Compatibility and Commutativity of Lock Modes

An important characteristic of concurrency control mechanisms is the level of concurrency that they support. In this paper, we study this problem in the context of non-two-phase locking protocols which are defined for data bases in which a directed acyclic graph structure is superimposed on the data items. A new lock mode is introduced called INV, with properties fundamentally different from lock modes previously studied and show how this allows increased concurrency. Through the introduction of the INV mode of locking, a new principle of the theory of data base concurrency control is enunciated. This principle involves the separation of the effects of cornmutativity (which relates to serializability) and compatibility (which relates to deadlock-freedom of data manipulation operations. It is shown how the level of concurrency in an existing very general protocol could be increased. Then how the extension affects the occurrence of deadlocks is examined. Certain conditions under which deadlock-freedom is maintained are identified , and simple methods for removing deadlocks in other situations are presented.