PROV-AQ: Provenance Access and Query

W3C Working Draft 19 June 2012

This version:
Latest published version:
Latest editor's draft:
Previous version:
Graham Klyne, University of Oxford
Paul Groth, VU University Amsterdam
Luc Moreau, University of Southampton
Olaf Hartig, Invited Expert
Yogesh Simmhan, Invited Expert
James Myers, Rensselaer Polytechnic Institute
Timothy Lebo, Rensselaer Polytechnic Institute
Khalid Belhajjame, University of Manchester
Simon Miles, Invited Expert


This document specifies how to use standard Web protocols, including HTTP, to obtain information about the provenance of resources on the Web. We describe both simple access mechanisms for locating provenance information associated with web pages or resources, and provenance query services for more complex deployments. This is part of the larger W3C Prov provenance framework.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document is part of a set of specifications produced by the W3C provenance working group to define interoperable interchange of provenance information in heterogeneous environments such as the Web. It describes the use of existing web mechanisms for discovery and retrieval of provenance information.

PROV Family of Specifications

The PROV family of specifications aims to define the various aspects that are necessary to achieve the vision of inter-operable interchange of provenance information in heterogeneous environments such as the Web. The specifications are as follows.

How to read the PROV Family of Specifications

Second Public Working Draft

This is the second public working draft. The changes focus on revising the provenance-service specification to provide better guidance to developers as well as introducing better naming conventions for the use of link headers in locating provenance.

This document was published by the Provenance Working Group as a Working Draft. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to public-prov-comments@w3.org (subscribe, archives). All feedback is welcome.

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 was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

1. Introduction

The Provenance Data Model [PROV-DM] and Provenance Ontology [PROV-O] specifications define how to represent provenance information in the World Wide Web.

This note describes how standard web protocols may be used to locate, retrieve and query provenance information:

1.1 Concepts

In defining the specification below, we make use of the following concepts.

Provenance information
refers to provenance represented in some fashion.
a URI denoting some provenance information.
Constrained resource
an aspect, version or instance of a resource, about which one may wish to present some provenance information. For example, a weather report for a given date may be an aspect of a resource that is maintained as the current weather report. A constrained resource is itself a resource, and may have it's own URI different from that of the original. See also [PROV-DM], and [WEBARCH] section 2.3.2.
a URI denoting a resource (including any constrained resource), which identifies that resource for the purpose of finding and expressing provenance information associated with it (see section 1.2 for discussion)
Provenance service
a service that provides provenance information given a target-URI.
the URI of a provenance service.
a resource in the general sense of "whatever might be identified by a URI", as described by the Architecture of the World Wide Web [WEBARCH], section 2.2. A resource may be associated with multiple instances or views (constrained resources) with differing provenance.

1.2 Provenance and resources

Fundamentally, provenance information is about resources. In general, resources may vary over time and context. E.g., a resource describing the weather in London changes from day-to-day, or a listing of restaurants near you will vary depending on your location. Provenance information, to be useful, must be persistent and not itself dependent on context. Yet we may still want to make provenance assertions about dynamic or context-dependent resources (e.g. a weather forecast for London on a particular day may have been derived from a particular set of Meteorological Office data).

Provenance descriptions of dynamic and context-dependent resources are possible through a notion of constrained resources. A constrained resource is simply a resource (in the sense defined by [WEBARCH], section 2.2) that is a contextualized view or instance of some other resource. For example, a W3C specification typically undergoes several public revisions before it is finalized. A URI that refers to the "current" revision might be thought of as denoting the specification throughout its lifetime. Each individual revision would also have its own target-URI denoting the specification at that particular stage in its development. Using these, we can make provenance assertions that a particular revision was published on a particular date, and was last modified by a particular editor. Target-URIs may use any URI scheme, and are not required to be dereferencable.

Requests for provenance about a resource may return provenance information that uses one or more target-URIs to refer to versions of that resource. Some given provenance information may use multiple target-URIs if there are assertions referring to the same underlying resource in different contexts. For example, provenance information describing a W3C document might include information about all revisions of the document using statements that use the different target-URIs of the various revisions.

In summary, provenance information may be not universally applicable to a resource, but may be expressed with respect to that resource in a restricted context (e.g. at a particular time). This restriction is itself just another resource (e.g. the weather forecast for a give date as opposed to the current weather forecast), with its own URI for referring to it within provenance information.

