Controlling concurrency using locking protocols

This paper is concerned with the problem of developing locking protocols for ensuring the consistency of database systems that are accessed concurrently by a number of independent transactions. It is assumed that the database is modelled by a directed acyclic graph whose vertices correspond to the database entities, and whose arcs correspond to certain locking restrictions. Several locking protocols are presented. The weak protocol is shown to ensure consistency and deadlock-freedom only for databases that are organized as trees. For the databases that are organized as directed acyclic graphs, the strong protocol is presented. Discussion of SHARED and EXCLUSIVE locks is also included.