We describe a hierarchical model for coordination of concurrent activities based on grouping actors into casts and coordinating casts by actors that are designated directors. The hierarchical model provides a simple, intuitive basis for actor communication and coordination. Casts serve as abstraction units for naming, migration, synchronization and load balancing. Messengers are actors used to send messages with special behaviour across casts. Moreover, an implementation of the hierarchical model does not require a reective run-time architecture. We present the operational semantics for our model and illustrate the model by two sample applications: an atomic multicast protocol and a messenger carrying remote exception-handling code. These applications have been implemented in Java, leveraging the existence of cross-platform, safe virtual machine implementations.

The mobile form of distributed computation, called mobile computation, is investigated from the viewpoints of computational model, programming language and implementation. The emphasis of this thesis compared with the related work is on analyzing existing mobile language systems from a rather implementational point of view. The contribution of this thesis is threefold. (1) We present a computational model in which dynamic transference of code, data and execution state can be represented. Several mobility mechanisms in existing mobile language systems are analyzed in the model. (2) Based on the analysis, a programming language for mobile computation is designed. The distinctive feature of the language is the facility to control the transparency of migration. (3) An implementation of the mobile programming language is presented. The language is implemented as an extension of the Java language. The implementation is so portable that the program runs on any standard Java inte...

Abstract. Scalability, reliability and adaptability are among the key requirements for the enactment of distributed workflows. In addition, system resources should be efficiently utilized. Central workflow engines and static analysis of workflow specifications are some of the important obstacles to meeting these requirements. We propose a fully decentralized approach to workflow enactment that is not subject to these obstacles. In addition, it supports automatic recovery. The approach is of continuation-passing style, where continuations, or the reminder of the executions, are passed along with asynchronous messages for workflow enactment. Two continuations are associated to an execution: a success continuation and a failure continuation. Recovery plans for workflows are automatically generated at runtime and included in failure continuations. A prototype is implemented. 1