Ordering vs timeliness: two facets of consistency?
Future directions in distributed computing
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Consistency criteria adopted for the management ofpersistent replicated objects in a distributed system define the degree of concurrency allowed among operationsaccessing objects. Several notions of consistency areknown from the literature, among them are causal consistency, causal serializability, and serializability. Inthis paper, we propose a generalizing algorithm for concurrency control in a transaction system that exhibitsa clean separation between policy and mechanism. Aconsistency criterion selected is manifested as a set ofrules forming the policy. The mechanism, however, remains unchanged regardless of the currently used policy. It implements causally consistent message deliveryand uses tokens and quorums of tokens to enforce access operation ordering according to the specified consistency criterion. An example of an application exploiting the advantages of dynamically switching among various consistency criteria concludes the paper.