Inside Microsoft SQL Server 2005: The Storage Engine (Solid Quality Learning)

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 conflicts by acquiring locks on data that is being read, so no other processes can modify that data. It also acquires locks on data being modified, so no other processes can access that data for either reading or modifying. In other words, readers block writers and writers block readers in a pessimistic concurrency environment.

Optimistic Concurrency

Optimistic concurrency assumes that there are sufficiently few conflicting data modification operations in the system that any single transaction is unlikely to modify data that another transaction is modifying. The default behavior of optimistic concurrency is to use row versioning to allow data readers to see the state of the data before the modification occurs. Older versions of data rows are saved, so a process reading data can see the data as it was when the process started reading and not be affected by any changes being made to that data. A process that modifies the data is unaffected by processes reading the data because the reader is accessing a saved version of the data rows. In other words, readers do not block writers and writers do not block readers. Writers can and will block writers, however, and this is what causes conflicts. SQL Server generates an error message when a conflict occurs, but it is up to the application to respond to that error.