The W3C RDF Data Access Working Group has released an initial Working Draft specification for RDF Data Access Use Cases and Requirements which "outlines use cases for RDF query languages and access protocols and their requirements, examining their design objectives." The Working Draft has been produced as part of the W3C Semantic Web Activity.
The RDF Data Access Working Group was chartered to "gather requirements and to define an HTTP and/or SOAP-based protocol for selecting instances of subgraphs from an RDF graph," paying special attention to the RDF Net API submission. The WG's work involves specification of a "language for the query and the use of RDF in some serialization for the returned results. The query langauge may have aspects of a path language similar to XPath (used for XML in XSLT and XQuery) and various RDF experimental path syntaxes."
The Use Cases and Requirements draft clarifies some of the objectives in the W3C design of an RDF query language and data access protocol. "Each use case describes a user-oriented context in which the RDF query language or protocol or both are used to solve a real problem. The use cases characterize some of the most important and most common motivations behind the development of existing RDF query languages and access protocols. The use cases, in turn, inform decisions about requirements, that is, the critical features that a standard RDF query language and data access protocol require, as well as design objectives that are not on the critical path."
Sample use cases include: finding an email address; finding information about motorcycle parts; finding unknown media objects; monitoring news events; avoiding traffic jams; discovering what people say about news stories; exploring the neighborhood; sharing vacation photos with a friend; finding input and output documents for test cases.
Technical requirements outlined in the draft are "features or characteristics of either the query language or data access protocol (or, in some cases, of both) that are expected to be in the specification." Examples include: RDF graph pattern matching; variable binding results; extensible value testing; subgraph results; local queries; optional match; limited datatype support; bookmarkable queries; bandwidth-efficient protocol; result limits.
"The RDF data model is a directed, labeled graph with edges labeled with URIs and nodes that are either unidentified, literals, or URIs. The W3C's Semantic Web Activity is based on RDF's flexibility as a means of representing data. While there are several standards covering RDF itself, there has not yet been any work done to create standards for querying or accessing RDF data. There is no formal, publicly standardized language for querying RDF information. Likewise, there is no formal, publicly standardized data access protocol for interacting with remote or local RDF storage servers."
The requirements sketched in the initial Working Draft are in development: the RDF Data Access Working Group has adopted some but not all of these requirements; several are still under discussion. The team invites feedback especially with respect to which use cases and requirements should be elaborated, clarified, removed, or added.
From the RDF Data Access Use Cases and Requirements Introduction
"The W3C's Semantic Web Activity is based on RDF's flexibility as a means of representing data. While there are several standards covering RDF itself, there has not yet been any work done to create standards for querying or accessing RDF data. There is no formal, publicly standardized language for querying RDF information. Likewise, there is no formal, publicly standardized data access protocol for interacting with remote or local RDF storage servers.
Despite the lack of standards, developers in commercial and in open source projects have created many query languages for RDF data. But these languages lack both a common syntax and a common semantics. In fact, the extant query languages cover a significant semantic range: from declarative, SQL-like languages, to path languages, to rule or production-like systems. The existing languages also exhibit a range of extensibility features and built-in capabilities, including inferencing and distributed query.
Further, there may be as many different methods of accessing remote RDF storage servers as there are distinct RDF storage server projects. Even where the basic access protocol is standardized in some sense — HTTP, SOAP, or XML-RPC — there is little common ground upon which to develop generic client support to access a wide variety of such servers.
The following use cases characterize some of the most important and most common motivations behind the development of existing RDF query languages and access protocols. The use cases, in turn, inform decisions about requirements, that is, the critical features that a standard RDF query language and data access protocol require, as well as design objectives that aren't on the critical path..."
