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The facts as we know them:

  • SRU (Search/Retrieve via URL) is a URL-based system for communication between information retrieval systems.
  • SRW (Search/Retreive Web Service) is a generic Web Service for communication between information retrieval systems. SRW 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.
  • CQL is the query language used by SRU/SRW.
  • All of the above fall under the set of standards called ZiNG (Z39.50 International: Next Generation).
  • These days, most people simply talk about support for SRU, but they may mean any combination of the ZiNG standards.

It is very easy to write basic SRW 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 SRW method, which
translates your query format into an CQL query (client) or a CLQ query
into your database query (server).


CQL allows you to specify the format of the result set (Dublin Core,
MARCXML, etc.). All servers must support Dublin Core and the SRW
Diagnostics format.

The query can be a list of attribute-value pairs, or a chunk of XML.

CQL makes a distinction between string search (exact match of a string)
and keyword search (finding all words of a string somewhere in the document).

SRU/W Results

The result set can contain state-preserving information, like the
original query or an IP address, for use with lightweight clients.

Records in the result set are encoded as strings, with angle brackets
escaped. This doesn't work very well for completely browser-based
clients using SRU, since you lose the ability to apply XPath
statements to returned records. This problem will be addressed in a
future version.

Servers can/should retain result sets, so they can be referenced
later, especially when the client is asking for multiple pages from a
set. A good way to force the server to keep a result set is to "touch"
it with the client, and refresh the time.

Misc SRU/SRU info

Authentication/encryption is not built into the protocol, and must be addressed
at a higher level. (See general web services literature.)

Contacts: Ralph LeVan (, Matthew Dovey

Comparison to other systems:

  • Xquery is based on knowing the structure of the data being searched. It doesn't work well for general-purpose searches.
  • SDLIP is complex, doesn't have a query language
  • DASL is linked to SQL, but pretty good
  • Z39.50 is complex and fragile
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