Beyond two-phase locking

Abstract

Many database systems maintain the consistency of the data by using a locking protocol to restrict access to data items. It has been previously shown that if no information is known about the method of accessing items in the database, then the two-phase protocol is optimal. However, the use of structural information about the database allows development of non-two-phase protocols, called graph protocols , that can potentially increase efficiency. Yannakakis developed a general class of protocols that included many of the graph protocols. Graph protocols either are only usable in certain types of databases or can incur the performance liability of cascading rollback. In this paper, it is demonstrated that if the system has a priori information as to which data items will be locked first by various transactions, a new graph protocol that is outside the previous classes of graph protocols and is applicable to arbitrarily structured databases can be constructed. This new protocol avoids cascading rollback and its accompanying performance degradation, and extends the class of serializable sequences allowed by non-two-phase protocols. This is the first protocol shown to be always as effective as the two-phase protocol, and it can be more effective for certain types of database systems.