Synthesizing Finite-State Protocols from Scenarios and Requirements

Scenarios, or Message Sequence Charts, offer an intuitive way of describing the desired behaviors of a distributed protocol. In this paper we propose a new way of specifying and synthesizing finite-state protocols using scenarios: we show that it is possible to automatically derive a distributed implementation from a set of scenarios augmented with a set of safety and liveness requirements, provided the given scenarios adequately cover all the states of the desired implementation. We first derive incomplete state machines from the given scenarios, and then synthesis corresponds to completing the transition relation of individual processes so that the global product meets the specified requirements. This completion problem, in general, has the same complexity, PSPACE, as the verification problem, but unlike the verification problem, is still hard (NP-complete) even for a constant number of processes. We present an algorithm for solving the completion problem, based on counterexampleguided inductive synthesis. We evaluate the proposed methodology for protocol specification and the effectiveness of the synthesis algorithm using the classical alternating-bit protocol, the VI cache-coherence protocol, and a consensus protocol.