Internationalized Resource Identifier

IRIs are an integral part of the Semantic Web, and constitute the bottom layer supporting the Semantic Web stack, providing the core of linkage and identification capabilities connecting 12

RDF with resources and the Web. In this section, we introduce the basic concepts of resource, IRI, their types, and related concepts.

2.2.1 Resource

Definition 2.1 . A resource can be anything that has an identity, [BLFM98, MBLF05, HP09]

ranging from a Web-page (with an identifying Web address) to a human being (with an iden-tifying name or social security number). More formally, resources can be organized in two main categories [Hal13b, HP09] (see Figure 2.2): i) information resources (a.k.a. Web IRI, or information IRI) used to designate and access electronic information on the Web (e.g., Web-pages and documents), and ii) non-information resources (a.k.a. Semantic Web IRIs, or non-information IRIs) referring to non-electronic information like physical entities (e.g., per-son named John, monument named Tour Eiffel, and academic institution named University of Pau) and abstract concepts (e.g., definition of academia, concept of democracy, etc.)

For instance, a Web page describing the University of Pau is an information resource, but the University of Pau itself (i.e., the academic institution) is a non-information resource.

Each resource would be identified through its own IRI: using a Web IRI (URL) for the former (e.g., http://www.univ-pau.fr/live/) and a Semantic Web IRI for the latter (e.g., http:

//dbpedia.org/resource/UPPA).

Information Resource (Web IRI)

Resource

Non-Information Resource (Semantic Web IRI)

Connected using RDF

Figure 2.2: Resource Classification

2.2.2 Uniform Resource Identifier - URI

Definition 2.2 . It is is a string of characters used to identify a resource. A URI can be further classified as a locator (URL), a name (URN), or both [BLFM98, MBLF05] as we show in Figure 2.3

• A Uniform Resource Locator (URL) is the most common type of URI, also known as Web address, used to locate information resources on the Web [BLFM98, MBLF05], e.g.,

• A Uniform Resource Name (URN) is another form of less commonly used URIs, using the urn scheme of the form urn:nsi:nss where nsi is the namespace identifier and nss a namespace string [BLFM98, MBLF05], e.g., urn:isbn:0451450523, providing the ISBN number of a certain book as Romeo et Juliet, but not a reference to the content of the book itself.

2.2.3 Internationalized Resource Identifier - IRI

Definition 2.3 . It extends the existing Uniform Resource Identifier (URI) scheme to iden-tify a resource, by allowing sequences of characters from the Unicode set, including Chinese, Japanese, and Korean, etc., as opposed to ASCII characters only with URI [DS04]

There is a mapping from IRIs to URIs, URLs or URNS when IRIs are used instead of them to identify resources. This mapping we show in the Euler Diagram of the relations between IRI, URI, URL, URN in Figure 2.3.

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Figure 2.3: This Euler diagram shows that an IRI is a URI, and URI is either a Uniform Resource Locator (URL), a Uniform Resource Name (URN), or both.

2.2.4 IRI Classification

IRIs can be used to designate different things [Boo03], referred to as different IRI types, and which we can classify in two categories: i) conventional IRIs, and ii) owner defined IRIs, as described in Figure 2.4.

1. Conventional IRI: An IRI is named based on a standard mechanism for specifying its identity, according to:

(a) Implementation: some Web applications (e.g., DBpedia) give a IRI depending on their implementation (see Figure 2.5.a). This category is classified as following:

IRI

Conventional IRI

Implementation IRI

Resource IRI Page IRI Data IRI Ontology IRI

Web/Information IRI

Naming Resources IRI

Document IRI

Document Representation

IRI

Concept IRI Id IRI Owner

defined IRI

Semantic Web/Non -information IRI Resource

Type IRI

Type user-defined classification

Figure 2.4: Taxonomy of IRI types.

• Resource: to name/identify the resource. For example: http://dbpedia.

org/resource/Luxembourg

• Page: to give the Web location of the resource. For example: http://dbpedia.

org/page/Luxembourg

• Data: to provide a representation of the resource (e.g., rdf, nt, json, etc.). For example: http://dbpedia.org/data/Luxembourg.rdf

• Ontology: to name the domain ontology for the resource. For example: http:

//dbpedia.org/ontology/Place

The IRI structure of each type based on implementation classification is described in Table 2.1.

Table 2.1: Conventional IRI types based on Implementation with their respective structures IRI type IRI structure

Resource http://{domain}/resource/{resource}

Page http://{domain}/page/{resource}

Data http://{domain}/data/{resource.file-extension}

Ontology http://{domain}/ontologies/{resource}

• Identifier IRI: to name/identify the resource. For example: http://www.

example.com/id/sebastien

• Document IRI: to identify the location of the resource. For example: http:

//www.example.com/doc/sebastien

• Document representation IRI: each document can have one or more repre-sentations (text, html, rdf, owl, etc.). For example: http://www.example.com/

doc/sebastien.rdf

• Concept IRI: to name the concept that models the resource. For example:

http://www.example.com/def/sebastien

• Ontology IRI: to name the domain ontology including the concept that models the resource, e.g., http://www.example.com/ontologies/person

The IRI structure of each type based on naming resources classification is described in Table 2.2.

Table 2.2: Conventional IRI types based on naming resources with their respective structures

IRI type IRI structure

Identifier http://{domain}/id/{concept}/{reference}

Document http://{domain}/doc/{concept}/{reference}

Doc. Representation http://{domain}/doc/{concept}/{reference}/{doc.file-extension}

Concept http://{domain}/def/{concept}

Ontology http://{domain}/ontologies/{concept}

2. Owner defined IRI: An IRI is named using a declaration without specifying its type in the path, while using the owner preferences for the naming. For example, http:

//toureiffel.fris an owner defined IRI since it does not follow any of the conventional IRI types described above, while identifying a certain resource: toureiffel.fr.

2.2.5 IRI Dereferencing

Definition 2.4 . IRI dereferencing is the process of looking up an IRI on the Web, i.e., accessing the resource referenced by the IRI, which usually comes down to accessing a Web-page or another kind of document representation of the resource available online [BLCC⁺06, BC06]

Also, note that non-information resources can be linked with information resources available on the Web using RDF (as described in the following sections, cf. Figure 2.2).

Figure 2.6 shows the dereferencing process of the resource Eiffel Tower in Web-pagehttp:

//www.example.org/EiffelTower. In terms of IRIs, this means issuing an HTTP request in order to retrieve the data pertaining to Eiffel Tower.

Resource: Luxembourg country

Resource IRI Page IRI

(Web location)

Document Representation(s) IRI

http://dbpedia.org/resource/Luxembourg http://dbpedia.org/page/Luxembourg

http://dbpedia.org/data/Luxembourg.nt http://dbpedia.org/data/Luxembourg.json

…

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(a)Luxembourg resource.

Resource: Sebastien person

Identifier IRI Concept IRI Document IRI

(Web location)

Document Representation(s) IRI

http://www.example.com/id/sebastien http://www.example.com/def/sebastien http://www.example.com/doc/sebastien

http://www.example.com/doc/sebastien.rdf http://www.example.com/doc/sebastien.nt

…

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(b)Sebastien resource.

Figure 2.5: Examples of IRI uses.

Figure 2.6: Dereferencing the resource Eiffel Tower in [Hal13a].

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