Concurrency Control and Locking
Concurrency control and locking is the mechanism used by DBMSs for the sharing of data. Atomicity, consistency, and isolation are achieved through concurrency control and locking. See ACID Properties.
When many people may be reading the same data item at the same time, it is usually necessary to ensure that only one application at a time can change a data item. Locking is a way to do this. Because of locking, all changes to a particular data item will be made in the correct order in a transaction. See isolation.
The amount of data that can be locked with the single instance or groups of instances defines the granularity of the lock. The types of granularity are illustrated here are:
Additional information on this subject along with examples of multi-user considerations can be found in the Object Database Handbook.
More detail for the current topic: Concurrency Control and Locking
More on the general topic: Basic Concepts for Using a DBMS