Optimistic Concurrency Control for Replicated Objects

The distributed applications are composed of multiple objects o_1, ..., o_n which are cooperating by exchanging messages through the communication network. Each object o_i supports abstract operations for manipulating the state of o_i. The objects have to be mutually consistent in the presence of multiple accesses to the objects. In the famous two-phase locking (2PL) protocol, the transactions lock the objects before computing the operations on the objects. Most systems adopt the strict 2PL protocol where the locks obtained are released at the end of the transactions in order to resolve the cascading abort. The optimistic concurrency control is discussed to reduce the overhead implied by the locking. Here, the transaction manipulates objects without locking the objects. When the transaction T ends up, T commits unless the objects manipulated by T are accessed by other transactions in the modes conflicting with T. In order to increase the reliability, availability, and performance of the system, the objects in the system are replicated. Here, it is critical to make the replicas of the object mutually consistent. Jing discusses an optimistic two-phase locking (O2PL) method to maintain the mutual consistency among the replicas. In the O2PL, all the replicas are locked by a transaction T in a read (Rlock) mode to write the object. When T commits, T tries to convert the Rlock mode , on the replicas to a write (Wlock) mode. If succeeded, T commits. Otherwise, T aborts. The distributed applications are modeled in an object-based concept. The objects support abstract operations. In this paper, each object is locked in an abstract mode corresponding to the abstract operation. The conflicting relation between the lock modes is defined based on the conflicting operations. In this paper, we propose a novel optimistic locking scheme for the replicated objects. The number of replicas to be locked depend on how strong the lock mode of the operation is and how frequently the operation is invoked.