Performing work efficiently in the presence of faults

Abstract

We consider a system of t synchronous processes that communicate only by sending messages to one another, and that together must perform $n$ independent units of work. Processes may fail by crashing; we want to guarantee that in every execution of the protocol in which at least one process survives, all n units of work will be performed. We consider three parameters: the number of messages sent, the total number of units of work performed (including multiplicities), and time. We present three protocols for solving the problem. All three are work-optimal, doing O(n+t) work. The first has moderate costs in the remaining two parameters, sending O(t\sqrt{t}) messages, and taking O(n+t) time. This protocol can be easily modified to run in any completely asynchronous system equipped with a failure detection mechanism. The second sends only O(t log{t}) messages, but its running time is large (exponential in n and t). The third is essentially time-optimal in the (usual) case in which there are no failures, and its time complexity degrades gracefully as the number of failures increases.