Conversation Types

We present a type theory for analyzing concurrent multiparty interactionsas found in service-oriented computing. Our theory introduces a novel andflexible type structure, able to uniformly describe both the internal and theinterface behavior of systems, referred respectively as choreographies and con-tracts in web-services terminology. The notion of conversation builds on thefundamental concept of session, but generalizes it along directions up to nowunexplored; in particular, conversation types discipline interactions in conversa-tions while accounting for dynamical join and leave of an unanticipated numberof participants. We prove that well-typed systems never violate the prescribedconversation constraints. We also present techniques to ensure progress of sys-tems involving several interleaved conversations, a previously open problem.