1.3 Interpreting provenance information

Any given provenance information may contain information about several resources, referring to them using their various target-URIs. Thus, when interpreting provenance information, it is important to be aware that statements about several resources may be present, and to be accordingly selective when using the information provided. (In some exceptional cases, it may be that the provenance information returned does not contain any information relating to a specific associated resource.)

Provenance information is not of itself guaranteed to be authoritative or correct. Trust in provenance information must be determined separately from trust in the original resource. Just as in the web at large, it is a user's responsibility to determine an appropriate level of trust in any other resource; e.g. based on the domain that serves it, or an associated digital signature. (See also section 7. Security considerations.)

2. Accessing provenance information

Web applications may access provenance information in the same way as any resource on the Web, by dereferencing its URI. Thus, any provenance information may be associated with a provenance-URI, and may be accessed by dereferencing that URI using web mechanisms.

How much or how little provenance information is returned in response to a retrieval request is a matter for the provenance provider application.

It may be useful to provide provenance information through a service interface. A REST protocol for provenance retrieval is defined in Section section 4. Provenance services.

When publishing provenance information, a corresponding provenance-URI or service-URI should be discoverable using one or more of the mechanisms described in section 3. Locating provenance information.

Some alternative practices for accessing provenance information are discussed in section 5. Best practice

3. Locating provenance information

When provenance information is a resource that can be accessed using web retrieval, one needs to know its provenance-URI to dereference. If this is known in advance, there is nothing more to specify. If a provenance-URI is not known then a mechanism to discover one must be based on information that is available to the would-be accessor. Likewise, provenance information may be exposed by a service. In this case, the service-URI needs to be known.

Provenance information about a resource may be provided by several parties other than the provider of that resource, each using different locations, and each with different concerns. (It is possible that these different parties may provide contradictory provenance information.)

Once provenance information about a resource is retrieved, one may also need to know how to locate information about that resource within the provenance information. This may be a constrained resource identified by a separate target-URI.

We start by considering mechanisms for the resource provider to indicate a provenance-URI or Service-URI along with a target-URI. Three mechanisms are described here:

These particular cases are selected as corresponding to primary current web protocol and data formats. Finally, in section 3.4 Arbitrary data, we discuss the case of a resource in an unspecified format which has been provided by some means other than HTTP.

The mechanisms specified for use with HTTP and HTML are similar to those proposed by POWDER [POWDER-DR] (sections 4.1.1 and 4.1.3).

3.1 Resource accessed by HTTP

For a resource accessible using HTTP, provenance information may be indicated using an HTTP Link header field, as defined by Web Linking (RFC 5988) [LINK-REL]. The Link header field is included in the HTTP response to a GET or HEAD operation (other HTTP operations are not excluded, but are not considered here).

A provenance link relation type for referencing provenance information is registered according to the template in section 6. IANA considerations, and may be used as shown:

Link: provenance-URI; rel="provenance"; anchor="target-URI"
When used in conjunction with an HTTP success response code (2xx), this HTTP header field indicates that provenance-URI is the URI of some provenance information associated with the requested resource and that the associated resource is identified within the referenced provenance information as target-URI. (See also section 1.3 Interpreting provenance information.)

If no anchor parameter is provided then the target-URI is assumed to be the URI of the resource, used in the corresponding HTTP request.

At this time, the meaning of these links returned with other HTTP response codes is not defined: future revisions of this specification may define interpretations for these.

An HTTP response may include multiple provenance link header fields, indicating a number of different provenance resources that are known to the responding server, each providing provenance information about the accessed resource.

The presence of a provenance link in an HTTP response does not preclude the possibility that other publishers may offer provenance information about the same resource. In such cases, discovery of the additional provenance information must use other means (e.g. see section 4. Provenance services).

Provenance resources indicated in this way are not guaranteed to be authoritative. Trust in the linked provenance information must be determined separately from trust in the original resource. Just as in the web at large, it is a user's responsibility to determine an appropriate level of trust in any other linked resource; e.g. based on the domain that serves it, or an associated digital signature. (See also section 7. Security considerations.)

