Diversity is essential to successful preservation. Requirements Librarians techniques for preserving access to published paper materials have been honed since 415 AD, when much of the world s then-current literature was lost in the Library of Alexandria fire. Access is restricted to protect the artifacts and to ensure preservation. Circulating library collections provide access, and explicit risks are taken with each artifact to achieve this goal. Copies are loaned to readers on a promise that they will eventually be returned.
Libraries circulating collections are a model fault-tolerant distributed system.
As a whole, the system is highly replicated and far more reliable than any individual component. There is no single point of failure, no central control to be subverted. There is a low degree of policy coherence between the replicas, and thus low systemic risk. The desired behavior of the system as a whole emerges as the participants take actions in their own local interests and cooperate in ad-hoc, informal ways. Librarians are more likely to have confidence in an electronic system if it works in a familiar way.