Before WAS 6.1 if you wanted to use multiple LDAPs, or combination of LDAP and database as user repository then only option that you had was to create a custom user repository. And developing custom user repository is very complex. Starting from WAS 6.1 you can use Federated User Repository, which allows you to map entries from multiple individual user repositories into a single virtual repository.
The federated repository consists of a single named realm, which is a set of independent user repositories. Each repository may be an entire external repository or, in the case of LDAP, a subtree within that repository. The root of each repository is mapped to something called a base entry within the federated repository, which is basically a starting point within the hierarchical namespace of the virtual realm.To the WebSphere Application Server runtime, there is still only one registry, and thus, all applications in the cell still share this one single registry
A federated repository contains a realm that can consist of identities in:
* The file-based repository that is built into the system.
* One or more external LDAP repositories.
* A JDBC accessible database repository.