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Inside Microsoft® SQL Server™ 2005, Fourth Edition by Kalen Delaney

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Concurrency Models

In either concurrency model, a conflict can occur if two processes try to modify the same data at the same time. The difference between the two models lies in whether conflicts can be avoided before they occur or can be dealt with in some manner after they occur.

Pessimistic Concurrency

With pessimistic concurrency, the default behavior is for SQL Server to acquire locks to block access to data that another process is using. Pessimistic concurrency assumes that enough data modification operations are in the system that any given read operation will likely be affected by a data modification made by another user. In other words, the system behaves pessimistically and assumes that a conflict will occur. Pessimistic concurrency avoids ...

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