Motivation Resource Description Framework (RDF) � Describe all resources � Make them machine-readable � Provide common framework among different applications How to do this? What is RDF? � Create a framework using an application- � Describes attributes about objects independent syntax: XML � Can point to other objects described by RDF � Creates a graph of resources � Resources are identified by URIs
What is a URI? Resource Description Framework (RDF) � Universal Resource Indicator � Consists of prefix and description � URL is a type of URI RDF/XML Syntax RDF/XML RDF/XML � RDF/XML is valid XML � The relationships are represented by nested XML tags � It uses a standardized set of tags to represent an RDF graph � Following the nesting of the tags is equivalent to traversing the graph � The RDF graph has node-arc-node triples that represent subject-predicate-object � <rdf:Description> is the tag used to begin a relationships node
RDF/XML RDF/XML � Example: � The tags use URI as attributes to identify <?xml version="1.0"?> what they describe <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:ex="http://example.org/stuff/1.0/"> � In the case of an arc or predicate the URI is <rdf:Description rdf:about="http://www.w3.org/TR/rdf-syntax-grammar" appended with a local reference dc:title="RDF/XML Syntax Specification (Revised)"> <ex:editor> � The URI can then be replaced with a <rdf:Description ex:fullName="Dave Beckett"> <ex:homePage rdf:resource="http://purl.org/net/dajobe/" /> namespace to shorten the tag length </rdf:Description> </ex:editor> </rdf:Description> </rdf:RDF> RDF RDF � RDF is intended for situations where information � A language for representing information needs to be processed by applications, rather then about resources in the World Wide Web. being only displayed to people � Based on XML syntax. � RDF is based on the idea of identifying things that � Used to describe the title, author ect for use Web identifiers (URI) webpage. � Describes things in simple terms of properties and property values. � The concept of a ‘web resource.’ � The resources of RDF are not fixed
Basic Concepts of RDF In Plain English � Subject-The resource we are talking about � http://www.example.org/index.html has a creator whose value is John Smith � Predicate-A property of that resource that we wish to describe � Object-The value of the predicate. The object we wish to associate with the predicate RDF Graph Representation � RDF is designed so machines can identify resources. � Querying RDF is based on the concept of a URI � Uses XML to describe all its information
Graph Representation Graph Representation � There can be a larger graph where there are multiple predicate/object tuples defining an object Tuples RDF � Or data can be defined in tuples: � Each triple corresponds to a single arc in the graph above � <http://www.example.org/index.html> <http://purl.org/dc/elements/1.1/creator> � RDF does not associate meaning any more then a <http://www.example.org/staffid/85740> database associates meaning to its table names and .<http://www.example.org/index.html> SQL. It simply defines a structure creation method <http://www.example.org/terms/creation-date> and a querying method. "August 16, 1999" � When data needs to be more structured it can be .<http://www.example.org/index.html> broken up into pieces for instance with an address <http://purl.org/dc/elements/1.1/language> "en" .
RDF Blank Nodes � Blank nodes are used to represent a more accurate description of a model. For instance john and johns address are not the same object and therefore there should be arcs that separate them from each other. � In the triples notation URI objects must be enclosed in angle brackets. � RDF uses blank nodes to represent n-ary replationships. From the triples listing above this is a 5 way relationship. Types Types � All of the previous objects have had no type. However in the example above the author most likely intends to mean 27 interpreted as a number. � In RDF we define an objects type by pairing it with a type definintion. For example with integers this is xsd:integer. � The xsd is shorthand for a descriptor of its type.
Types Types � We can write this in the triples representation � Technically the type associations have no as follows: actual meaning since the RDF document only associates the name of the type but � ex:index.html exterms:creation-date "1999- does not enforce it. The type is only used to 08-16"^^xsd:date . describe the intended meaning of an object. � RDF has no built in datatypes of its own. Similar to soap however the user can describe any object datatype by associating a type to it. Applications Resource Description Framework (RDF) � RDF is used for storing metadata � We use metadata all the time � Searching the library � Looking up a phone number in the Yellow Pages � It is possible to perform them without metadata Applications � Search the books one by one � Dial every possible phone number until you get the right one � These searches are made easy because of metadata
How Does RDF Make Life Easier? RDF and Search Engines � RDF can be used for: � Currently search engines operate on a “brute force” discovery method – Search engines � RDF allows for an efficient, structured and organized way to – Web Services search for information � Search engines like Vivisimo will become more ubiquitous – Internal relationships between items because RDF allows for an efficient querying of metadata – Web Mining – Higher precision in resource discovery than with full text – Standardized metadata access search engines – Industry standards � The aim: to provide a uniform query capability for – Searches resource discovery – Storage � Possible because of the way RDF can structure and – Interoperability organize metadata so it is easily understandable by programs RDF and Web Services RDF and Web Mining � RDF provides a structured way to describe both the semantics � RDF can enhance web mining and syntax of your web service. � With a standardized way to represent � As seen with the project, organizing and finding out what the format and syntax of a web service call is a somewhat arduous metadata, the process of extracting patterns process and “massaging” this data becomes much � With RDF, this discovery process could be made much easier easier and much more seamless � Less resource intensive
RDF and Multimedia Databases RDF – An Example � Currently most multimedia databases either rely on •Metadata information about the text searches of captions of an image or on picture properties of the actual content of the file •This information can � Image – global color histogram, layout be efficiently and � Audio – length, frequency signature conveniently � Metadata about multimedia files can be created with searched RDF and this can provide a much more powerful way for searching through multimedia databases � The problem: While this metadata is great for searching, who is going to input this metadata? � Automated metadata generators are far from perfect Some examples of where RDF can be used. RDF Examples � Describing properties for shopping items, like price and availability � Describing time schedules for web events � Describing information about web pages, like created and modified date, title, and author � Describing content and rating for web pictures � Describing content for search engines � Describing electronic libraries
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