Abstract
This document specifies use cases, requirements, and objectives for an
RDF query language and data access
protocol. It suggests how an RDF query language and data access protocol
could be used in the construction of novel, useful Semantic Web
applications in areas like web publishing, personal information
management, transportation, and tourism.
This is a firstsecond Public Working Draft of the Data Access Use Cases
and Requirements for review by W3C Members and other interested
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Publication as a Working Draft does not imply endorsement by
the W3C Membership. This is a draft document and may be updated,
replaced or obsoleted by other documents at any time. It is
inappropriate to cite this document as other than work in
progress.
This document has been produced by the RDF Data Access Working
Group as part of the Semantic Web Activity in the
W3C Technology & Society
Domain. It reflects the best effort of the editor to incorporate
input from various members of the WG, but is not yet endorsed by the
WG as a whole. In particular, the requirementsdesign objectives
are in development. The status of each requirementdesign objective indicates whether
it has been adopted by the WG. The requirements have
all been accepted by the working group.
This document was produced under the 5 February 2004
W3C Patent Policy. The Working Group maintains a public list of
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Essential Claim(s) with respect to this specification should disclose the
information in accordance with section
6 of the W3C Patent Policy.
Per section
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document.
Table of Contents
- Introduction
-
Use Cases
-
Requirements
-
Design Objectives
- Related Technologies and Standards
- Acknowledgments
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.
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. However, it
is not necessarily the case that the query language or data access protocol
will directly address all of these use cases. (Some of the use cases
contain illustrative RDF in Notation 3 form; consult Primer: Getting into the semantic
web and RDF using N3 or Notation3: A Rough Guide to N3
for more details about N3.)
George wants to send email to a person named "Johnny Lee Outlaw".
George's personal address book, which includes contact information for a
"Johnny Lee Outlaw", is stored in RDF using the FOAF Vocabulary Specification.
Figure 1: FOAF Address Book ( SVG ) -->@prefix foaf: <http://xmlns.com/foaf/0.1/> .
[]
foaf:name "Johnny Lee Outlaw" ;
foaf:mbox <mailto:jlow@example.com> .
Figure One: A Fragment of a FOAF Address
Book
George's email client queries his local address book service and, since
there is only one match, uses the query's result to populate the
To: field.
Motivates: RDF Graph Pattern
Matching, Variable Binding Results.
Endeavour, a dealer specializing in new and antiqueBritish motorcycles, maintains a
database that describes spare and replacement parts, including their
properties and relationships. Ev, a repair person who specializes in
newTriumph bikes, is working on an ailing Speed Triple motorcycle when a
diagnostic tool produces a report identifying a defect in the fuel
management system.
@prefix triumph: <http://triumph.info/schema/#><http://triumph.example/schema/#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
<http://triumph.info/part/0d92ie433><http://triumph.example/part/0d92ie433>
rdf:type triumph:part ;
rdfs:label "Accelerator Cable MK3" ;
triumph:depends-on <http://triumph.info/part/329i2dk39><http://triumph.example/part/329i2dk39> ;
triumph:part-for <http://triumph.info/2004/SpeedTriple><http://triumph.example/2004/SpeedTriple> ;
triumph:part-number "LCD 100-04BSPT" .
<http://triumph.info/part/329i2dk39><http://triumph.example/part/329i2dk39>
rdfs:label "Mounting Bracket" ;
triumph:requires
[ triumph:has-number "4" ;
triumph:part-number "149028ab-MT" ;
triumph:type triumph:screwx
] .
Figure Two: A Fragment of the Endeavour Parts
Database
Ev uses a query interface to the parts database to ask about the
defective part. In response to her query, Ev receives a human-readable
description of the part, which provides enough information to obtain a
replacement part and tells her about other, dependent parts that must be
replaced at the same time.
Motivates: Subgraph Results,
Optional Match, Human-friendly
Syntax , User-specifiable Serialization.
Smiley works for a multinational media conglomerate. As part of his job
as an editor of foreign market compilations, he needs to be notified
whenever the conglomerate's knowledge bases contain information about new
media objects—books, movies, and pop music—matching various properties:
title, author, and price point.
@prefix baf: <http://big-accounting-firm.com/scheme/1.0/#><http://big-accounting-firm.example/scheme/1.0/#> .
@prefix bmc: <http://big-media-conglomerate.com/ontology/#><http://big-media-conglomerate.example/ontology/#> .
@prefix dc: <http://purl.org/dc/elements/1.1/> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
[]
baf:dollarPrice "29.99" ;
bmc:objectName "J to the LO" ;
dc:author <http://big-media.com/author/1929/><http://big-media.example/author/1929/> .
