Add to ORKGWe give a formal account of a calculus for modeling service-based systems, suitable to describe both service composition (orchestration) and the protocol that services run when invoked (conversation). The calculus includes primitives for defining and for invoking services, for isolating conversations between requesters and providers of services, and primitives for orchestrating services, that is, to make use of existent services as building blocks to accomplish complex tasks. The calculus is equipped with a reduction and a labeled transition semantics; an equivalence result relates the two. To give an hint on how the structuring mechanisms of the language can be exploited for static analysis we present a simple type system guaranteeing the ...