HP 3000 Manuals HP 3000 Manuals
Chapter 5 Concurrency Control through Locks and Isolation Levels Concurrency control is the process of regulating access to the same data by multiple transactions operating in the same DBEnvironment. Without regulation, a database could easily become inconsistent or corrupt. Consider what can happen if two or more concurrent users access the same data without any concurrency control. For example, one user could delete a row while another user is in the process of updating it. Or one user might update a row, and a second user might make a decision based on the update, then the first user might decide to roll back the update, at which point the second user's decision becomes invalid. To avoid problems of this type, it is important to regulate the kinds of access to database tables available to concurrent users. This chapter describes the methods employed by ALLBASE/SQL to provide concurrency control for multiuser DBEnvironments. A section is devoted to each of the following topics: * Defining Transactions * Understanding ALLBASE/SQL Data Access * Use of Locking by Transactions * Defining Isolation Levels between Transactions * Details of Locking * What Determines Lock Types * Scope and Duration of Locks * Examples of Obtaining and Releasing Locks * Resolving Conflicts among Concurrent Transactions * Monitoring Locking with SQLMON The techniques of concurrency control described in this chapter are normally implemented through application programs, though you can use some of them interactively as well. Concurrency is a complex subject. If you are a new user of relational technology or of ALLBASE/SQL, you should read the entire chapter before attempting to use any of the special features described here.