The last overlay that we will examine in this section is the uniqueness overlay. The uniqueness overlay enforces uniqueness for a configurable set of attributes in the directory. It prevents attributes in different records from containing the same values. This is desirable, for example, when working with the
uid attribute, where we would clearly not want to have the same UID for multiple users in the system. By default, SLAPD only enforces uniqueness when it comes to DNs—no two DNs may be the same. But other attribute values are unchecked. Using the uniqueness overlay, we can specify which attributes we want SLAPD to ensure uniqueness for.
The first step in configuring the uniqueness overlay is to load the module: