Modelling causal connections between objects

The conventional object-oriented paradigm provides client-server message passing as the primary means of interaction between objects. Although this is appropriate in situations where one objects requests a service from another object, there are other situations where one object, the observer, depends on state-changes or actions occurring at another object, the target. In those situations, the object-oriented paradigm requires the target object to notify the observer objects, despite the fact that the target object does not benefit from the interaction. This inverted interaction scheme leads to problems such as increased coupling and decreased understandability, maintainability and reusability of the resulting classes. In the analysis of object interactions one can identify four roles: the benefitor, the establisher, the sender and the receiver. The aforementioned problems result from the fact that in client-server interaction, the benefitor and the sender are not always the same object. To address this, we propose the notion of causal connections, an alternative interaction mechanism complementing the traditional client-server interaction. Two implementations of causal connections are described; one in the context of C++ and another using the layered object model. It is shown that causal connections solve the problems mentioned above.