Combining generality and practicality in a conit-based continuous consistency model for wide-area replication

Replication is a key approach to scaling wide-area applications. However, the overhead associated with large-scale replication quickly becomes prohibitive across wide-area networks. One effective approach to addressing this limitation is to allow applications to dynamically trade reduced consistency for increased performance and availability. Although extensive study has been performed on relaxed consistency models in traditional replicated databases, none of the models can simultaneously achieve the following two typically conflicting requirements imposed by wide-area applications: generality (capturing application-specific consistency semantics) and practicality (enabling efficient application-independent consistency protocols to be designed and providing natural ways to express application semantics). We propose a conit-based continuous consistency model designed to simultaneously achieve generality and practicality. Our conit theory provides generality, where application-specific consistency requirements are exported as conits. Practicality is achieved by using a simple, spanning set of metrics for conit consistency and by using a per-write weight specification. We demonstrate the generality of our model through representative wide-area applications and by showing that a number of existing models can be expressed as instances of our model. Our efficient, application-independent consistency protocols and prototype implementation verify its practicality.