We formulate the problem of query evaluation over a relational database (EDB) supplemented by function-free horn clause rules (IDB) as a system of cooperating processes communicating by message passing shared memory is not required making this approach suitable for distributed systems this modularization offers flexibility of implementation and opportunities to use existing operating system feature to enhance performance as well as providing a step toward practical parallel implementation the technique is based on top-down construction of a rule/ goal graph followed by a mixed top-down and bottom-up evaluation strategy employing sideways information passing to restrict the computation to relevant or at least potentially relevant portions of intermediate relations termination is guaranteed but the termination conditions are global in general and their distributed asynchronous detection requires care we describe an efficient protocol to accomplish this we describe a greedy information passing strategy and introduce the monotone flow property for rules we discuss the relation of these concepts to acyclic database schemas and qual trees