Locking and Row Versioning
The SQL Server Database Engine uses the following mechanisms to ensure the integrity of transactions and maintain the consistency of databases when multiple users are accessing data at the same time:
Each transaction requests locks of different types on the resources, such as rows, pages, or tables, on which the transaction is dependent. The locks block other transactions from modifying the resources in a way that would cause problems for the transaction requesting the lock. Each transaction frees its locks when it no longer has a dependency on the locked resources.
When a row versioning-based isolation level is enabled, the Database Engine maintains versions of each row that is modified. Applications can specify that a transaction use the row versions to view data as it existed at the start of the transaction or query instead of protecting all reads with locks. By using row versioning, the chance that a read operation will block other transactions is greatly reduced.
Locking and row versioning prevent users from reading uncommitted data and prevent multiple users from attempting to change the same data at the same time. Without locking or row versioning, queries executed against that data could produce unexpected results by returning data that has not yet been committed in the database.
Applications can choose transaction isolation levels, which define the level of protection for the transaction from modifications made by other transactions. Table-level hints can be specified for individual Transact-SQL statements to further tailor behavior to fit the requirements of the application.
Users can control row versioning implementation by enabling or disabling database options. For more information, see Enabling Row Versioning-Based Isolation Levels and Using Row Versioning-based Isolation Levels.