ALF — Enforcement of multiplicity

[Sridhar Iyengar] 20. In a number of places in the spec (see Page 2-261,
para 3 for an example, Page 2-266 314 next to last paragraph...) , the
following statement 20. In a number of places in the spec (see Page
2-appears "The semantics of adding a value that violates the
multiplicity of an attribute is undefined". Why did the submitters not
choose to treat this event as a violation of well formedness rules and
raise an exception (for example by invoking an exceptionAction)? For
vendors that implement UML tools that implement these constraints, it
would be better if the semantics of actions handles these violations
consistently. At some point in time when enough tools implement the
Action Semantics spec issues such as these will become more important
for interoperability reasons. Imagine a programming language interface
(MOF 2 IDL or JMI) used to manipulate a UML metamodel server which has
been extended to support the Actions package. It would be good if
conformant client implementations treated multiplicity violations
consistently. (the fact that the spec leaves this undefined is one way
to be consistent I suppose!). Should the spec reviewer assume that for
those parts of the spec where the semantics is explicitly marked as
undefined, we should raise a red flag for modelers using these
capabilities because those models 'may not be executable'?

See also Page 2-266, next to last para. 'creating a link that violates
20. In a number of places in the spec (see Page 2-maximum multiplicities
has undefined semantics'.. 'modeler must determine when minimum
multiplicity associations should be enforced'. There isn't a standard
way (I know of), this can be done consistently in UML. May be this will
get sorted out as part of UML2 as part of the OCL Metamodel RFP.

Multiplicity constraints are very popular in UML and we should look at
providing some clear guidelines of when and how these (and other
constraints) are checked or ignored in UML.

Finally in Page 128 of revised submission the following text "When a
semantic variation point' is mentioned.

24. ClearAssociationAction class. I suggest that handling
multiplicity be a semantic variation point as opposed to making the
arbitrary choice that minimum muliplicity be violated when links of the
association in which the object participats is destroyed.This could for
example be handled by tags that can be customized (preferably at the
package level) to (a) ignore multiplciity constraints (b) enforce them
and raise an exception if the constraint is violated. I did notice the
the choice to ignore minimum multiplicity is being consistently made -
so this will minimize confusion.

Decline