Global Protocols under Rendezvous Synchrony: From Realizability to Type Checking

Abstract: Global protocol specifications are the starting point of top-down verification methodologies, and serve as a blueprint for synthesizing local specifications that guarantee the correctness of distributed implementations. In this work, we study global protocol specifications targeting distributed processes that communicate via rendezvous synchrony. We obtain the following positive results for the synchronous realizability problem: (a) realizability is decidable for global protocols over a transitive concurrency alphabet in 2-EXPTIME in the size of the protocol, and in 3-EXPTIME in the size of the alphabet, and (b) realizability is decidable in EXPTIME for global protocols that unambiguously represent their trace language. Unambiguous global protocols encompass fragments of directed and sender-driven choice protocols studied in prior work. Further, our reductions admit the corollary that the synchronous verification problem is solvable with the same complexity, where a candidate realization is included as part of the input. We then prove a negative result stating that synchronous realizability is undecidable in general. Finally, we propose a type system to check pi-calculus processes against local specifications in the form of synchronous communicating state machines. Our type system is the first to support processes with local mixed choice in the presence of session interleaving and~delegation.