UML22 — Meaning of navigability

The resolution to issue 6460 in the InfrastructureLibrary specification indicates "Implementation can support traversal across non-navigable ends, but it is not required. Once an object is found by traversal, messages can be sent to it like any other object." This statement may lead to interoperability problems between implementations, is not included in the adopted Superstructure specification, and contradicts constraint [4] for ReadLinkAction which states the end must be navigable. Infrastructure also does not define what it means to send messages to an object so it is not clear what these statements actually mean.

It is possible that the resolution to issue 6243 traded coupling between navigability and property ownership for coupling between navigability and tool implementations. Navigability no longer has any well-defined semantics and becomes simply a hint to tool implementors that the traversal should be efficient.

I believe this is quite unfortunate and can be avoided by decoupling tool implementations that manipulate models from the meaning of the models themselves. Navigability should continue to mean semantically traversable as specified by ReadLinkAction. This will establish an interoperable meaning across all tools and preserve an important and commonly used semantic. If tools wish to support efficient traversal to non-navigable ends for their purposes, they should feel free to do so. This can be done by maintaining additional information in associations for the non-navigable ends for the tools purpose, or by using crawlers that examine the model and cache information for specific tool purposes. This is manipulating the model for very different purposes than the meaning of the model itself. If it is desired to have some standard means of indicating to tool vendors where non-navigable association ends should be efficiently traversable, this should be done by a separate property perhaps available through the standard profile. It should not be coupled with the semantic meaning of navigability.

Discussion
This issue has already been resolved by, or no longer applies to, the UML 2.5 Beta 1 specification.
Disposition: Closed - No Change