The Contract Net: A Formalism for the Control of Distributed Problem Solving

Distributed processing offers the potential for high speed, reliable computation, together with a means to handle applications that have a natural spatial distribution. In this paper, distributed processing is defined as processing that is characterized by physical decomposition of the processor into relatively Independent processor nodes. Recent advances in LSI technology, expected to result in single silicon wafers with at least 10 6 active elements by 1981 [Noyce, 1976], indicate that it is reasonable to contemplate designs which incorporate large networks of single chip processor nodes. In this paper we examine the control of problem solving in such an environment, where most information is local to a node, and relatively little information is shared by the complete network. Individual nodes correspond to "experts" which cooperate to complete a top level task (analogous to the "beings" of [Lenat, 1975]). The distributed processor is thus to be composed of a network of "loosely-coupled", asynchronous nodes, with distributed executive control, a flexible interconnection mechanism, and a minimum of centralized, shared resources. This puts the emphasis on "coarse grain" parallelism, in which individual nodes are primarily involved with computation (large kernel tasks), pausing only occasionally to communicate with other nodes. The CONTRACT NET represents a formalism in which to express the control of problem solving in a distributed processor architecture. Individual tasks are dealt with as contracts. A node that generates a task broadcasts Its existence to the other nodes in the net as a contract announcement, and acts as the contract manager forthe duration of that contract. Bids are then received from potential contractors, which are simply other nodes in the net.An award is made to one node which assumes responsibility for the execution of that contract. Subcontracts may be-let in turn as warranted by task size or a requirement for special expertise or data not in the possession of the contractor. When a contract has been executed, a report is transmitted to the contract manager. Contracts may be announced via general broadcast, limited broadcast, or point-to-point communications mechanisms, depending on information about relevant contractors available to the contract manager. If, for example, a manager has knowledge about the location of particular data, then its contract announcement will be directed to the node(s) believed to possess that data, so that the complete network is not needlessly involved. Contracting effectively distributes control throughout the network, thus allowing for flexibility and reliability. Decisions about what …