Distributed Universality

Abstract

A notion of a universal construction suited to distributed computing has been introduced by Herlihy in his celebrated paper "Wait-free synchronization" (ACM Trans Program Lang Syst 13(1):124---149, 1991). A universal construction is an algorithm that can be used to wait-free implement any object defined by a sequential specification. Herlihy's paper shows that the basic system model, which supports only atomic read/write registers, has to be enriched with consensus objects to allow the design of universal constructions. The generalized notion of a k-universal construction has been recently introduced by Gafni and Guerraoui (Proceedings of 22nd international conference on concurrency theory (CONCUR'11), Springer LNCS 6901, pp 17---27, 2011). A k-universal construction is an algorithm that can be used to simultaneously implement k objects (instead of just one object), with the guarantee that at least one of the k constructed objects progresses forever. While Herlihy's universal construction relies on atomic registers and consensus objects, a k-universal construction relies on atomic registers and k-simultaneous consensus objects (which are wait-free equivalent to k-set agreement objects in the read/write system model). This paper significantly extends the universality results introduced by Herlihy and Gafni---Guerraoui. In particular, we present a k-universal construction which satisfies the following five desired properties, which are not satisfied by the previous k-universal construction: (1) among the k objects that are constructed, at least$$\ell $$l objects (and not just one) are guaranteed to progress forever; (2) the progress condition for processes is wait-freedom, which means that each correct process executes an infinite number of operations on each object that progresses forever; (3) if any of the k constructed objects stops progressing, all its copies (one at each process) stop in the same state; (4) the proposed construction is contention-aware, in the sense that it uses only read/write registers in the absence of contention; and (5) it is generous with respect to the obstruction-freedom progress condition, which means that each process is able to complete any one of its pending operations on the k objects if all the other processes hold still long enough. The proposed construction, which is based on new design principles, is called a $$(k,\ell )$$(k,l)-universal construction. It uses a natural extension of k-simultaneous consensus objects, called $$(k,\ell )$$(k,l)-simultaneous consensus objects ($$(k,\ell )$$(k,l)-SC). Together with atomic registers, $$(k,\ell )$$(k,l)-SC objects are shown to be necessary and sufficient for building a $$(k,\ell )$$(k,l)-universal construction, and, in that sense, $$(k,\ell )$$(k,l)-SC objects are $$(k,\ell )$$(k,l)-$$ {universal}$$universal.