One of the unique features of eDirectory is its ability to maintain the tight referential integrity. Any object Classes derived from Top will have a reference attribute in its class definition. This is a hidden attribute added to all the referenced objects that are internally maintained by eDirectory. Background processes keep running to check the links between the referenced object and the referencing objects.
If the referenced object is from a different partition than the one held locally in the server, an external reference to that object will be created locally in the external reference partition. An external reference is a representation of an object existing in the eDirectory tree. However, it is not a copy of the object and its assigned attributes.
Though we can remove the Reference attribute from eDirectory, currently, the class definitions are untouched to maintain the backward compatibility in the tree.
This chapter explains the changes and possible upgrade scenarios in eDirectory 9.1.