Token-Based Sequential Consistency in Asynchronous Distributed Systems

A concurrent object is an object that can be concurrently accessed by several processes. Sequential consistency is a consistency criterion for such objects. It informally states that a multiprocess program executes correctly if its results could have been produced by executing that program on a single processor system. (Sequential consistency is weaker than atomic consistency-the usual consistency criterion- as it does not refer to real-time.)The paper proposes a new surprisingly simple protocol that ensures sequential consistency when the shared memory abstraction is supported by the local memories of nodes that can communicate only by exchanging messages through reliable channels. The protocol nicely combines in a simple way the use a token with cached values. It has the noteworthy property to never invalidate cached values, thereby providing fast read operations (i.e., a process has never to wait to get a correct value of a shared object). Additionally, The paper presents a simple token navigation protocol.