The WHOIS++ protocol is a distributed directory system, originally designed to provide a "white pages" search mechanism to find humans, but which could actually be used for arbitrary information retrieval tasks.
The WHOIS protocol is still widely used to allow domain ownership records in the Internet to be easily queried.
WHOIS++ attempted to address some of the short comings in the original WHOIS protocol that had become apparent over the years.
Whilst WHOIS++ as a white pages directory service never really took off compared to competitors such as X.500, it did gain a notable amount of use in the United Kingdom as the underlying search and retrieval protocol of a number of subject based gateways funded as part of the Jisc Electronic library programme.
The use of centroids to provide forward knowledge and query routing allowed a subject gateway to not only provide resources to academic users from their own database but also point them at other JISC funded subject gateways that might have useful information.