Concurrency control and locking is the mechanism used by DBMSs for the sharing of data. Atomicity, consistency, and isolation are achieved through concurrency control and locking. See ACID Properties.
When many people may be reading the same data item at the same time, it is usually necessary to ensure that only one application at a time can change a data item. Locking is a way to do this. Because of locking, all changes to a particular data item will be made in the correct order in a transaction. See isolation.
The amount of data that can be locked with the single instance or groups of instances defines the granularity of the lock. The types of granularity are illustrated here are:
Additional information on this subject along with examples of multi-user considerations can be found in the Object Database Handbook.
More Detail on Concurrency Control and Locking
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The Savvy Manager's Guide
Douglas K Barry is also the author of a book that explains Web Services, service-oriented architecture, and Cloud Computing in an easy-to-understand, non-technical manner.
Web Services, Service-Oriented Architectures, and Cloud Computing: The Savvy Manager's Guide (Second Edition)
by Douglas K Barry with David Dick
This is a guide for the savvy manager who wants to capitalize on the wave of change that is occurring with Web Services, service-oriented architecture, and—more recently—Cloud Computing. The changes wrought by these technologies will require both a basic grasp of the technologies and an effective way to deal with how these changes will affect the people who build and use the systems in our organizations. This book covers both issues. Managers at all levels of all organizations must be aware of both the changes that we are now seeing and ways to deal with issues created by those changes.