Coordination Requirements Expressed in Types for Active Objects

An object's type is usually regarded as a contract between the object and each of its users. However, in concurrent (and sometimes also in sequential) systems it is more useful to regard a type as a contract between an object and the unity of all users: The users must be coordinated before sending messages to the object. Types in the proposed model express requirements on the coordination of users; objects must accept messages only in pre-speciied orders. The model ensures statically that objects behave as speciied by their types, and users are coordinated appropriately.