1.5 AD schema

The schema contains the definition of all the objects in the AD. Every object has its own class and a set of attributes (also called properties) associated with it. A class is the unit of storage in the AD. This means that for an object to be stored in the AD, it must first have a class, which defines how it is stored and what attributes it may have. Attributes hold the value of a particular property in the object. Attributes are typed, which means that their value is stored as a string, octet, numeric, and so on. Attributes may be multivalued, which means that an attribute may be stored using multiple values. For example, a user can have multiple telephone numbers.

The internal structure of the AD can be compared with a relational DB. Objects are stored in a large table. Each object is stored in a row within the table, and the different attributes for objects are stored in the columns in the table. When an attribute is not used, it does not occupy any space within the DB. For example, if the telephone number attribute for a user object is not filled, the column will be empty, and no space will be allocated to store the attribute inside the DB. This feature is necessary because the number of attributes associated with an object may be considerable. For example, the class user went from a few attributes in Windows NT 4.0 to more than...