Figure Three: Big Media Conglomerate Knowledge
Base
Smiley uses his web browser to create a query that will be executed
regularly against the conglomerate's knowledge bases. Whenever there are
new matches for Smiley's query, he receives an email with URIs to resources
about the new matches; and Smiley's personal RSS feed is also updated with
the new matches, since he uses an RSS aggregator to gather news every
day.
Since Smiley's query will operate over knowledge bases structured by at
least four different ontologies—the result of his conglomerate's rapid
expansion—Karla, the staff programmer for Smiley's group, makes sure that
knowledge bases in question contain appropriate
rdfs:subPropertyOf assertions. For example, Smiley's query
uses the predicate media:ObjectName, which will also find
properties like dc:title, doi:title, and mods.mods:titleInfo.
Motivates: Human-friendly Syntax,
Provenance , User-specifiable SerializationAggregation Graphs, Aggregate
Query, Additional Semantic Information.
Kate wants to see all the television programs that feature information
about the Japanese baseball player Ichiro. She wants her personal digital
recorder (PDR) to record every television show about Japanese baseball
automatically using the Electronic Program Guides (EPGs). She also wants an
index page for each week's recorded items.
Her RDF-enabled PDR periodically executes a query against the RDF
version of its EPGs, and continues to execute the query every day for new
items to record.
Motivates: Bookmarkable Queries ,Result Limits, Aggregate Query, Addressable Query
Results.
Niel has to drive every day from home to his office during heavy rush
hour traffic in Atlanta, GA, in his new car, which has Bluetooth and
wireless Internet access. Using his cell phone, Niel requests that his car
query public RDF storage servers on the Web for a description of current
Atlanta road construction projects, traffic jams, and roads affected by
inclement weather.
Based on the information retrieved efficiently from the public RDF
servers, Niel uses the mapping program in his cell phone to plan a
different route to work, cutting his commute time by 10%.
Motivates: Bandwidth-efficient
Protocol, Result Limits.
Abelard, an independent publisher of web publications, wants to query
RSS feed aggregators in order to track RDF assertions people make about
articles and stories in his publications. Abelard's client software
includes support for three different RDF query languages.
Heloise manages one of the servers that Abelard wants to query. Her
server publishes a machine readable description of its capabilities,
including the query languages it supports, in RDF. It negotiates withAbelard's client in order to choose the most appropriateasks
Heloise's server whether it supports his preferred query language that they have in common.language.
Abelard's client software also negotiates with the other servers and uses a
common transport protocol to retrieve the results of his queries.
Motivates: Human-friendly Syntax,
ProvenanceAggregation Graphs, Aggregate
Query, Yes-No Queries.
José knows that the U.S. Census Bureau provides interesting geographic
data in its public domain TIGER database. José attends a
conference in Washington, DC, at the new convention center, and he stays in
a hotel nearby. José wants to find out the latitude, longitude, name, and
type of everything within one mile of the convention center, as well as all
events occurring during his stay, so that he can plan his meals and
sightseeing time accordingly.
Rather than working with the TIGER database files directly, José sends a
query to the Census Bureau's new RDF storage server and requests that his
client pass the query results to an XSLT transformation service so that he
can print the resulting XHTML.
Motivates: Extensible Value
Testing, Limited Datatype Support, Human-friendly Syntax.
Frannie and Zoe, old college friends, live in different countries and
keep in daily contact via IRC. Zoe wrote an IRC bot that they use to make
assertions—which the bot stores as RDF—about photographs of their family,
friends, and vacations. Frannie wants to be able to republish some of these
assertions in a human readable form on her weblog. Zoe tells her about a
server that accepts and agrees to host documents that describe what they
say about web resources, and their IRC bot sends those documents
periodically to the server.
Frannie programs her weblog software to query the server that hosts
their annotations for vacation images that co-depict her family members
with Zoe's family members, as well as for things Zoe and Franny have said
about those images. Frannie uses the XSLT processor built into her weblog
software to transform the query results into XHTML for display in her
weblog.
Motivates: Variable Binding
Results, Non-existent Triples, Aggregate Query.
Nada, a Semantic Web developer, has a bug report from a valued user
indicating that a software tool is incorrectly emitting the N3
representation of some of the RDF core test cases. Nada wants to create a
list of input and output documents for each of the approved test cases,
filtering only for those which have an "approved" status, from the RDF core
test suite. The list of tests resides in a single file.
Nada can programmatically process the RDF core manifest file with a
result which is one line per input/output pair so that a script can easily
be written to create the next stage, namely, reading the input document,
writing it and checking it.
Motivates: RDF Graph Pattern
Matching, Variable Results, Local
Queries , User-specifiable Serialization.
