Abstracts in English and Danish.

Cover title.

"August 1994."

Thesis (Ph. D.)--Aalborg University, 1994.

Includes bibliographical references.

Abstract: "Future distributed systems will be so-called open distributed heterogeneous systems. Applications for such systems must be able to interact across architectural barriers, in form of differences in representation of data at the hardware level. These systems should persist the termination of a particular application and offer resources to applications. Application interaction must be flexible enough to support reconfigurations and extensions over time, and architectural barriers should not be visible; the runtime system should translate transparently between different representations. This is lacking in existing systems, which must support transparent heterogeneity and better flexibility in naming interaction partners. A new object-based concurrent coordination paradigm called ActorSpace based on Actors is proposed. It offers a set of coordination primitives providing transparent heterogeneity, which combined with a sequential programming language forms a concurrent programming language. Objects (actors) perform local computation, and coordination by explicit sending asynchronous messages. In addition to object identifiers, ActorSpace includes patterns as specifications of message destinations. Patterns are evaluated in explicit contexts (actorspaces), which contain attributes of objects made explicitly visible. Patterns are matched at runtime against the attributes in a set of actorspaces, and a message is then sent to one or all of the potential receivers. Both actors and actorspaces may be made visible, thus supporting a hierarchical name space. Visibility changes are explicit and authenticated: legal capabilities for objects must be presented when changing visibilities. A formal definition of ActorSpace is given through an operational semantics, defining the exact meaning of ActorSpace primitives. A prototype is designed and implemented, consisting of a compiler and a runtime system. The runtime system consists of components responsible for multitasking actors, interpreting actor definitions, carrying out ActorSpace primitives, and maintaining coherence between several runtime systems distributed across a set of nodes. Its performance is evaluated by a set of tests, revealing both time consumption of ActorSpace primitives and various overheads present in the implementation. Finally potential concrete extensions to ActorSpace are discussed, concerning explicit specifications of the synchronization of interaction and increasing the customizability of ActorSpace, and the remaining unsolved problems in ActorSpace is presented."

Supported in part by a joint research fellowship from #Arhus University and Aalborg University

Supported in part by the Danish Research Academy. V910219