3.1.1 Specifying Provenance Services

The resource provider may indicate that provenance information about the resource is provided by a provenance service. This is done through the use of a provenance-service link relation type following the same pattern as above:

Link: provenance-service-URI; anchor="target-URI"; rel="provenance-service"

The provenance-service link identifies the service-URI. Dereferencing this URI yields a service description that provides further information to enable a client to determine a provenance-URI or retrieve provenance information for a resource; see section 4. Provenance services for more details.

There may be multiple provenance-service link header fields, and these may appear in an HTTP response together with provenance link header fields (though, in simple cases, we anticipate that provenance and provenance-service link relations will not be used together).

3.2 Resource represented as HTML

For a document presented as HTML or XHTML, without regard for how it has been obtained, provenance information may be associated with a resource by adding a <link> element to the HTML <head> section. Two new link relation types for referencing provenance information are registered according to the template in section 6. IANA considerations, and may be used as shown:

  <html xmlns="http://www.w3.org/1999/xhtml">
        <link rel="provenance" href="provenance-URI">
        <link rel="anchor" href="target-URI">
        <title>Welcome to example.com</title>

The provenance-URI given by the provenance link element identifies the provenance-URI for the document.

The target-URI given by the anchor link element specifies an identifier for the document that may be used within the provenance information when referring to the document.

An HTML document header may include multiple provenance link elements, indicating a number of different provenance sources that are known to the creator of the document, each of which may provide provenance information about the document.

If no "anchor" link element is provided then the target-URI is assumed to be the URI of the document. It is recommended that this convention be used only when the document is static and has a stable URI that is reasonably expected to be available to anyone accessing the document (e.g. when delivered from a web server, or as part of a MIME structure containing content identifiers [RFC2392]).

3.2.1 Specifying Provenance Services

The document creator may specify that the provenance information about the document is provided by a provenance service. This is done through the use of a third link relation type following the same pattern as above:

  <html xmlns="http://www.w3.org/1999/xhtml">
        <link rel="provenance-service" href="service-URI">
        <link rel="anchor" href="target-URI">
        <title>Welcome to example.com</title>

The provenance-service link element identifies the service-URI. Dereferencing this URI yields a service description that provides further information to enable a client to access provenance information for a resource; see section 4. Provenance services for more details.

There may be multiple provenance-service link elements, and these may appear in the same document as provenance link elements (though, in simple cases, we anticipate that provenance and provenance-service link relations would not be used together).

3.3 Resource represented as RDF

If a resource is represented as RDF (in any of its recognized syntaxes, including RDFa), it may contain references to its own provenance using additional RDF statements. For this purpose three new RDF properties, prov:hasProvenance, prov:hasAnchor, and prov:hasProvenanceService, are defined as follows. (These terms may be used to indicate provenance of other resources too, but discussion of such usage is beyond the scope of this section.)

The RDF property prov:hasProvenance is defined as a relation between two resources, where the object of the property is a resource that provides provenance information about the subject resource. Multiple prov:hasProvenance assertions may be made about a subject resource. This property corresponds to a provenance link relation used with an HTTP Link header field, or HTML <link> element.

Property prov:hasAnchor specifies a target-URI used in the provenance information to refer to the containing RDF document. This corresponds to use of the anchor parameter in an HTTP provenance Link header field, or an anchor link relation in an HTML <link> element, which similarly indicate a URI used by the provenance information to refer to the described document.

Property prov:hasProvenanceService specifies a service-URI associated with the RDF document for possible access to provenance information. This property corresponds to a provenance-service link relation used with an HTTP Link header field, or HTML <link> element.

@prefix prov: <http://www.w3.org/ns/prov#>

<> dcterms:title      "Welcome to example.com" ;
   prov:hasAnchor     <http://example.com/data/resource.rdf> ;
   prov:hasProvenance <http://example.com/provenance/resource.rdf> ;
   prov:hasProvenanceService <http://example.com/provenance-service/> .
   (RDF data)

3.4 Arbitrary data

If a resource is represented using a data format other than HTML or RDF, and no URI for the resource is known, provenance discovery becomes trickier to achieve. This specification does not define a specific mechanism for such arbitrary resources, but this section discusses some of the options that might be considered.

