Unlike most attributes in Directory Server, computed attributes do not have values stored in the Directory Server database. Computed attributes become available when the values are requested. Therefore, the attribute values must be computed at the time of the request, every time the values are requested.
In general, therefore, computed attributes should be quick to process. Computed attributes that depend on large internal searches, or even access to data not contained in the directory, can be expensive to generate. Therefore, such attributes are not ideal candidates for computed attributes.
Instead computed attributes that generate values quickly from the data at hand are more typical. For example, you might use a computed attribute to count or otherwise process the values of other attributes in an entry. This chapter shows a simple, artificial example that uses the API to get the current time when reading an entry.