of
> > > indicating progress (especially interesting for long tasks
> > > like live migrations) would be useful. Would prefer a
> > > mechanism that worked universally for the API.
> > >
> > > Sam
> > >
> > >
> > >
> > >
> > > Extensions
> > > State control
> > > State
> > >
> > >
> > > Transitions
> > >
> > >
> > > Description
> > >
> > >
> > > aborting
> > >
> > >
> > > aborted
> > >
> > >
> > > The resource
> > > encountered an error
> > > and is aborting
> > >
> > >
> > > aborted
> > >
> > >
> > > n/a
> > >
> > >
> > > The resource
> > > encountered an error
> > > and has aborted
> > >
> > >
> > > active
> > >
> > >
> > > pause, restart,
> > > stop, suspend
> > >
> > >
> > > The resource is
> > > active
> > >
> > >
> > > resuming
> > >
> > >
> > > aborting, active
> > >
> > >
> > > The resource is
> > > becoming active and
> > > restoring state
> > >
> > >
> > > pausing
> > >
> > >
> > > aborting, paused
> > >
> > >
> > > The resource is
> > > preparing to refuse
> > > new requests
> > >
> > >
> > > paused
> > >
> > >
> > > aborting, resume
> > >
> > >
> > > The resource is
> > > refusing new
> > > requests
> > >
> > >
> > > starting
> > >
> > >
> > > aborting, active
> > >
> > >
> > > The resource is
> > > becoming active
> > >
> > >
> > > stopped
> > >
> > >
> > > start
> > >
> > >
> > > The resource is
> > > inactive and has no
> > > saved state
> > >
> > >
> > > stopping
> > >
> > >
> > > stopped, aborting
> > >
> > >
> > > The resource is
> > > becoming inactive
> > > and destroying state
> > >
> > >
> > > suspended
> > >
> > >
> > > resume, stop
> > >
> > >
> > > The resource is
> > > inactive and has
> > > saved state
> > >
> > >
> > >
> > > Note: Stable states and user transitions in bold.
> > >
> > >
> > >
> > >
> > >
> > -------------------------------------------------------------
> > > Intel Ireland Limited (Branch)
> > > Collinstown Industrial Park, Leixlip, County Kildare,
> > Ireland
> > > Registered Number: E902934
> > >
> > > This e-mail and any attachments may contain confidential
> > material for
> > > the sole use of the intended recipient(s). Any review or
> > distribution
> > > by others is strictly prohibited. If you are not the
> > intended
> > > recipient, please contact the sender and delete all copies.
> > >
> > > _______________________________________________
> > > occi-wg mailing list
> > > occi-wg at ogf.org
> > > http://www.ogf.org/mailman/listinfo/occi-wg
> > --
> > Thijs Metsch Tel: +49 (0)941 3075-122 (x60122)
> > http://blogs.sun.com/intheclouds
> > Software Engineer Grid Computing
> > Sun Microsystems GmbH
> > Dr.-Leo-Ritter-Str. 7 mailto:thijs.metsch at sun.com
> > D-93049 Regensburg http://www.sun.com
> >
> > -------------------------------------------------------------
> > Intel Ireland Limited (Branch)
> > Collinstown Industrial Park, Leixlip, County Kildare, Ireland
> > Registered Number: E902934
> >
> > This e-mail and any attachments may contain confidential material for
> > the sole use of the intended recipient(s). Any review or distribution
> > by others is strictly prohibited. If you are not the intended
> > recipient, please contact the sender and delete all copies.
> >
> >
> >
> >
> >
> > -------------------------------------------------------------
> > Intel Ireland Limited (Branch)
> > Collinstown Industrial Park, Leixlip, County Kildare, Ireland
> > Registered Number: E902934
> >
> > This e-mail and any attachments may contain confidential material for
> > the sole use of the intended recipient(s). Any review or distribution
> > by others is strictly prohibited. If you are not the intended
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--
Nothing is ever easy.
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