1. A method of updating schema in a directory system, the method comprising: providing the directory system having a reference layer adapted to provide directory functionality for a first data store and an intermediate layer adapted to provide supplemental directory functionality for a second data store, wherein the intermediate layer is provided between the reference layer and a client, and wherein: the reference layer comprises a first directory having a fixed directory information tree associated with the first data store and a first schema that is not extensible; and the intermediate layer comprises a second directory having an extensible directory information tree associated with the second data store and a second schema that is extensible; determining that a new attribute type is not defined in the first schema; registering the new attribute type in the second schema in response to said determining, the supplemental directory functionality of the intermediate layer using the second schema to make the first schema associated with the first directory appear extensible; and providing a client view of the directory system that presents the first directory as having information stored in the first directory and the second directory even though at least a value associated with the new attribute type is stored in the second directory but not in the first directory.