For formats which have provision for including metadata within the file (e.g. JPEG images, PDF documents, etc.), use the format-specific metadata to include a target-URI, provenance-URI and/or service-URI. Format-specific metadata provision might also be used to include provenance information directly in the resource.

Use a generic packaging format that can combine an arbitrary data file with a separate metadata file in a known format, such as RDF. At this time, it is not clear what format that should be, but some possible candidates are:

4. Provenance services

This section describes a REST-based protocol [REST] for provenance services with facilities for the retrieval of provenance information. The protocol specifies HTTP operations for retrieval of provenance information from a provenance service. It follows the approach of the SPARQL Graph Store HTTP Protocol [SPARQL-HTTP].

The introduction of this protocol is motivated by the following possible considerations:


This protocol combines the target-URI with the service-URI to formulate an HTTP GET request, according to the following convention:

  GET /provenance/service?target=http://www.example.com/entity HTTP/1.1
  Host: example.com

The embedded target-URI (http://www.example.com/entity) identifies the resource for which provenance information is to be returned. Any server that implements this protocol and receives a request URI in this form should return provenance information for the resource-URI embedded in the query component where that URI is the result of percent-decoding the value associated with the provenance-resource key. The embedded URI must be an absolute URI and the server must respond with a 400 Bad Request if it is not. If the supplied resource-URI includes a fragment identifier, the '#' must be %-encoded as %23 when constructing the provenance-URI value; similarly, any '&' character in the resource-URI must be %-encoded as %26.

If the provenance information identified in the request does not exist in the server, a 404 Not Found response code should be returned.

The format of returned provenance information is not defined here, but may be established through content type negotiation using Accept: header fields in the HTTP request. A provenance service should be capable of returning RDF using the vocabulary defined by [PROV-O], in any standard RDF serialization (e.g. RDF/XML), or any other standard serialization of the Provenance Model specification [PROV-DM]. Services must identify the Content-Type of the information returned.

4.2 Provenance service discovery

This specification does not define any specific mechanism for discovering provenance services. Applications may use any appropriate mechanism, including but not limited to: prior configuration, search engines, service registries, etc.

To facilitate service discovery, we recommend that RDF publication of service descriptions uses the provenance service type prov:ProvenanceService, defined by the provenance ontology [PROV-O]. The RDF service description example below in section 4.3 Provenance service description shows this use.

@@TODO: sync up term definitions with provenance ontology specification.

4.3 Provenance service description

Dereferencing a provenance service URI should yield a provenance service description. This is to be compatible with the constraints of [REST-APIs]. The provenance service description should be available as RDF (in any of its common serializations, and determined through HTTP content negotiation), and it should contain RDF statements of the form:

  <service-URI> a prov:ProvenanceService ;
    prov:provenanceUriTemplate "service-URI?target={+uri}" .

where service-URI is the URI of the provenance service. Note that the object of the prov:provenanceUriTemplate statement is a literal text value, not a URI.

A client may retrieve this service description and extract the associated value for prov:provenance-URI-template. This value is a string containing a URI template [URI-template] (level 2). A URI for the desired provenance information is obtained by expanding the URI template with the variable uri set to the resource-URI for which provenance is required. If the target-URI contains '#' or '&' these must be %-escaped as %23 or %26 respectively before template expansion.

@@TODO: sync up term definitions with provenance ontology specification.

5. Best practice

5.1 Using SPARQL for provenance queries

Simply identifying and retrieving provenance information as a resource on the Web may not always meet the requirements of a particular application or service, e.g.:

A provenance query service provides an alternative way to access provenance information and/or provenance-URIs. An application will need a URI for the provenance query service, and some relevant information about the resource whose provenance is to be accessed.

The details of a provenance query service is an implementation choice, but for interoperability between different providers and users we recommend use of SPARQL [RDF-SPARQL-PROTOCOL] [RDF-SPARQL-QUERY]. The query service URI would then be the URI of a SPARQL protocol service (often referred to as a "SPARQL endpoint"). The following subsections provide examples for what are considered to be some plausible common scenarios for using SPARQL, and are not intended to cover all possibilities.

A SPARQL protocol service description may be published using the SPARQL 1.1 Service Description vocabulary [SPARQL-SD].

The following subsections illustrate use cases for querying a SPARQL-based provenance query service.

