SQL Server® 2008 Administration: Real World Skills for MCITP Certification and Beyond

Blocks, Locks, and Deadlocks

Concurrency, within the realm of database systems, is defined as the condition where multiple users or processes are accessing the same database at the same time. Because this condition will result in conflicts over resource access, some mechanism must be in place to address these conflicts. Conflicts will occur if two processes attempt to modify, or even access, the exact same data at the same time. Because two or more processes cannot possibly modify the same data at the same time, something must exist that provides the illusion of complete concurrency to the client applications while truly providing individual access to the data. In SQL Server, this illusion is created using locks. Locks result in blocks and may ...