Control Flow vs. Object Flow

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Michael Campbell

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Jun 11, 2018, 4:25:52 PM6/11/18
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Hi all, 

Just trying to learn some of these patterns to understand the need for both. 

Why would we need a control flow?

Can the control flow and object flow overlap? 

Thanks. 

FAUDOU raphael

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Jun 11, 2018, 8:45:20 PM6/11/18
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Hi Michael,

you need a control flow when you want to express a precedence constraint between two action nodes. 
Those actions may represent process, algorithm or function steps.
Control flow provides explicit execution logics. Without it you cannot simulate your activity diagram except you get simple data flow without loop: in that case data flows provide implicit logics (data produced before it is consumed) and can sometimes be enough to support simulation.
best regards,
raphael
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Remy Fannader

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Jun 11, 2018, 8:45:24 PM6/11/18
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Two different semantics: control flows induce some change of state (objects, activity, expectation), object flows are neutral. So a control flow can also carry objects, but object flows are just that.

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James Towers

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Jun 12, 2018, 5:58:35 PM6/12/18
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I do not believe this is correct. My understanding is that control flows may only carry control tokens. 

The reason for having  them is that not all activities require input object tokens, and so do not have incoming object flows. Additionally not all activities ‘start’ on the presentation of input object tokens, but rather need separate control flows - a pump may not ‘pump’ simply because there is water at it’s input (object flow) but only when there is water at the input and it has received a ‘start pumping’ signal (control flow).


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Remy Fannader

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Jun 13, 2018, 1:08:03 AM6/13/18
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James, how to represent a request with an object as parameter then ?

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James Towers

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Jun 15, 2018, 8:24:15 PM6/15/18
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Well this depends on context, but assuming we are talking about activities (on which the original question was based) then an object-flow is sufficient. If this is the only incoming edge to the action, then the activity will 'execute' once the minimum number of object-tokens arrive at the incoming object-node, therefor also passing 'control' to the action. 




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Remy Fannader

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Jun 17, 2018, 1:49:02 AM6/17/18
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That's called escapism: you simply assume that there is no need to represent a request (control) can be sent with an object. Good luck with users that want to do that.

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James Towers

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Jun 17, 2018, 10:33:12 PM6/17/18
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Not at all. A control flow has no objects, just events (the arrival of tokens), an object flow has by definition object tokens and events (the arrival of those tokens). So you can model:

1) Events alone (Control Flows)
2) Events with Objects (Object Flows) <- Which is the scenario you describe.
3) Combinations of the two - thus allowing control to transfer on a different event to the arrival of the object token(s). 

The only thing not covered by these two is the availability of objects without events, but then that's not a flow. Object's which are always available (values) would be properties of the owning classifier (in this case probably a Block)

I concede SysML doesn't use particularly intuitive names when it comes to Control Flow and Object Flow (since both actually influence the transition of control), but the semantics you describe are available, and actually you could model everything just using Object Flows if you defined your own Control Token Block.

Remy Fannader

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Jun 18, 2018, 9:56:10 PM6/18/18
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"Event" meaning some change of state, "event with object" is by definition a control flow. There should be no argument here because it's not a matter of opinion but of semantic consistency.

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