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:
Understanding ALLBASE/SQL Data Access
Use of Locking by Transactions
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.