5.1.1 Find a provenance-URI given a target-URI

If the requester has a target-URI, a simple SPARQL query may be used to return the corresponding provenance-URI. E.g., if the original resource has a target-URI http://example.org/resource:

@prefix prov: <http://www.w3c.org/ns/prov#>
SELECT ?provenance_uri WHERE
  <http://example.org/resource> prov:hasProvenance ?provenance_uri

5.1.2 Find Provenance-URI given identifying information about a resource

If the requester has identifying information that is not the URI of the original resource, then they will need to construct a more elaborate query to locate a resource description and obtain its provenance-URI(s). The nature of identifying information that can be used in this way will depend upon the third party service used, further definition of which is out of scope for this specification. For example, a query for a document identified by a DOI, say 1234.5678, using the PRISM vocabulary [PRISM] recommended by FaBio [FABIO], might look like this:

@prefix prov: <http://www.w3c.org/ns/prov#>
@prefix prism: <http://prismstandard.org/namespaces/basic/2.0/>
SELECT ?provenance_uri WHERE
  [ prism:doi "1234.5678" ] prov:hasProvenance ?provenance_uri

5.1.3 Obtain provenance information directly given a target-URI

This scenario retrieves provenance information directly given the URI of a resource, and may be useful where the provenance information has not been assigned a specific URI, or when the calling application is interested only in specific elements of provenance information.

If the original resource has a URI http://example.org/resource, a SPARQL query for provenance information might look like this:

@prefix prov: <http://www.w3c.org/ns/prov#>
SELECT ?generationStartTime WHERE {
    <http://example.org/resource> prov:wasGeneratedBy ?activity .
    ?activity prov:startedAtTime ?generationStartTime .

This query extracts a "generation start time" for an artifact by following links to the start time of the activity which generated it.

5.2 Incremental Provenance Retrieval

Provenance information may be large. While this specification does not define how to implement scalable provenance systems, it does allow for publishers to make available provenance in an incremental fashion. We now discuss two possibilities for incremental provenance retrieval.

5.2.1 Via Web Retrieval

Publishers are not required to publish all the provenance information associated with a given resource at a particular provenance-URI. The amount of provenance information exposed is application dependent. However, it is possible to incrementally retrieve (i.e. walk the provenance graph) by progressively looking up provenance information using HTTP. The pattern is as follows:

  1. For a given resource (resource-URI) retrieve it's associated provenance-URI-1 and its associated target-URI-1 using a returned HTTP Link: header field (section 3.1 Resource accessed by HTTP)
  2. Dereference provenance-URI-1
  3. Navigate the provenance information
  4. When reaching a dead-end during navigation, that is on encountering a reference to a resource (target-URI-2) with no provided provenance information, find its provenance-URI and continue from Step 2. (Note: an HTTP HEAD request for target-URI-2 may be used to obtain the Link: headers without retrieving the resource representation.)

To reduce the overhead of multiple HTTP requests, a provenance information publishers are encouraged to link entities to their associated provenance information using the prov:hasProvenance predicate. Thus, the same pattern above applies, except instead of having to retrieve a new Link header field, one can immediately access the resource's associated provenance.

The same approach can be adopted when using the provenance service API (section 4. Provenance services). However, instead of performing an HTTP HEAD or GET against a resource one queries the provenance service using the given target-URI.

5.2.2 Via SPARQL Queries

Provenance information may be made available using a SPARQL endpoint (section 5.1 Using SPARQL for provenance queries) [RDF-SPARQL-PROTOCOL] [RDF-SPARQL-QUERY]. Using SPARQL queries, provenance can be selectively retrieved using combinations of filters and or path queries.

6. IANA considerations

This document requests registration of new link relations, per section-6.2.1 of RFC 5988.

@@TODO The following templates should be completed and submitted to link-relations@ietf.org:

7. Security considerations

Provenance is central to establishing trust in data. If provenance information is corrupted, it may lead agents (human or software) to draw inappropriate and possibly harmful conclusions. Therefore, care is needed to ensure that the integrity of provenance information is maintained.

Secure HTTP (https) should be used across unsecured networks when accessing provenance information that may be used as a basis for trust decisions, or to obtain a provenance URI for same.

