Quorum Systems Access Protocol

Quorum systems are tools for increasing the availability and eeciency of replicated services. A quorum system for a universe of servers is a collection of subsets of servers, each pair of which intersect. Intuitively, each quorum can operate on behalf of the system, thus increasing its availability and performance, while the intersection property guarantees that operations done on distinct quorums preserve consistency. The motivation for quorum systems stems from the need to make critical missions performed by machines reliable. The only way to increase the reliability of a service, aside from using in-trinsically more robust hardware, is via replication. To make a service robust, it can be installed on multiple identical servers, each one of which holds a copy of the service state and performs read/write operations on it. This allows the system to provide information and perform operations even if some machines fail or communication links go down. Unfortunately, replication incurs a cost in the need to maintain the servers consistent. To enhance the availability and performance of a replicated service, Giiord and Thomas introduced in 1979 3,10] the usage of votes assigned to each server, such that a majority of the sum of votes is suucient to perform operations. More generally, quorum systems are deened formally as follows: Quorum system: Assume a universe U of servers, jUj = n, and an arbitrary number of clients. A quorum system Q 2 U is a set of subsets of U, every pair of which intersect. Each Q 2 Q is called a quorum. To demonstrate the usability of quorum systems in constructing replicated services, quorums are used here to implement a multi-writer multi-reader atomic shared variable. Quorums have also been used in various mutual exclusion protocols, to achieve Consensus, and in commit protocols. In our application, clients perform read and write operations on a variable x that is replicated at each server in the universe U. A copy of the variable x is stored at each server, along with a times-tamp value t. Timestamps are assigned by a client to each replica of the variable when the client writes the replica. Diierent clients choose distinct timestamps, e.g., by choosing integers appended with the name of c in the low-order bits. The read and write operations are implemented as follows. Write: For a client c to write the value v, it queries each server in some quorum Q to obtain a …