Clarification of Two Phase Locking in Concurrent Transaction Processing.

The two phase locking protocol for serializability in concurrent transaction processing is simile to comprehend but is difficult to formalize. The definition of the two phase locking class given by Papadimitriou assumes certain facts about the transaction model, which are not obvious to the reader, and dues not clarify the occurrence of events in the log of database accesses. In this paper, we propose a formal definition of the two phase locking class derived from the semantic description of the two phase locking protocol, and prove that this definition is equivalent to that given by Papadimitriou. The contributions of the paper are as follows. 1) A precise definition of the two phase locking. 2) A clarification of the occurrence and the order of all events such as lock points, unlock points, read operations, and write operations of conflicting transactions. 3) By relaxing some conditions in the new definition, we derive a new class called restricted-non-two-phase locking (RN2PL) which is a superset. of the class two-phase locking (2PL) but a subset of the class D-serializable (DSR) given by Papadimitriou. [ABSTRACT FROM AUTHOR]