RDF was designed to provide a common way to describe information so it can be read and understood by computer applications.
RDF descriptions are not designed to be displayed on the web.
RDF documents are written in XML. The XML language used by RDF is called RDF/XML.
By using XML, RDF information can easily be exchanged between different types of computers using different types of operating systems and application languages.
The RDF language is a part of the W3C's Semantic Web Activity. W3C's "Semantic Web Vision" is a future where:
RDF uses Web identifiers (URIs) to identify resources.
RDF describes resources with properties and property values.
RDF identifies things using Web identifiers (URIs), and describes resources with properties and property values.
Explanation of Resource, Property, and Property value:
The following RDF document could describe the resource "https://www.w3schools.com/rdf":
The example above is simplified. Namespaces are omitted.
The combination of a Resource, a Property, and a Property value forms a Statement (known as the subject, predicate and object of a Statement).
Let's look at some example statements to get a better understanding:
Statement: "The author of https://www.w3schools.com/rdf is Jan Egil Refsnes".
Statement: "The homepage of https://www.w3schools.com/rdf is https://www.w3schools.com".
Here are two records from a CD-list:
| Title | Artist | Country | Company | Price | Year |
|---|---|---|---|---|---|
| Empire Burlesque | Bob Dylan | USA | Columbia | 10.90 | 1985 |
| Hide your heart | Bonnie Tyler | UK | CBS Records | 9.90 | 1988 |
Below is a few lines from an RDF document:
The first line of the RDF document is the XML declaration. The XML declaration is followed by the root element of RDF documents: .
The xmlns:rdf namespace, specifies that elements with the rdf prefix are from the namespace "http://www.w3.org/1999/02/22-rdf-syntax-ns#".
The xmlns:cd namespace, specifies that elements with the cd prefix are from the namespace "http://www.recshop.fake/cd#".
The element contains the description of the resource identified by the rdf:about attribute.
The elements: , , , etc. are properties of the resource.
W3C's RDF Validation Service is useful when learning RDF. Here you can experiment with RDF files.
The online RDF Validator parses your RDF document, checks your syntax, and generates tabular and graphical views of your RDF document.
Copy and paste the example below into W3C's RDF validator:
The main elements of RDF are the root element, , and the element, which identifies a resource.
is the root element of an RDF document. It defines the XML document to be an RDF document. It also contains a reference to the RDF namespace:
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
. Description goes here.
The element identifies a resource with the about attribute.
The element contains elements that describe the resource:
The elements, artist, country, company, price, and year, are defined in the http://www.recshop.fake/cd# namespace. This namespace is outside RDF (and not a part of RDF). RDF defines only the framework. The elements, artist, country, company, price, and year, must be defined by someone else (company, organization, person, etc).
The property elements can also be defined as attributes (instead of elements):
rdf:about="http://www.recshop.fake/cd/Empire Burlesque"
cd:artist="Bob Dylan" cd:country="USA"
cd:company="Columbia" cd:price="10.90"
cd:year="1985" />
The property elements can also be defined as resources:
In the example above, the property artist does not have a value, but a reference to a resource containing information about the artist.
RDF containers are used to describe group of things.
The following RDF elements are used to describe groups: , , and .
The element is used to describe a list of values that do not have to be in a specific order.
The element may contain duplicate values.
The element is used to describe an ordered list of values (For example, in alphabetical order).
The element may contain duplicate values.
The element is used to describe a list of alternative values (the user can select only one of the values).
In the examples above we have talked about "list of values" when describing the container elements. In RDF these "list of values" are called members.
So, we have the following:
RDF collections describe groups that can ONLY contain the specified members.
As seen in the previous chapter, a container says that the containing resources are members - it does not say that other members are not allowed.
RDF collections are used to describe groups that can ONLY contain the specified members.
A collection is described by the attribute rdf:parseType="Collection".
RDF Schema (RDFS) is an extension to RDF.
RDF describes resources with classes, properties, and values.
In addition, RDF also needs a way to define application-specific classes and properties. Application-specific classes and properties must be defined using extensions to RDF.
One such extension is RDF Schema.
RDF Schema does not provide actual application-specific classes and properties.
Instead RDF Schema provides the framework to describe application-specific classes and properties.
Classes in RDF Schema are much like classes in object oriented programming languages. This allows resources to be defined as instances of classes, and subclasses of classes.
