Literature Survey Report On Quorum Systems

This report summarizes the state of the art in the research area of quorum systems as represented by the papers [PM01], [ES00], [MRW97], and [AMR+00]. Quorum systems are used in fault tolerant distributed computing systems for ensuring the availability of a replicated service and replicated data, even if the replicated servers fail by crashing benignly or in a byzantine, i.e. arbitrary way. Quorums are subsets of the servers with the condition that every pair of subsets intersect, so that each quorum can make a decision on behalf of all servers. In the papers new quorum systems with improved quorum size, load and availability are introduced and their properties mathematically proved. There are lower bounds for both load and availability, and there are quorum systems that are optimal in respect to these properties. Two of the papers deal with the emulation of shared memory that can be accessed by servers in a distributed system. Quorums are used to grant read or write access to the shared memory. The problems that arise are how to reconfigure the quorum system in case of a failure, and how to tolerate byzantine failures. Two papers are about byzantine fault tolerance, i.e. tolerating value faults. Quorum systems that tolerate byzantine failures must suffice stronger requirements for the intersection properties. The summarized papers are analysed and compared, so that a complete picture of the research area of quorum systems is provided.