Self-Stabilization of Wait-Free Shared Memory Objects

Past research on fault tolerant distributed systems has focussed either on processor failures, ranging from benign crash failures to the malicious Byzantine failure types, or on transient memory errors, which can suddenly corrupt the state of the system. It is an interesting question whether one can device highly fault tolerant distributed protocols that tolerate both processor failures as well as transient memory errors. To answer this question we consider self-stabilizing wait-free shared memory objects. These objects occur naturally in distributed systems in which both processors and memory may be faulty. Our contribution in this paper is threefold. First, we propose a general definition of a self-stabilizing wait-free shared memory object that expresses safety guarantees even in the face of processor failures. We prove that within this framework one cannot construct a self-stabilizing single-reader single-writer regular bit from single-reader single-writer safe bits-..

Self-stabilization of wait-free shared memory objects

This paper proposes a general definition of self-stabilizing wait-free shared memory objects. The definition ensures that, even in the face of processor failures, every execution after a transient memory failure is linearizable except for an a priori bounded number of actions. Shared registers have been used extensively as communication medium in self-stabilizing protocols. As an application of our theory, we therefore focus on self-stabilizing implementation of such registers, thus providing a large body of previous research with a more solid foundation. In particular, we prove that one cannot construct a self-stabilizing single-reader single-writer regular bit from self-stabilizing single-reader single-writer safe bits, using only a single bit for the writer. This leads us to postulate a self-stabilizing dual-reader single-writer safe bit as the minimal hardware needed to achieve self-stabilizing wait-free interprocess communication and synchronization. Based on this hardware, adaptations of well-known wait-free implementations of regular and atomic shared registers are proven to be self-stabilizing. # 2002 Elsevier Science (USA) Key Words: shared memory; wait-free constructions; self-stabilization; fault tolerance; distributed computing

Information Infrastructures in Distributed Environments: Algorithms for Mobile Networks and Resource Allocation

A distributed system is a collection of computing entities that communicate with each other to solve some problem. Distributed systems impact almost every aspect of daily life (e.g., cellular networks and the Internet); however, it is hard to develop services on top of distributed systems due to the unreliable nature of computing entities and communication. As handheld devices with wireless communication capabilities become increasingly popular, the task of providing services becomes even more challenging since dynamics, such as mobility, may cause the network topology to change frequently. One way to ease this task is to develop collections of information infrastructures which can serve as building blocks to design more complicated services and can be analyzed independently. The first part of the dissertation considers the dining philosophers problem (a generalization of the mutual exclusion problem) in static networks. A solution to the dining philosophers problem can be utilized when there is a need to prevent multiple nodes from accessing some shared resource simultaneously. We present two algorithms that solve the dining philosophers problem. The first algorithm considers an asynchronous message-passing model while the second one considers an asynchronous shared-memory model. Both algorithms are crash fault-tolerant in the sense that a node crash only affects its local neighborhood in the network. We utilize failure detectors (system services that provide some information about crash failures in the system) to achieve such crash fault-tolerance. In addition to crash fault-tolerance, the first algorithm provides fairness in accessing shared resources and the second algorithm tolerates transient failures (unexpected corruptions to the system state). Considering the message-passing model, we also provide a reduction such that given a crash fault-tolerant solution to our dining philosophers problem, we implement the failure detector that we have utilized to solve our dining philosophers problem. This reduction serves as the first step towards identifying the minimum information regarding crash failures that is required to solve the dining philosophers problem at hand. In the second part of this dissertation, we present information infrastructures for mobile ad hoc networks. In particular, we present solutions to the following problems in mobile ad hoc environments: (1) maintaining neighbor knowledge, (2) neighbor detection, and (3) leader election. The solutions to (1) and (3) consider a system with perfectly synchronized clocks while the solution to (2) considers a system with bounded clock drift. Services such as neighbor detection and maintaining neighbor knowledge can serve as a building block for applications that require point-to-point communication. A solution to the leader election problem can be used whenever there is a need for a unique coordinator in the system to perform a special task