The NASA Astrophysics Data System: Architecture
  The powerful discovery capabilities available in the ADS bibliographic services are possible thanks to the design of a flexible search and retrieval system based on a relational database model. Bibliographic records are stored as a corpus of structured documents containing fielded data and metadata, while discipline-specific knowledge is segregated in a set of files independent of the bibliographic data itself.   The creation and management of links to both internal and external resources associated with each bibliography in the database is made possible by representing them as a set of document properties and their attributes.   To improve global access to the ADS data holdings, a number of mirror sites have been created by cloning the database contents and software on a variety of hardware and software platforms.   The procedures used to create and manage the database and its mirrors have been written as a set of scripts that can be run in either an interactive or unsupervised fashion.   The ADS can be accessed at http://adswww.harvard.edu 
A. Accomazzi, G. Eichhorn, M. J. Kurtz, C. S. Grant and S. S. Murray   (Harvard-Smithsonian Center for Astrophysics, Cambridge)