Coordination Requirements Expressed in Types for Active Objects

                          Franz Puntigam
    Institut für Computersprachen, Technische Universität Wien
           Argentinierstraße 8, A-1040 Vienna, Austria
             E-mail: franz@complang.tuwien.ac.at

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-specified
orders. The model ensures statically that objects behave as specified by
their types, and users are coordinated appropriately.

Keywords: type model, concurrency, active objects