Formal Techniques in the Development of Blackboard Systems

The blackboard architecture has been an essentially informal construct from its inception. This has led to diierent, though essentially similar, interpretations of the original metaphor. In a recent book, the author presented a mathematical speciication of a blackboard system shell. The exercise was successful and suggested a method for developing blackboard systems with the aid of formal methods. Given the application of blackboard systems to real-time and safety-critical problems, better guarantees of correct functioning than that provided by a working prototype are required. This paper describes a formal method for the development of blackboard systems. The method is based, in part, on an informal one. Apart from the diierence in emphasis (formal rather than informal), the new approach rests upon a formal deenition of the architecture. The essential idea is that the mapping between the formal model of the problem domain (which is intended to be similar to the formal models proposed by, for example, Hayes) and the blackboard shell should be supported by formal proofs of correctness in a way identical to formal software engineering. As part of this mapping process, formal semantics must be given to the attributes that appear on the blackboard, as well as to the abstraction levels themselves. These formal semantics give a guarantee that the objects represented on the blackboard satisfy a variety of properties that are determined by the formal domain model. The semantics also determines the range of legal transformations that may be eeected by Knowledge Sources|this is of importance when developing the nal control structure. The control structure for the resulting system is derived by considering the blackboard as being transformed from an initial state to a set of potential nal states. Each of the nal states represents one of the possible answers that the system may produce. The paths from initial to nal states can be treated either as traces or by regarding the entire control problem as formalizable in a temporal logic. 1 We present an informal method for constructing blackboard systems. The informal method forms the basis of the formal method whose initial stages are then described. We outline the formal treatment of control and suggest the use of temporal logic as a tool for reasoning about control. The paper ends with a review of the method.