In many distributed applications, processes synchronize with one another in a complex way and execute for a long period of time. Atomic transactions are inadequate for designing reliable applications with these characteristics, because transactions restrict the types of synchronization than can be specified. An alternative approach that exploits behavior specified in a hierarchical finite-state machine (FSM) model is proposed. A set of general conditions that ensures the correctness of recovery is identified. These general conditions permit combinations of different types of recovery methods to be used in a recovery. They also enable one to enhance recovery efficiency by exploiting permutation and substitution of operations allowed by the behavior specification. It is shown that existing recovery techniques, including those that exploit application semantics, satisfy these conditions for correctness of recovery.>

We explore four classic problems in concurrent computing (election, mutual exclusion, consensus, and naming) when the number of processes which may participate is unbounded. Partial information about the number of processes actually participating and the concurrency level is shown to affect the computability and complexity of solving these problems when using only atomic registers. We survey and generalize work carried out in models with known bounds on the number of processes, and prove several new results. These include improved bounds for election when participation is required and a new adaptive starvation-free mutual exclusion algorithm for unbounded concurrency. We also survey results in models with shared objects stronger than atomic registers, such as test&set bits, semaphores or read–modify–write registers, and update them for the unbounded case.

A bilaterally symmetric figure on an arbitrarily oriented plane viewed under orthographic projection yields a “skew symmetric” figure whose axes of symmetry and skew constrain the orientation of the underlying plane. Detection of skewed symmetries and extraction of axis orientations constitute a necessary practical issue that has not received much attention. We develop a strong necessary condition on the axes of symmetry and skew in a planar skew symmetric figure which constrains, them to lie on a one-dimensional locus in a two-dimensional space of orientation pairs. The practicality of our technique is demonstrated with illustrations from an experimental testbed. A heuristic symmetry evaluator is used in conjunction with our constraint to determine axes of skewed symmetry.

Distributed systems must provide certain fundamental facilities, including communication, protection, resource management, reliability and process (computation) abstraction. The authors describe the design of HPC, an object-oriented model of interprocess relationships for distributed systems which addresses all of these fundamental services. The major novelties of HPC lie in the extension of the process abstraction to collections of processes and the provision of a rich set of structuring mechanisms for distributed computations. An important aspect of the model is that it results in the ability to maintain and exploit execution context for managing processes in a distributed computation.