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Wait-free Byzantine Agreement College of Computer Science

Summary: Wait-free Byzantine Agreement
Paul Attie
College of Computer Science
Northeastern University
We study the problem of devising wait-free consensus protocols that tolerate Byzantine failures. We
show that wait-free Byzantine agreement cannot be achieved even in the presence of a single faulty (i.e.,
Byzantine) process. We further show that wait-free weak Byzantine agreement cannot be achieved if
shared objects are \resettable," i.e., can be reset to an initial state from any of their reachable states
by an appropriate sequence of operations. Resettable objects are preferable to non-resettable objects,
since they can be reused more easily. We introduce a non-resettable object, a \sticky register," which is
a straightforward generalization of the sticky type of [Jay93], and use it devise a solution to wait-free
weak Byzantine agreement that tolerates any number of faulty processes. Our impossibility results hold
even if we can limit any process (including Byzantine processes) to invoking objects in a particular xed
set (i.e., impose a particular \access pattern"). Finally, we study the problem of repetitive wait-free weak
Byzantine agreement: how to solve a sequence of successive instances of agreement. A straightforward
solution uses an unbounded number of sticky registers|one for each instance of agreement. We show
how to bound the number of registers needed by using failure detectors and weakening the wait-freedom
property of the solution.
1 Introduction


Source: Attie, Paul - Department of Computer Science, American University of Beirut


Collections: Computer Technologies and Information Sciences