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Ada 95 is an expressive concurrent programming language with which it is possible to build complex multi-tasking applications. Much of the complexity of these applications stem from the interactions between the tasks. This paper argues that model checking tools are now mature enough that they can be used by engineers to verify the logical correctness of their tasking algorithms. The paper illustrates the approach by showing the correctness of an Ada implementation of the atomic action protocol.

The purpose of this note is to propose a model for building fault tolerant systems. We present an approach based on the object paradigm. To ensure system consistency in the event of failure we provide two basic mechamisms, the persistent state of an object and the recovery unit. Methods are called within recovery units. To define global checkpoints, we build dynamic distributed atomic actions from dependencies between recovery units. One implementation of this model is then briefly presented.

This paper presents an overview of an integrated programming language and system designed to support the construction and maintenance of distributed programs: programs in which modules reside and execute at communicating, but geographically distinct, nodes. The language is intended to support a class of applications in which the manipulation and preservation of long-lived, on-line, distributed data is important. The language addresses the writing of robust programs that survive hardware failures without loss of distributed information and that provide highly concurrent access to that information while preserving its consistency. Several new linguistic constructs are provided; among them are atomic actions, and modules called guardians that survive node failures.

A request manager is described that provides a set of communication primitives supporting the cooperation between processes requesting service and processes providing service. These communication primitives allow the initiation and termination of atomic actions at remote sites. They also maintain recoverable action state tables, which can be used to recall uncompleted actions during restart recovery. The presented request manager is a good foundation on which to develop application oriented high-level primitives as well as language level primitives. Besides a detailed description of the services offered by the request manager, the paper develops a request management protocol that, no matter what happens, insures the atomic property of actions. Evaluation nets, which are a modified form of Petri nets, are applied to describe this protocol.

Argus is a programming language and system developed to support the construction and execution of distributed programs. This paper describes the implementation of Argus, with particular emphasis on the way we implement atomic actions, because this is where Argus differs most from other implemented systems. The paper also discusses the performance of Argus. The cost of actions is quite reasonable, indicating that action systems like Argus are practical.

In this paper, a new approach to coordinating accesses to shared data objects is described. We have observed that synchronization of accesses to shared data and recovering the state of such data in the case of failures are really two sides of the same problem—implementing atomic actions on multiple data items. We describe a mechanism that solves both problems simultaneously in a way that is compatible with requirements of decentralized systems. In particular, the correct construction of a new atomic action can be done without knowledge of all other atomic actions in the system that might execute concurrently. Further, the mechanisms degrade gracefully if parts of the system fail—only those atomic actions that require resources in failed parts of the system are prevented from executing.