Another paper from Dan Rehak. A useful comparison of machine interfaces for search.

There are several available interface specifications, protocols and APIs for repository search and information retrieval. This document compares key characteristics of these to inform selection or profiling of one or more of these specifications for use within a CORDRA(TM) implementation or within Federated CORDRA. [Draft: Search Interface Protocols and Specifications]

CORDRA is working towards specifications for federated repositories.

Via Ralph LeVan.