1 Berendt: Advanced databases, 1st semester 2012/2013, 1 Advanced databases – The Semantic Web Bettina Berendt Katholieke Universiteit Leuven, Department of Computer Science Last update: 10 October 2012

2 Berendt: Advanced databases, 1st semester 2012/2013, 2 Agenda The Semantic Web: Motivation and overview Very brief recap of XML (& why it’s not semantic) RDF and RDFS OWL and ontologies Linked (Open) Data (LOD) Storing, accessing and combining SW data

3 Berendt: Advanced databases, 1st semester 2012/2013, 3 The original vision The entertainment system was belting out the Beatles' "We Can Work It Out" when the phone rang. When Pete answered, his phone turned the sound down by sending a message to all the other local devices that had a volume control. His sister, Lucy, was on the line from the doctor's office: "Mom needs to see a specialist and then has to have a series of physical therapy sessions. Biweekly or something. I'm going to have my agent set up the appointments." Pete immediately agreed to share the chauffeuring. At the doctor's office, Lucy instructed her Semantic Web agent through her handheld Web browser. The agent promptly retrieved information about Mom's prescribed treatment from the doctor's agent, looked up several lists of providers, and checked for the ones in-plan for Mom's insurance within a 20-mile radius of her home and with a rating of excellent or very good on trusted rating services. It then began trying to find a match between available appointment times (supplied by the agents of individual providers through their Web sites) and Pete's and Lucy's busy schedules. (The emphasized keywords indicate terms whose semantics, or meaning, were defined for the agent through the Semantic Web.) Tim Berners-Lee, James Hendler and Ora Lassila (2001). The Semantic Web. A new form of Web content that is meaningful to computers will unleash a revolution of new possibilities. Scientific American. 84A9809EC588EF21http:// 84A9809EC588EF21

4 Berendt: Advanced databases, 1st semester 2012/2013, 4 Questions 1. [concrete] What (meta-)data & procedures would be needed to solve this problem? 2. [general] What do you find works poorly on the Web today when you look for information?

5 Berendt: Advanced databases, 1st semester 2012/2013, 5 The Semantic Web: overview n The semantic web is an evolving extension of the World Wide Web in which web content can be expressed not only in natural language, but also in a format that can be read and used by software agents, thus permitting them to find, share and integrate information more easily.World Wide Web web contentnatural languagereadsoftware agentsintegrate n It derives from W3C director Sir Tim Berners-Lee's vision of the Web as a universal medium for data, information, and knowledge exchange.W3CSir Tim Berners-Leedatainformationknowledge n At its core, the semantic web comprises a philosophy, a set of design principles, collaborative working groups, and a variety of enabling technologies.working groups n Some elements of the semantic web are expressed as prospective future possibilities that have yet to be implemented or realized. n Other elements of the semantic web are expressed in formal specifications. n Some of these include Resource Description Framework (RDF), a variety of data interchange formats (e.g. RDF/XML, N3, Turtle, N-Triples), and notations such as RDF Schema (RDFS) and the Web Ontology Language (OWL), all of which are intended to provide a formal description of concepts, terms, and relationships within a given knowledge domain.Resource Description FrameworkRDF/XMLN3TurtleN-TriplesRDF SchemaWeb Ontology Languageformal descriptionconcepts termsrelationshipsknowledge domain

6 Berendt: Advanced databases, 1st semester 2012/2013, 6 The Semantic Web layer cake (T. Berners-Lee talk at XML 2000) RDF: W3C Rec OWL: W3C Rec OWL2: W3C Rec. 2009

7 Berendt: Advanced databases, 1st semester 2012/2013, 7 BTW: Semantic non-interoperability has real consequences...

8 Berendt: Advanced databases, 1st semester 2012/2013, 8 Working example: People and their relations

9 Berendt: Advanced databases, 1st semester 2012/2013, 9 Approach 1: Centralised

10 Berendt: Advanced databases, 1st semester 2012/2013, 10 Approach 2: Decentralised / open

11 Berendt: Advanced databases, 1st semester 2012/2013, 11 FOAF (Friend of a Friend) n a machine-readable ontology describing persons, their activities and their relations to other people and objects.machine-readableontologypersons n Anyone can use FOAF to describe him or herself. n FOAF is an extension to RDF and is defined using OWL.RDFOWL n Computers may use these FOAF profiles to find, for example, all people living in Europe, or to list all people both you and a friend of you know. n This is accomplished by defining relationships between people. n Each profile has a unique identifier (such as the person's addresses, a Jabber ID, or a URI of the homepage or weblog of the person), which is used when defining these relationships. addressesJabberURI n The FOAF project, which defines and extends the vocabulary of a FOAF profile, was started in 2000 by Libby Miller and Dan Brickley.2000Libby MillerDan Brickley l n „possibly the single most prevalent use of Semantic Web technologies so far“ – blog software exporting FOAF + RSS (Paolillo et al., 2005)

