Normality: A Consistency Condition for Concurrent Objects

Linearizability is a consistency condition for concurrent objects (objects shared by concurrent processes) that exploits the semantics of abstract data types. It provides the illusion that each operation applied by concurrent processes takes eeect instantaneously at some point between the beginning and the end of its execution. When compared with other consistency conditions (such as sequential consistency) Linearizability satisses the Locality property (i.e, a system is linearizable if each object taken individually is linearizable) and the Non-Blocking property (i.e., termination of an invoked operation does not depend on other pending invocations). Those are noteworthy properties as they allow concurrent systems to be designed and constructed in a modular fashion. This paper introduces a consistency condition called Normality that is less constraining than Linearizability (in the sense it does not refer to a global real-time order) and still satisses Locality and Non-Blocking. As it does not refer to a global real-time, Normality is well-suited to objects supported by asynchronous distributed systems and can consequently be seen as an adaptation of Linearizability for these systems.