Erasmus Jones, a professor, wants to find some cases, of both)learning materials for
his seminar on Renaissance humanism. He is using a recommended web site
that are expected to be inprovides descriptions of learning materials; he performs a search at
the specification. 3.1 RDF Graph Pattern Matchingsite, chosing the query language mustgeneral subject area, student learning level, and
provides some keywords. The results include materials returned from
multiple learning repositories, where the capability to restrict matches onsubject and learning levels have
been matched across multiple educational metadata vocabularies, including
predicates from the Dublin
Core Metadata Element Set and the UK Learning Object Metadata
Framework specifications.
Motivates: Aggregation Graphs,
Aggregate Query.
Esther, a queried graph by providingprogrammmer for a graph pattern, which consists of one or morenew social networking site based on FOAF, has written an RDF triple patterns,crawler which
follows foaf:knows links to be satisfieddetermine the publicly available
properties of new people it will invite into the network. While processing
a new FOAF resource, it finds an rdf:Property referring to a
URI that it has not seen before. The crawler queries an ontology server to
see if the property's domain(s) and range(s) are ones that it has already
encountered, so that it can track where it first discovered this property
and use the property in future searches.
Motivates: Aggregation Graphs,
Additional Semantic Information.
Peter is developing a medical knowledge base using OWL/RDF in
collaboration with medical domain experts. The knowledge base is used
within electronic patient
records. To facilitate collaboration and avoid duplication, the team is
using a federated
ebXML Registry to store the knowledge base they are building.
When adding a new concept to the knowledge base, Peter uses a registry
browser application to search the ebXML Registry for similar or related
concepts. The registry browser
allows Peter to choose a
parameterized query from a set of preconfigured parameterized queries
and offers a form that Peter uses to enter the query parameters.
Peter enters a few parameters and issues the query. The ebXML Registry
returns a large number of
matching results. Peter narrows his search by reissuing the query with
additional parameters until he find concepts that are most relevant to his
concept. Peter then drills
down and browses these concepts, as well as their related concepts and
metadata, to determine whether to add his new concept.
Motivates: RDF Graph Pattern
Matching, Variable Binding Results, Streaming Results.
Lyndie works for a firm that creates market research reports for
corporations that have contracts with the US federal government. She has
access to an RDF repository, which contains information about accounting
firms, corporations, and their customers:
@prefix baf: <http://big-accounting-firm.example/scheme/1.0/#>.
@prefix xsd: <http://www.w3.org/2001/XMLSchema#>.
<http://www.pwc.com/> baf:hasName "PriceWaterhouseCoopers"^^xsd:string.
<http://www.boeing.com/> baf:hasName "Boeing"^^xsd:string.
<http://www.labor.gov/> baf:hasName "US Department of Labor"^^xsd:string.
<http://www.pwc.com/> baf:accountsFor <http://www.boeing.com/>.
<http://www.boeing.com/> baf:hasCustomer <http://www.labor.gov/> .
Figure Four: Accounting Repository
Fragment
Lyndie wants to query this RDF repository in order to find the names of
accounting firms that do accounts for suppliers of the Department of Labor
or that do accounts for the Department of Labor itself.
Motivates: RDF Graph Pattern
Matching—Disjunction.
Marty wants to learn which of the ten biggest grossing Hollywood movies
of all time also had soundtracks among the ten biggest grossing film
soundtracks of all time. Imagine that some future version of the IMDB site
exposes its information about movies as RDF. Further imagine that the CDDB
site does the same for its information about music. Marty then writes a
query to find the titles of the ten biggest grossing films. He uses the
results of that query to query CDDB in order to filter the films that did
not have top 10 soundtracks.
Motivates: Querying Multiple
Sources.
3. Requirements
Technical requirements 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.
The query language must include the capability to restrict matches on a
queried graph by providing a graph pattern, which consists of one or more
RDF triple patterns, to be satisfied in a query.
Status: Accepted 2004-05-11.
It must be possible for queries to return zero or more bindings of
variables. Each set of bindings is one way that the query can be satisfied
by the queried graph.
Status: Accepted 2004-05-11.
The query language must make it possible—whether through function calls,
namespaces, or in some other way—to calculate and test values
extensibly.
Many application domains have specific value testing requirements; for
example: the concept of "distance" in geospatial data or calculating the
gravitational attraction of two masses, given their mass and the distance
between them. Value testing may be more efficient when domain specific
functions are available for use.
Status: Accepted 2004-05-04.
It must be possible for query results to be returned as a subgraph of
the original queried graph that the query matches. Status: Pending. 3.4a Subgraph Results (variant) It must be possible to select an entailed subgraph of a queried graph, in which case the query results are an RDFgraph.
Status: Pending.Accepted 2004-06-15
The query language must be suitable for use in accessing local RDF
data—that is, from the same machine or same system process. Status: Accepted 2004-05-04 . 3.6 Optional Match It must be possible to express a query that does not fail when some specified part of the query fails to match. Any such triples matched by this optional part, or variable bindings caused by this optional part, can be returned inis, from the results, if requested.same machine or same system process.
