Version 1.1, 20th November 2003

Background

The SRW Initiative, building on Z39.50 along with web technologies, recognizes the importance of Z39.50 (as currently defined and deployed) for business communication, and focuses on getting information to the user. SRW provides semantics for searching databases containing metadata and objects, both text and non-text. Building on Z39.50 semantics enables the creation of gateways to existing Z39.50 systems while reducing the barriers to new information providers, to make their resources available via a standard search and retrieve service.

SRW defines a web service combining several Z39.50 features, most notably, the Search, Present, Sort and Scan Services. Additional features/services may be added later or defined later as new web services.

Z39.50 Concepts Retained in SRW

• Result Sets
• Abstract Access points
• Abstract Record schemas
• Explain
• Diagnostics

SRW Features which Differ from Z39.50

• Result Set Named by Server
  In contrast to Z39.50 where the client names the result set, for SRW the server assigns the result set id. After a server executes a query it may include in the response a result set name. (If the server does not intend that the result set will remain reasonably static, then it will not supply a result set name.) The server may include a result set idle time value indicating a projected (not guaranteed) length of time that the result set will remain available if it is not referenced.
  
  
• Connections, Sessions, State
  There is no explicit concept of connection, session, or state. Each invocation of the Search/Retrieve service will be a request/response sequence, via an XML/SOAP/RPC message using HTTP POST. Different invocations will not be related to one-another (in any way that's visible to the protocol). A result set created by one invocation may be referenced by a subsequent invocation, however it is the responsibility of the application (not the protocol) to ensure that the subsequent invocation is authenticated/authorized to access the result set. (An authentication token is introduced in the protocol which may be used for this purpose.)
  
  
• No distinction between server and database
  SRW does not distinguish between a server and a database; the philosophy is that the database distinction is just another search criterion, and search criteria is the province of the query. It is hoped that elimination of the database concept will effect significant simplification (since the multiple-database concept in Z39.50 has caused such complexity), for example Explain will be significantly simplified (and hopefully it will therefore become more widely implemented).
  
  
• Single record syntax
  All SRW records are retrieved according to a single record syntax (XML) and therefore the Z39.50 concept of record syntax is not meaningful in SRW. The Z39.50 concepts of element set/specification and schema are represented by XML schemas. The following record schemas will be distinguished in SRW: Dublin Core, Onix, MODS, and MarcXml.
  
  
• String Query
  SRW specifies string queries. The query language, CQL ("Common Query Language"), is a human-readable-string query-representation based loosely on CCL (however just the query, no commands) with access points defined. The CQL syntax will include the result set name, and will support both the capability to qualify a result set (e.g. "records in result set 'A' where title is 'B' ") and to specify only a result set name (e.g. "records in result set 'A'") analogous to a Z39.50 Present.
  
  
• Flat Access Points
  Flat access points are defined, rather than utilizing attribute vectors as in traditional Z39.50. For example, consider 'title - word' and 'title - phrase'. In SRW these would be represented as distinct access points (rather than two attribute combinations with the same Use attribute and different qualifying attributes).
  
  
• Static Explain
  Explain information will be static (not based on the Z39.50 Explain concept of searching an Explain database for specific information), Explain information will include supported access points and record schemas. The Explain simplification also owes in large part to the SRW simplification discarding multiple databases and record syntaxes, and it is hoped that there will be more motivation to implement the SRW version of Explain (than there was to implement the Z39.50-1995 Explain) because of the substantial simplification.
  
  
• XML instead of ASN.1.
  XML will be used for abstract syntax as well as encoding. ASN.1/BER is not used.