- SRU (Search/Retrieve via URL) is a URL-based system for communication between information retrieval systems. SRU is based on Z39.50, but removes a lot of the old baggage that no one was using. If any of the old stuff is needed, it can be implemented as a separate web service. This is just search and retrieve.
- SRW (Search/Retreive Web Service) is being renamed "SRU via SOAP".
- CQL is the query language used by SRU. (Contextual Query Language)
- All of the above used to fall under the set of standards called ZiNG (Z39.50 International: Next Generation), but they are now all being renamed SRU. (Although the SRU XML objects still use the SRW namespace.)
A list of available SRU servers is maintained by OCLC.
It is very easy to write basic SRU implementations. SOAP toolkits can use a WSDL description to generate protocol code. Then all we have to
do is implement the single client or server SRU method, which translates your query format into an CQL query (client) or a CLQ query into your database query (server).