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Subject: Re: [soa-rm-ra] some additional considerations in the servicedescription write-up


I agree. I do not think that dependencies are the same as preconditions -- although I guess that you can frame a dependency as a precondition.

To me, dependencies hints at resource dependencies: to use the airline reservation service, the airline must be solvent :)

There is inherently some guessing in determining reachability: just because it is here today does not mean it will not be gone tomorrow.

I thought that we looked at this in the RM

Frank

On Sep 5, 2008, at 2:40 PM, Ken Laskey wrote:

As I try to make use of the service description content, I've found a few things that could probably use more attention.

- The model shows the Process Model has Dependencies and the Action Model has Preconditions.  The PR1 text has these as being analogous but the difference is not clear.  I would propose the following:
  -- Leave the text on Preconditions (lines 1321-1325 in PR1) as is

      An action may have preconditions where a Precondition is something that needs to be in place before an action can occur, e.g. confirmation of a precursor action.  Whether preconditions are satisfied is evaluated when someone tries to perform the action and not before. Presence for an action means someone can initiate it and is independent of whether the preconditions are satisfied.  However, the successful completion of the action may depend on whether its preconditions were satisfied.

  -- Reconsider Dependencies (lines 1331-1336 in PR1)

   Analogous to the relationship between actions and preconditions, the Process Model may imply Dependencies for the indicated process to proceed.  Whereas a precondition applies to individual actions whenever the action is performed, the dependencies on a process model indicate  conditions that are required by the process as a whole.  For example, a process that includes use of simulation resources may require a valid account against which the costs of the simulation can be billed.  This may not apply to any particular action comprising the process, but constrains the process as a whole.  Dependencies related to the process model do not affect the presence of a service although these may affect whether the business function successfully completes.

- Section 4.1.1.2.2 talks about Reachability, and the last paragraph (lines 1177-1181 in PR1) discusses service presence.  What also needs to be noted is that presence is a prerequisite for reachability but it isn't enough -- a service may have presence but the prospective consumer loses connectivity, and so the consumer loses reachability.  A ping demonstrates testing presence and reachability; an average service availability only conveys how often the service is here and not how often the consumer can get to it.  I'm not sure what I want to say on this in the RA, but I think it is worth noting what should be the obvious but had previously eluded me.

Ken

------------------------------------------------------------------------------------------
Ken Laskey
MITRE Corporation, M/S H305     phone:  703-983-7934
7515 Colshire Drive                        fax:        703-983-1379
McLean VA 22102-7508


smime.p7s



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