A generic model of consistency guarantees for replicated services

High availability, scalability, and reliability of services can be provided by replication. However, distributed systems suffer from network partitioning, which reduces availability and/or consistency. The choice between availability and consistency boils down to distinguishing between pessimistic and optimistic approaches to replication. This paper proposes a new model of coexistence of pessimistic and optimistic replication, enabling the user to balance between availability and consistency. In this approach a client can decide on the degree of optimism appropriate for the application. This is achieved by specifying operation modes which are intended to describe client expectations.