Re: [AM] modelling the state of a service

Ian

It seems to me that this is a classic example of where the notion of a
calculated state is useful.

It would be used as follows:

- Each Node would have its own state (perhaps modelled with a state machine
for each node).

- The Service would be modelled with a state machine that has a derived
state, calculated from the states of the Nodes. If the count of nodes that
are down is two (or more) the calculated state of the Service is "service
compromised", otherwise it is "service OK". This state machine determines
what can and cannot happen at the Service level.

Calculated states (which are conceptually just like calculated or derived
attributes) have equal expressive power to guards, but avoid the transition
explosion you describe.

Calculated states are not (unfortunately) supported in any sensible way by
UML.

I can provide more information if you are interested -- let me know
privately.

Rgds
Ashley

For more information about AM, visit the Agile Modeling Home Page at www.agilemodeling.com
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