When retrieving a provenance URI from a document, steps should be taken to ensure the document itself is an accurate copy of the original whose author is being trusted (e.g. signature checking, or verifying its checksum against an author-provided secure web service).

@@TODO ... privacy, access control to provenance (note to self: discussed in Edinburgh linked data provenance workshop). In particular, note that the fact that a resource is openly accessible does not mean that its provenance information should also be.

@@TODO Expand on trust-in-provenance issues. Suggested by Curt: Just as provenance information can help determine trust of the information content of a resource, provenance information related to the provenance itself ("provenance of provenance") can help determine trust of the provenance.

A. Acknowledgements

The editors acknowledge the contribution and review from members of the provenance working group.

Many thanks to Robin Berjon for making our lives so much easier with his cool ReSpec tool.

B. Names added to prov: namespace

This specification defines the following additional names in the provenance namespace.

The provenance namespace URI is http://www.w3c.org/ns/prov#.

nameDescriptionDefinition ref
ProvenanceServiceClass for a service described by a provenance service description...
hasAnchorIndicates anchor URI for a potentially dynamic resource instance...
hasProvenanceRelates a resource to its provenance...
hasProvenanceServiceRelates a resource to a provenance service...
provenanceUriTemplateRelates a provenance service to a URI template string for constructing provenance-URIs...

C. References

C.1 Normative references

No normative references.

C.2 Informative references

D. Shotton; S. Peroni. FaBiO, the FRBR-aligned Bibliographic Ontology. June 2011. URL: http://speroni.web.cs.unibo.it/cgi-bin/lode/req.py?req=http:/purl.org/spar/fabio#namespacedeclarations
M. Nottingham, Web Linking, October 2010, Internet RFC 5988. URL: http://www.ietf.org/rfc/rfc5988.txt
Kevin Smith; Phil Archer; Andrea Perego. Protocol for Web Description Resources (POWDER): Description Resources. 14 November 2008. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2008/WD-powder-dr-20081114
International Digital Enterprise Alliance, Inc. PRISM: Publishing Requirements for Industry Standard Metadata. February 2008. URL: http://www.prismstandard.org/specifications/2.0/PRISM_prism_namespace_2.0.pdf
L. Moreau; P. Missier. The PROV Data Model and Abstract Syntax Notation. 2011, Work in progress. URL: http://www.w3.org/TR/prov-dm/
S. Sahoo; D. McGuinness. PROV Ontology Model. 2011, Work in progress. URL: http://dvcs.w3.org/hg/prov/raw-file/default/ontology/ProvenanceFormalModel.html
Lee Feigenbaum; Kendall Grant Clark; Elias Torres. SPARQL Protocol for RDF. 15 January 2008. W3C Recommendation. URL: http://www.w3.org/TR/2008/REC-rdf-sparql-protocol-20080115
Andy Seaborne; Eric Prud'hommeaux. SPARQL Query Language for RDF. 15 January 2008. W3C Recommendation. URL: http://www.w3.org/TR/2008/REC-rdf-sparql-query-20080115
R. Fielding. Representational State Transfer (REST). 2000, Ph.D. dissertation. URL: http://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arch_style.htm
R. Fielding. REST APIs must be hypertext driven. October 2008 (blog post), URL: http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hypertext-driven
E. Levinson. The MIME Multipart/Related Content-type. August 1998. Internet RFC 2387. URL: http://www.ietf.org/rfc/rfc2387.txt
E. Levinson. Content-ID and Message-ID Uniform Resource Locators. August 1998. Internet RFC 2392. URL: http://www.ietf.org/rfc/rfc2392.txt
Chimezie Ogbuji. SPARQL 1.1 Service Description. 2011, Work in progress. URL: http://www.w3.org/TR/sparql11-http-rdf-update/
G. T. Williams. SPARQL 1.1 Service Description. 2011, Work in progress. URL: http://www.w3.org/TR/sparql11-service-description/
J. Gregorio; R. Fielding; M. Hadley; M. Nottingham; D. Orchard. URI Template. March 2012, Internet RFC 6570. URL: http://tools.ietf.org/html/rfc6570
Norman Walsh; Ian Jacobs. Architecture of the World Wide Web, Volume One. 15 December 2004. W3C Recommendation. URL: http://www.w3.org/TR/2004/REC-webarch-20041215/