FUML11 — Having a fork node as initial enabled node does not work

If a fork node is identified as initial enabled node in the method ActivityNodeActivationGroup.run(ActivityNodeActivationList), and the method ActivityNodeActivation.receiveOffer() is called for the ForkNodeActivation, the method ForkNodeActivation.fire(TokenList) does not produce any forked tokens (because there is no real incoming token for the fork node) and because of this, the method call this.sendOffers(forkedTokens) (last statement of the method ForkNodeActivationActivation.fire(TokenList)) does not result in offers sent to successor nodes but this should be possible.

A fork node is required to have a single incoming edge (this is Constraint [1] under Subclause 12.3.30 ForkNode in the UML Superstructure spec, constraints from which also apply to fUML). The intent is that a fork node only generates forked tokens based on tokens offered to it on that one incoming edge.

More specifically, consider that the only way a fork node can be initially enabled (in fUML) is if the source for its one incoming edge is either outside the activity node activation group containing the fork node activation or it is in the same activation group but not yet activated.

The first case occurs when the fork node is contained a structured node but is the target of an edge crossing into the structured node from a source outside it. In this case, receiveOffer is called on the (enabled) fork node activation when the structured node fires, and this operation in turn calls takeOfferedTokens, which accepts any tokens offered on the incoming edge into the fork node. If tokens have been offered on that edge previously to the structured node firing, then the fork node activation will create forked tokens for them and offer them on the outgoing edges from the fork node. On the other hand, if no tokens have been offered, then the fork node activation does nothing further, which is correct.

The second case occurs when the fork node is part of a conditional node or a loop node and the source of the incoming edge is an action or pin within the same node. The executable nodes for a conditional or loop node are divided up into test and body parts, which are activated incrementally per the semantics of the containing nodes. Contained control nodes, however, are always activated unconditionally (or, in the case of a loop, on each iteration). If a fork node is thus activated before the source of its incoming edge is, then it is not possible for anything to have been offered on that edge yet and, therefore, it is correct that the fork node should not offer anything on its outgoing edges. If that source node is later activated and does eventually offer something on the edge to the fork node, then that will trigger another call to receiveOffer on the fork node activation, which will result in forked nodes being offered on outgoing edges as appropriate.

So, the current semantics for fork nodes are actually correct.

Revised Text:
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Disposition: Closed, No Change