Re: UMLsemantics questions

Hi and thanks for your reply. Please see my comments below. I would be
very interested to hear your ideas on them.

I am confused. B-( If C has an object state machine to describe
behavior, then the only behavior operations would be state machine
actions associated with states (Moore FSM model) or transitions (Mealy
FSM model). Transitions would be triggered by events and events are
generated outside the FSM. (Purists consider self-directed events
generated by a machine's own actions to be a no-no.)

In the UML is it not possible to have the behaviour of an object
specified as both actions on transitions of the object statechart as
well as methods of the object class? For example in Rhapsody, for a
class one can specify event receptions (or signals for asynchronous
comm.), triggered operations (or calls for synchronous comm.), or
primitive operations. The reception of events and triggered operations
are queued by the object statechart and can trigger transitions of the
statechart. The same is not true for primitive operations. I believe
the OMG UML specification (which cannot claim to fully understand) has
similar notions. This has lead me to wonder about the semantics of
receiving a primitve operation call.

Thank you for the remainder of your reply.

Pouria Shaker

On Jan 23, 1:01 pm, "H. S. Lahman" <h.lah...@xxxxxxxxxxx> wrote:
Responding to Pourias...

consider a class "C" with operation "o" and a method to implement this
operation. also suppose that "C" has a statechart that specifies the
reactive behaviour of its instances, but no transitions of this
statechart are triggered by "o".I am confused. B-( If C has an object state machine to describe
behavior, then the only behavior operations would be state machine
actions associated with states (Moore FSM model) or transitions (Mealy
FSM model). Transitions would be triggered by events and events are
generated outside the FSM. (Purists consider self-directed events
generated by a machine's own actions to be a no-no.)

Therefore "o" is either: (A) a behavior responsibility implemented as a
state machine action triggered by a transition or (B) a knowledge
responsibility implemented as a synchronous service to be executed
synchronously. [A synchronous service does not generate events (trigger
transitions) or modify the state machine state.]

if the statechart of an instance of "C" is blocked in the middle of a
run-to-completion step waiting for a synchronous call action to return,
and receives a request from another object to invoke "o", will it
execute the method of "o" or queue the request until the its
run-to-completion step is complete?At the OOA/D level one assumes there is an event queue that controls
when events are consumed. One of the relevant rules the modeler can
assume is that only one event at a time will be consumed for any state
machine. Thus a C state machine will consume only one event at time to
completion of the action triggered by the transition. So if "o" is a
behavior responsibility, it will not get triggered until the current
action completes.

If "o" is a synchronous service and concurrency is possible, then one
needs to provide infrastructure in the concurrency implementation to
ensure that invoking "o" is blocked for other objects while any C event
is being consumed. (An object synchronous service could access the same
object knowledge as the state action so data integrity requires blocking
if "o" is invoked externally.)

However, concurrency is not directly an OOA/D issue. (One can express
parallel processing constraints via Activity Diagrams.) That's because
concurrency implementation is a pure OOP issue since it depends upon
tactical facilities like threads, semaphores, hardware interrupts,
etc..) So one doesn't need to worry about "o" being invoked by another
object.

For example, when using object state machines in the OOA/D one assumes a
single event queue exists in the implementation. (The event queue never
appears explicitly in the OOA/D model.) The rules for that queue ensure
only one event is consumed at a time. If another object is to invoke
"o" it would do so from one of its actions. But the single queue ensure
only one action at a time is executing.

However, a sort of poor man's time slicing can be implemented during OOP
quite efficiently at the action level by providing multiple event queues
to service different groups of object state machines. Now one can have
time slicing by doing a round robin of popping an event from each queue
in turn. Or one can have concurrency on multiple processors by allowing
each queue to pop events in parallel.

If one does concurrency this way, one needs to ensure proper blocking of
knowledge access. One would do that at the OOP level with a set of
semaphores that would pause particular event queues for blocking or
would block invocation of particular synchronous services. One can
pretty much handle that by having the event queues talk to each other as
they consume events.

So all one really has to worry about at the OOA/D level is solving the
asynchronous message problem with proper handshaking to ensure the
objects will be in the right state to accept events. If that
asynchronous communication is provided correctly for the single event
queue assumption, it will be <relatively> easy to map it into a
concurrent environment at OOP time using threads, multiple event queues,
and whatnot. [It is also crucial that one distinguishes between state
actions (behavior) and synchronous services (knowledge access).
Limiting synchronous services to knowledge access allows one to map
blocking scope to action scope, which makes data integrity a /lot/ easier.]

*************
There is nothing wrong with me that could
not be cured by a capful of Drano.

H. S. Lahman
h...@xxxxxxxxxxxxxxxxx
Pathfinder Solutionshttp://www.pathfindermda.com
blog:http://pathfinderpeople.blogs.com/hslahman
"Model-Based Translation: The Next Step in Agile Development". Email
i...@xxxxxxxxxxxxxxxxx for your copy.
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