An Algorithm for Replicated Objects with Efficient Reads

The problem. We consider the problem of implementing a consistent replicated object in a partially synchronous message passing distributed system susceptible to process and communication failures. The object is a generic shared resource, such as a data structure, a file, or a lock. The processes implementing the replicated object access it by applying operations to it at unpredictable times and potentially concurrently.1 The object should be linearizable: it should behave as if each operation applied to it takes effect at a distinct instant in time during the interval between its invocation and its response. The main reason for replicating an object is fault tolerance; if a copy of the object becomes inaccessible, the object can still be accessed through the other copies. Another reason for replicating an object is performance; a process that requires the object can access its local copy. The benefit of accessing a local copy, however, must be weighed against the cost of keeping the copies consistent.