Oracle Essentials: Oracle9i, Oracle8i and Oracle8, Second Edition

Chapter 7. Multiuser Concurrency

All information systems fulfill a single purpose: to collect, store, and retrieve information. As systems grow to handle many different users with many different needs, problems can arise as a result of the conflicting demands for concurrent access to the same data.

Concurrent user access is one of the most central and vexing issues for applications utilizing databases. Concurrency can affect two of the most important facets of any application: the underlying integrity of the data and the performance of the application system.

As Ken Jacobs, Vice President at Oracle, put it in his classic paper entitled “Transaction Control and Oracle7,” a multiuser database must be able to handle concurrently executing transactions in a way that “ensure(s) predictable and reproducible results.” This goal is the core issue of data integrity, which, in turn, is the foundation of any database system.

As multiple users access the same data, there is always the possibility that one user’s changes to a specific piece of data will be unwittingly overwritten by another user’s changes. If this situation occurs, the accuracy of the information in the database is compromised, which can render the data useless or, even worse, misleading. At the same time, the techniques used to prevent this type of loss can dramatically reduce the performance of an application system, as users wait for other users to complete their work before continuing. These techniques act like a traffic ...