Tuesday, April 1, 2014

Guard Conditions of UML Decision Nodes

In the beginning of the year I attended a requirements engineering class where we discussed - among other topics - activity diagrams. One element of an activity diagram is the decision node, the diamond shape, where you can branch the flow of activities depending on guarding condition for each outgoing edge. There we stumbled upon one - at least to me - completely new aspect on the guard conditions.

According to the UML specification the guard condition protects the entry of an edge. If the condition is not fulfilled, the edge is no entered. Before the above mentioned discussion I always assumed that an edge without a condition always evaluates to true and is always evaluated at last of all outgoing edged and is therefore entered only if none of the other edges' condition applies. But that is wrong!

The truth is, that according to the UML specification the order of the "processing" of the edges is not determined and therefore may be completely random. So if you define an outgoing edge of a decision node without a guarding condition, it may be the case, the edge is entered although another edge's condition may have been met but would be evaluated after the edge without a condition.

Conclusion is, if you want to have a determined behavior of your activities, always define all of the guard conditions of your outgoing edges of decision nodes.
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