In wide-area settings, unpredictable events, such as flash crowds caused by nearly instantaneous popularity of services, can cause servers that are expected to respond quickly to instead suddenly respond slowly. This presents a problem for achieving consistently good performance in quorum-based distributed systems, in which clients must choose which quorums (sets of servers) to access. Typically,clients are motivated to choose quorums containing the servers that respond fastest. Often, these may be the closest servers, but when the closest servers are particularly slow to respond, e.g., because of a changed workload, servers that are farther may actually respond faster. In this paper, we show how clients can locally change their quorum selections efficiently such that the overall system performance rapidly converges to that of the best global strategy for the current conditions. Moreover, we discuss how to benefit even when changes in quorums must be accompanied by expensive state-transfer operations.

A quorum system is a collection of sets (quorums) of servers,where any two quorums intersect. Quorum-based protocols underly modern edge-computing architectures and throughput-scalable service implementations. In this paper we propose new algorithms for placing quorums in wide-area networks and tuning which quorums clients access, so as to optimize clients' average response time in quorum-based protocols. We examine scenarios in which the service is lightly loaded and hence network latency is the dominant delay, and in which client-induced load contributes significantly to the delay that clients observe.In each case, we evaluate our algorithms on topologies ranging from 50 to over 150 wide-area locations.

A quorum system over a universe of logical elements is a collection of subsets (quorums) of elements, any two of which intersect. In numerous distributed algorithms, the elements of the universe reside on the nodes of a physical network and the participating nodes access the system by contacting every element in some quorum, potentially causing the added network congestion induced by these quorum accesses to play a limiting factor in the performance of the algorithm.In this paper we initiate the study of algorithms to place universe elements on the nodes of a physical network so as to minimize the network congestion that results from quorum accesses, while also ensuring that no physical node is overloaded by access requests from clients. We consider two models, one in which communication routes can be chosen arbitrarily and one in which they are fixed in advance. We show that in either model, the optimal congestion (with respect to the load constraints) cannot be approximated to any factor (unless P=NP). However, we show that at most doubling the load on nodes allows us to achieve a congestion that is close to this optimal value. We also shed some light on the extent to which element migration can reduce congestion in this context.

A quorum system is a family of sets (themselves called quorums), each pair of which intersect. In many distributed algorithms, the basic unit accessed by a client is a quorum of nodes. Such algorithms are used for applications such as mutual exclusion, data replication, and dissemination of information. However, accessing spread-out quorums causes access delays that we would like to minimize. Furthermore, every member of the quorum incurs processing load to handle quorum accesses by clients.In this paper we study the problem of placing quorums in a physical network so as to minimize the delay that clients incur by accessing quorums, and while respecting each physical node's capacity (in terms of the load of client requests it can handle). We provide approximation algorithms for this problem for two natural measures of delay (the max-delay and total-delay). All our algorithms ensure that each node's load is within a constant factor of its capacity, and minimize delay to within a constant factor of the optimal delay for all capacity-respecting solutions. We also provide better approximations for several well-known quorum systems.