Abstract An object type is usually regarded as a contract between an object and each of its clients. 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 set of all clients: when the object acts as a shared resource, the clients must be coordinated before sending messages to the object. Process types of active objects specify constraints on the ordering of messages. Static type checking ensures proper coordination of clients so that objects receive messages only in acceptable orderings. As presented in this article, the process type system is based on a simple computation model where active objects communicate via asynchronous message passing. Copyright © 2001 John Wiley & Sons, Ltd.

Rapid growth in the area of multidatabase systems (MDBSs), which involve both the access of global data and distributed transaction processing, has created a need for programming languages that provide communication reliability and powerful synchronization. The requirements of MDBSs are reviewed, and the Vienna Parallel Logic programming language is presented. The ways to realize an MDBS communication protocol in this language are discussed. The Vienna Parallel Logic language incorporates a concurrent logic language, as well as features of both distributed operating systems and database management systems. These features combine to support the communication and synchronization required by distributed transaction processing. The Vienna Parallel Logic language is suitable for use as a general-purpose distributed programming and coordination language.>