Inside Microsoft® SQL Server™ 2005

Isolation Levels

Isolation levels allow you to control the consistency level that you will get when manipulating data, bearing in mind that multiple processes might be running concurrently. SQL Server 2000 gave you four isolation levels (read uncommitted, read committed, repeatable read, and serializable). SQL Server 2005 adds two new isolation levels (snapshot and read committed snapshot).

The different isolation levels control the level of consistency that you will get when manipulating data mainly by controlling the way readers behave. You can improve consistency at the cost of worsening concurrency, and vice versa. Technically, isolation levels improve consistency by increasing the duration of locks. Of course, the other side of the coin is ...