Essential RDF concepts

Turtle RDF notation will be used throughout the documentation. However the the application files do not need to be written in turtle notation.

There are a few RDF concepts that are essential to understanding how the application is defined. These are:

• The Triple Pattern
• RDF classes
• RDF properties
• rdfs domains
• rdfs ranges

The Triple Pattern

RDF data is written in a triple pattern that has a specific order. The items in the pattern form a sentence composed of a subject, predicate and object. Throughout the documentation and in the framework these items will be referred to repeatly.

Example

subject	predicate	object
John’s	phone number is	555-555-1234
<http://.../john>	schema:telephone	“555-555-1235”^^xsd:string

The subject, in SQL/Relational database terms, is a uuid (universal unique identifier).

Note

The subject/IRI/URI value itself may contain human readable information but it is not necessary that it does. Sometimes it causes confusion if it does! For example. If you look at <http://.../Topic/Battle-of-Gettysburg> it may lead one to think that we are refering to the Battle of Gettysburg as a topic to be used for categorization. However, the Battle of Gettysburg can a calendar event, a battle, a historical event, etc. It would be better to list it solely as <http://.../Battle-of-Gettysburg>. This will avoid creating multiple URIs/IRIs for one particular concept.

Warning

Blanknodes have a subject value that is a variable specific to the triplestore. Blanknode subjects will NOT function as uuids. Blanknodes are used to group properties together for a particular subject i.e. if you have the subject/IRI/URI <http://.../john> as above and you want to add an address for him you can group all the address properties in one blanknode as below:

<http://.../john> schema:address [
        schema:Country  "USA";
        schema:addressLocality  "Mountain View";
        schema:addressRegion    "CA";
        schema:postalCode       Text    "94043";
        schema:streetAddress    "1600 Amphitheatre Pkwy"
   ] .

The predicate is a RDF vocabulary term that is in the form of URI/IRI that describes the relationship between the subject and object

The object is a either another IRI, a blankenode or a literal (a string, date, number etc)

RDF classes

A class represents a category of subjects. Examples of classes are organizations, people, and events. When a subject that is identified as a class it is one instance of the larger category and shares many properties with other subjects.

schema:Organization is a RDF class for describing organization.

Identifying <http://example.com/org#a> rdf:type schema:Organization means that <http://example.com/orf#a> is an organization.

RDF properties

RDF properties are used as the predicates of the triple pattern. In general they are used to describe the relationships between RDF classes.

Note

A property can be used in multiple classes. i.e. schema:name can be used with schema:Person, schema:Organization, etc.

Warning

Throughout the code property and predicate are used interchangeably.

rdfs:domains

The rdfs:domain declaration is essential to defining an application within the framework. The domain statement tells what type of rdf:Class should be in the subject position of a triple. For example:

schema:email rdfs:domain schema:Person .

The above statement means that when the schema:email property is used that the subject of the triple should be a schema:Person.

<http://example.com/john> rdf:type schema:Person ;
                      schema:email "john@example.com"^^xsd:string .

<http://example.com/john> is a Person and we are in compliance of the specified domain rule.

<http://example.com/moby-dick> rdf:type schema:Book ;
                      schema:email "myemail@example.com"^^xsd:string .

The above triples are a violation of the specified domain rule. The point is not that a book can’t have an email, but rather that the specified statement says that only a schema:Person should use the property schema:email.

Warning

There are RDF rules that allow for looking at subclasses and so forth but for the rdfframework you must specifically specify each class the property will be used in.

To use the schema:email property for a Person, Book and Organization specify them explicitly:

schema:email rdfs:domain schema:Person,
                     schema:Book,
                     schema:Organization .

rdfs:ranges

The rdfs:range tells us what type of rdf:Class should occupy the object position of the triple pattern.:

schema:email rdfs:range xsd:string

This means that when ever a schema:email property is used it should have a string as the object. Having an URI in the object position would be a violation of the rule.

Note

For the rdfframework be as specific as possible for specification of the range value. If you want a date value to be stored in the object position use xsd:date. Do not specify the range as a literal.

If you want more than one option for the range specifically specify it.:

schema:email rdfs:range xsd:string,
                    yourNs:EmailObject .

Next we will look at defining our classes and properties for the framework.

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