# Friday, 25 November 2005

Contracts and Inheritance anomaly.. A solution?

Despite the dozens of concurrent object languages born in the research laboratories few have seen practical use: objects and concurrency constructs have failed to work well together.

This happenes because the object oriented methodology and the synchronization of concurrent activities are orthogonal concepts. The puprose of Concurrent Object Oriented Language is to find a way to resolve automatically this orthogonality.

The famous anomaly in the combination of synchronisation of concurrent activities and inheritance is well known as the Inheritance Anomaly: when inheriting from a base class to add a new method, unrelated to the other methods in the class, concurrency constructs force the redefinition of all other methods of a class.

There is not "solution" to the inheritance anomaly: as long as synchronization constructs and application code are interwinded, the problem stands.
However, we can mitigate it separating the two as much as possible. If we could put no synchronization code at all, obviously the anomaly is solved.
Therefore, we use synchronization constract not only to check the temporal behaviour of objects, but to "orchestrate" them. To put it simple, a method is not executed till its precondition is statisfied; assertions are turned into synchronization primitives. A method will not return until its postcondition is satisfied (so we have a rendez-vous point). This approach make it possible to write monitor-like code using only precondition and poscondition, without writing code in the method body.

Of course, not all scenarios can be modeled using only synchronization contracts, but temporal logic should be powerful enough for many common scenarios. Next post, after all this speaking, we will finally see a real application in a common scnario.