A Torus Quorum Protocol for Distributed Mutual Exclusion

Designing efficient and fault-tolerant algorithms for solving the mutual exclusion problem is an important issue in distributed systems. Many algorithms have been proposed in the literature, and they are being broadly classified into two types: token-based and permissionbased [5, 21]. In a token-based algorithm, the system uses a unique token to represent the privilege for the node possessing it to enter the critical section. In a permissionbased algorithm, a node wishing to enter the critical section needs to request permissions from a subset of other nodes, called a request set. Earlier results typified by [14, 19] allow the nodes in the request sets to grant multiple permissions, but use logical clocks and timestamps to ensure mutual exclusion and freedom of deadlocks. A different approach typified by [16] requires that each node in a request set grant one permission at a time. Such a request set is called a quorum, and the family of all quorums forms a coterie if certain minimality condition is satisfied. Quorum-based algorithms guarantee mutual exclusion because each pair of the quorums have a non-empty intersection. The main advantage of quorum-based algorithms is their resilience to node and network failures, because any quorum with fully operational nodes can grant permissions, improving the system’s availability. In this paper, we are mainly interested in quorum-based algorithms for solving the distributed mutual exclusion problem, although similar techniques have been applied to solving the replicated data control problem [12, Chap.7].