Static and Dynamic Adaptation of Transactional Consistency

Consistency criteria adopted for the management of persistent replicated objects in a distributed system de ne the degree of concurrency allowed among operations accessing objects Several notions of consistency are known from the literature among them are causal con sistency causal serializability and serializability In this paper we propose a generalizing algorithm for con currency control in a transaction system that exhibits a clean separation between policy and mechanism A consistency criterion selected is manifested as a set of rules forming the policy The mechanism however re mains unchanged regardless of the currently used pol icy It implements causally consistent message delivery and uses tokens and quorums of tokens to enforce ac cess operation ordering according to the speci ed consis tency criterion An example of an application exploiting the advantages of dynamically switching among various consistency criteria concludes the paper