Concurrency and availability as dual properties of replicated atomic data

Abstract

A replicated data object is a typed object that is stored redundantly at multiple locations in a distributed system. Each of the object's operations has a set of quorums, which are sets of sites whose cooperation is needed to execute that operation. A quorum assignment associates each operation with its set of quorums. An operation's quorums determine its availability, and the constraints governing an object's quorum assignments determine the range of availability properties realizable by replication.In this paper, the restrictions on quorum assignment imposed by three kinds of atomicity mechanisms found in the literature are analyzed: (1) serial schemes, in which replication and atomicity are implemented independently at different levels in the system, (2) static schemes, in which the transaction serialization order is predetermined, and (3) hybrid schemes in which the serialization order emerges dynamically.The following results are derived: (1) Although serial schemes place the strongest restrictions on concurrency, they place the weakest restrictions on availability. (2) Although hybrid and static mechanisms place incomparable restrictions on concurrency, hybrid mechanisms place weaker restrictions on availability. (3) Bounding the maximum depth of transaction nesting strengthens restrictions on concurrency for all classes, but weakens restrictions on availability for hybrid schemes only. Concurrency and availability are best considered as dual properties: A complete analysis of an atomicity mechanism should take both into account.