  The need to federate repositories emerges in two distinctive scenarios. In
one scenario, scalability-related problems in the operation of a repository
reach a point beyond which continued service requires parallelization and hence
federation of the repository infrastructure. In the other scenario, multiple
distributed repositories manage collections of interest to certain communities
or applications, and federation is an approach to present a unified perspective
across these repositories. The high-level, 3-Tier aDORe federation architecture
can be used as a guideline to federate repositories in both cases. This paper
describes the architecture, consisting of core interfaces for federated
repositories in Tier-1, two shared infrastructure components in Tier-2, and a
single-point of access to the federation in Tier-3. The paper also illustrates
two large-scale deployments of the aDORe federation architecture: the aDORe
Archive repository (over 100,000,000 digital objects) at the Los Alamos
National Laboratory and the Ghent University Image Repository federation
(multiple terabytes of image files).