Status: Pending.Accepted 2004-05-04.
(variant)It must be possible to express a query with optional parts suchthat the querydoes not fail to matchwhen one or more optional partssome
specified part of the query fails to match. Any such triples matched by
this optional part, or variable bindings caused by this optional part, can
be returned in the results, if requested.
Status: Pending.Accepted 2004-07-15.
The query language must include support for a subset of XSDW3C XML Schema
datatypes and operations on those datatypes.
Status: Accepted 2004-05-11.
It must be possible to express a query as a URI.specify an upper bound on the resultnumber of dereferencingquery
results returned.
(Note: The resource identified by this URI will be a representationWorking Group has discussed and is aware
of the query results. This formconnection between result limits and result sorting, as well as the
implementation costs of sorting and the query is not assumed totradeoffs between client and server
computing power per user.)
Status: Accepted 2004-07-15.
It must be humanly readable. This requirement does not preclude other mechanismspossible, when returning multiple unordered results, for issuing queries. Status: Pending. 3.9 Bandwidth-efficient Protocolthe
access protocol design shall address bandwidth utilization issues;client to request that is, it shall allow for at leastresults be streamed. When the client requests
streaming results, all the data in one result format that does not make excessive use of network bandwidth for a given collection of results. Status: Pending. 3.10 Result Limits Itmust be possible toavailable to specify an upper bound onthe
number of query results returned.client before all the data for the next result.
Status: Pending. 3.10a IterativeAccepted 2004-06-29.
The query (variant) Itlanguage must be possibleinclude the capability to handle large result setsrestrict matches on a
queried graph based on a disjunction of any size by iterating over the result set and fetching it in chunks.graph patterns, at least one of
which must be satisfied.
Status: Pending.Accepted 2004-07-16.
4. Design Objectives
Design objectives, which may be features or characteristics of the
eventual design, differ from requirements in that the specification may be
complete if none, some, or all of them are achieved.
There must be a text-based form of the query language which can be read and written by users of the language. Status: Pending. 4.2 Provenanceread
and written easily by users of the language.
Status: Accepted 2004-07-15.
RDF can be used for data integration and aggregation. RDF repositories
are built by merging RDF triples from several other RDF repositories or
from non-RDF sources converted to RDF. Such an aggregations can be real or
virtual.
It shouldmust be possible for the query resultslanguage and protocol to includeallow an RDF
repository to expose the source from which a query server collected a
triple or provenance information.subgraph.
Status: Pending.
It shouldmust be possible to query for the non-existence of one or more
triples or triple patterns. Status: Pending. 4.4 User-specifiable Serialization It should be possible to specify the serialization format of query results; this design objective is meant to be orthogonal topatterns in the semantics of query results, whether subgraph, variable bindings, or some other type.queried graph.
Status: Pending.Accepted 2004-07-15.
It should be possible to specify two or more RDF graphs against which a
query shall be executed; that is, the result of an aggregate query is the
merge of the results of executing the query on each of two or more
graphs.
Status: Pending.
It should be possible for a query to specify which of the available RDF
graphs it is to be executed against. If more than one RDF graph is
specified, the result is as if the query had been executed against the
merge of the specified RDF
graphs. Query processors with a single available RDF graph trivially
satisfy this objective.
Status: Pending.
It should be possible for knowledge encoded in other semantic
languages—for example: RDFS, OWL, and SWRL—to affect the results of queries
executed against RDF graphs.
It should be possible for a query to indicate that the answers should
take into account knowledge encoded in RDF semantic extensions such as
RDFS, OWL, etc.
Status: Pending.
The access protocol design shall address bandwidth utilization issues;
that is, it shall allow for at least one result format that does not make
excessive use of network bandwidth for a given collection of results.
Status: Accepted.
It should be possible for a query to perform substring searches of RDF
string literals.
Status: Accepted Accepted
2004-07-16.
It must be possible in the query language to express yes-no questions
straightforwardly.
Status: Accepted 2004-07-15.
A common pattern of access is to send a query, which is like a
question, to a remote service which evaluates it and returns the
results. This access pattern fits naturally into the architecture of the
Web by making query results addressable resources.
It must be possible for query results to be addressed in URI space.
Status: Accepted Accepted
2004-07-16.
See the survey of existing RDF query language implementations: "RDF Query Survey", as
well as the "RDF Query and Rules
Framework".
The editor acknowledges all of the members of the Data Access Working Group for
aid and assistance in preparing the present document, especially Andy
Seaborne, Yoshio Fukushige, Bryan Thompson, Howard Katz, Dave Beckett, Dan
Connolly, and Eric Prud'hommeaux. The editor also acknowledges the support
of his University of Maryland MIND Lab colleagues, especially Bijan Parsia
and James Hendler.
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