The following example demonstrates some of the RDFS facilities:
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xml:base="http://www.animals.fake/animals#">
In the example above, the resource "horse" is a subclass of the class "animal".
Since an RDFS class is an RDF resource we can abbreviate the example above by using rdfs:Class instead of rdf:Description, and drop the rdf:type information:
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xml:base="http://www.animals.fake/animals#">
The Dublin Core Metadata Initiative (DCMI) has created some predefined properties for describing documents.
RDF is metadata (data about data). RDF is used to describe information resources.
The Dublin Core is a set of predefined properties for describing documents.
The first Dublin Core properties were defined at the Metadata Workshop in Dublin, Ohio in 1995 and is currently maintained by the Dublin Core Metadata Initiative.
| Property | Definition |
|---|---|
| Contributor | An entity responsible for making contributions to the content of the resource |
| Coverage | The extent or scope of the content of the resource |
| Creator | An entity primarily responsible for making the content of the resource |
| Format | The physical or digital manifestation of the resource |
| Date | A date of an event in the lifecycle of the resource |
| Description | An account of the content of the resource |
| Identifier | An unambiguous reference to the resource within a given context |
| Language | A language of the intellectual content of the resource |
| Publisher | An entity responsible for making the resource available |
| Relation | A reference to a related resource |
| Rights | Information about rights held in and over the resource |
| Source | A Reference to a resource from which the present resource is derived |
| Subject | A topic of the content of the resource |
| Title | A name given to the resource |
| Type | The nature or genre of the content of the resource |
A quick look at the table above indicates that RDF is ideal for representing Dublin Core information.
The following example demonstrates the use of some of the Dublin Core properties in an RDF document:
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:dc= "http://purl.org/dc/elements/1.1/">
The recommended file extension for RDF files is .rdf. However, the extension .xml is often used to provide compatibility with old xml parsers.
The MIME type should be "application/rdf+xml".
| Element | Class of | Subclass of |
|---|---|---|
| rdfs:Class | All classes | |
| rdfs:Datatype | Data types | Class |
| rdfs:Resource | All resources | Class |
| rdfs:Container | Containers | Resource |
| rdfs:Literal | Literal values (text and numbers) | Resource |
| rdf:List | Lists | Resource |
| rdf:Property | Properties | Resource |
| rdf:Statement | Statements | Resource |
| rdf:Alt | Containers of alternatives | Container |
| rdf:Bag | Unordered containers | Container |
| rdf:Seq | Ordered containers | Container |
| rdfs:ContainerMembershipProperty | Container membership properties | Property |
| rdf:XMLLiteral | XML literal values | Literal |
| Element | Domain | Range | Description |
|---|---|---|---|
| rdfs:domain | Property | Class | The domain of the resource |
| rdfs:range | Property | Class | The range of the resource |
| rdfs:subPropertyOf | Property | Property | The property is a sub property of a property |
| rdfs:subClassOf | Class | Class | The resource is a subclass of a class |
| rdfs:comment | Resource | Literal | The human readable description of the resource |
| rdfs:label | Resource | Literal | The human readable label (name) of the resource |
| rdfs:isDefinedBy | Resource | Resource | The definition of the resource |
| rdfs:seeAlso | Resource | Resource | The additional information about the resource |
| rdfs:member | Resource | Resource | The member of the resource |
| rdf:first | List | Resource | |
| rdf:rest | List | List | |
| rdf:subject | Statement | Resource | The subject of the resource in an RDF Statement |
| rdf:predicate | Statement | Resource | The predicate of the resource in an RDF Statement |
| rdf:object | Statement | Resource | The object of the resource in an RDF Statement |
| rdf:value | Resource | Resource | The property used for values |
| rdf:type | Resource | Class | The resource is an instance of a class |
| Attribute | Description |
|---|---|
| rdf:about | Defines the resource being described |
| rdf:Description | Container for the description of a resource |
| rdf:resource | Defines a resource to identify a property |
| rdf:datatype | Defines the data type of an element |
| rdf:ID | Defines the ID of an element |
| rdf:li | Defines a list |
| rdf:_n | Defines a node |
| rdf:nodeID | Defines the ID of an element node |
| rdf:parseType | Defines how an element should be parsed |
| rdf:RDF | The root of an RDF document |
| xml:base | Defines the XML base |
| xml:lang | Defines the language of the element content |