About Class Of Service - Red Hat DIRECTORY SERVER 8.1 - DEPLOYMENT Deployment Manual

Chapter 4. Designing the Directory Tree

4.3.3. About Class of Service

A class of service (CoS) shares attributes between entries in a way that is invisible to applications.
With CoS, some attribute values may not be stored with the entry itself. Instead, they are generated by
class of service logic as the entry is sent to the client application.
For example, the directory contains thousands of entries that all share the common attribute
facsimileTelephoneNumber. Traditionally, to change the fax number required updating each entry
individually, a large job for administrators that runs the risk of not updating all entries. With CoS, the
attribute value can be generated dynamically. The facsimileTelephoneNumber attribute is stored
in one location, and each entry retrieves its fax number attribute from that location. For the application,
these attributes appear just like all other attributes, despite not actually being stored on the entries
themselves.
Each CoS is comprised of the several entries in the directory:
• The CoS definition entry identifies the type of CoS. It is stored as an LDAP subentry below the
branch it affects.
• The template entry contains a list of the shared attribute values. Changes to the template entry
attribute values are automatically applied to all the entries sharing the attribute.
The CoS definition entry and the template entry interact to provide attribute values to their target
entries, the entries within their scope. The value they provide depends upon the following:
• The entry's DN (different portions of the directory tree might contain different CoS).
• A service class attribute value stored with the entry.
The absence of a service class attribute can imply a specific default CoS.
• The attribute value stored in the CoS template entry.
Each CoS template entry supplies the attribute value for a particular CoS.
• The object class of the entry.
CoS attribute values are generated only when an entry contains an object class allowing the
attribute when schema checking is turned on; otherwise, all attribute values are generated.
• The attribute stored in some particular entry in the directory tree.
Types of CoS
There are three different types of CoS depending on how the value of the dynamic attributes is to be
generated:
• Pointer CoS identifies the template entry using the template DN only. There may be only one
template DN for each pointer CoS. A pointer CoS applies to all entries within the scope of the
template entry.
• Indirect CoS identifies the template entry using the value of one of the target entry's attributes. The
target entry's attribute must contain the DN of an existing entry.
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