Page type: faculty

Faculty Research Interests : Sam Toueg Sam Toueg;Professor;Ph.D., Princeton University, 1979;Research Interests;My research interests include distributed computing, fault-tolerance;and real-time. I work on methodologies, paradigms, and algorithms for;fault-tolerant distributed systems, in both message-passing and;shared-memory systems. My long-term goal is to bridge the gap;between theoretical results and the need for efficient and practical;solutions.;In collaboration with;Tushar Chandra;and;Prasad Jayanti,;two Ph.D.;Computer Science students, we continued our work on;unreliable failure;detectors for message-passing systems,;and on wait-free objects for shared-memory systems.;A fundamental result of fault-tolerant distributed computing;states that the Consensus problem cannot be solved (with a;deterministic algorithm) in asynchronous systems. This impossibility;result is due to the inherent difficulty of determining whether a;process has crashed (or is merely very slow) in such a system. In;our work, we were able to determine exactly how much information;about failures is necessary and sufficient to solve Consensus. We;first showed one can use W, an unreliable failure detector that can;make an infinite number of mistakes, to solve Consensus in systems;with a majority of correct processes. We then proved that to solve;Consensus, any failure detector has to provide at least as much;information about failures as W. Thus, W is the weakest failure;detector for solving Consensus in asynchronous systems with a;majority of correct processes. We are now exploring the practicality;of implementing W, and of applications that rely on W for their;correctness.;A concurrent system consists of processes communicating via shared;objects. A shared object is wait-free if each process that accesses;this object is guaranteed to get a response even if all the other;processes crash. We are now exploring wait-free hierarchies of;object types, where each object (type) is assigned to a level that;corresponds to its ability in implementing other wait-free objects.;In particular, Prasad Jayanti has shown that a well-known hierarchy;(Herlihy's) is not robust: Informally, in this hierarchy there is an;object at level 2 that can be used to implement wait-free objects at;any level. We are now exploring the question of whether;robust wait-free hierarchies exist.;Selected Publications; Bracha, G., and S. Toueg.;Asynchronous consensus and broadcast protocols.; Journal of the ACM, vol. 32, 10, 1985, 824-840.; Srikanth, T. K., and S. Toueg.;Optimal clock synchronization.; Journal of the ACM, vol. 34, 3, 1987, 626-645.; El Abbadi, A., and S. Toueg.;Maintaining availability in partitioned replicated databases.; ACM Transactions on Database Systems, vol. 14, 2, 1989, 264-290.; Neiger, G., and S. Toueg.;Automatically increasing the fault-tolerance of distributed algorithms.; Journal of Algorithms, vol. 11, 3, 1990, 374-419.; Chandra, T., and S. Toueg.;Unreliable failure;detectors for asynchronous systems.; Proceedings 10th ACM Symposium on Principles of Distributed Computing;.;August 1991, Montreal, Canada, 257-272.; Chandra, T.,;V. Hadzilacos; and S. Toueg.;The weakest failure detector;for solving consensus.; Proceedings 11th ACM Symposium on Principles of Distributed Computing; ,;August 1992, Vancouver, Canada, 147-158.;Jayanti, P.,;Chandra, T.,;and S. Toueg.;Fault-tolerant wait-free shared objects.; Proceedings 33rd IEEE Symposium on Foundations of Computer Science,;October 1992, Pittsburgh, Pennsylvania, 157-166.; Neiger, G., and S. Toueg.;Simulating synchronized clocks and common knowledge in distributed systems.; Journal of the ACM, vol. 40, 2, 1993, 334-367.;