12 Berendt: Advanced databases, 1st semester 2012/2013, 12 FOAF example (1) <rdf:RDF xmlns:rdf=" xmlns:foaf=" xmlns:rdfs=" Jimmy Wales Jimbo

13 Berendt: Advanced databases, 1st semester 2012/2013, 13 FOAF example (2) Angela Beesley Social-web inferences

14 Berendt: Advanced databases, 1st semester 2012/2013, 14 FOAF extensions (1) <rdf:RDF xmlns:rdf=" xmlns:foaf=" xmlns:rel=" Spiderman Green Goblin

15 Berendt: Advanced databases, 1st semester 2012/2013, 15 FOAF extensions (2) Peter Parker Harry Osborn Norman Osborn

16 Berendt: Advanced databases, 1st semester 2012/2013, 16 FOAF multimedia (1) <rdf:RDF xmlns:rdf=" xmlns:foaf=" xmlns:dc=" Peter Parker Spiderman

17 Berendt: Advanced databases, 1st semester 2012/2013, 17 FOAF multimedia (2) Green Goblin Battle on the Statue Of Liberty

18 Berendt: Advanced databases, 1st semester 2012/2013, 18 What inferences? Ex.: A social-network analysis of LiveJournal FOAF entries (Paolillo et al., 2005) n Interests over time remain similar n Friends over time remain similar n But: the manner in which people elect friends and interests in their LiveJournal profiles is sharply different.... [These differences] represent fundamentally different social behaviors. n What does this mean for recommender systems?

19 Berendt: Advanced databases, 1st semester 2012/2013, 19 Agenda The Semantic Web: Motivation and overview Very brief recap of XML (& why it’s not semantic) RDF and RDFS OWL and ontologies Linked (Open) Data (LOD) Storing, accessing and combining SW data

20 Berendt: Advanced databases, 1st semester 2012/2013, 20 You have data … How should you structure it? medium-altitude, long-endurance unmanned aerial vehicle 14.7 meters 512 kilograms 70 knots Here's some data about an aircraft: 400 nautical miles

21 Berendt: Advanced databases, 1st semester 2012/2013, 21 The XML approach is to "wrap" each data item in start/end tags 14.8 meters 512 kilograms 70 knots 400 nautical miles medium-altitude, long-endurance unmanned aerial vehicle RQ-1.xml and define this data schema, e.g. in a DTD

22 Berendt: Advanced databases, 1st semester 2012/2013, 22 XML Terminology 14.8 meters Start tag End tag Data Element

23 Berendt: Advanced databases, 1st semester 2012/2013, 23 Why use XML? n It is a universally accepted standard way of structuring data (syntax). n It is a W3C recommendation (W3C = World Wide Web Consortium) n The marketplace supports it with a lot of free/inexpensive tools. n The alternative to using XML is to define your own proprietary data syntax, and then build your own proprietary tools to support the proprietary syntax (Not a very appealing idea).

24 Berendt: Advanced databases, 1st semester 2012/2013, 24 But: What is this XML snippet talking about, i.e., what are the semantics? … What is a Predator?

25 Berendt: Advanced databases, 1st semester 2012/2013, 25 Predator - which one? n Predator: a medium-altitude, long-endurance unmanned aerial vehicle system. n Predator : one that victimizes, plunders, or destroys, especially for one's own gain. n Predator : an organism that lives by preying on other organisms. n Predator: a company which specializes in camouflage attire. n Predator: a video game. n Predator: software for machine networking. n Predator: a chain of paintball stores.

26 Berendt: Advanced databases, 1st semester 2012/2013, 26 A little more flexibility through namespaces <myThings xmlns:h= xmlns:f=" OL231-b 14.8 metres Panthera antelopes

27 Berendt: Advanced databases, 1st semester 2012/2013, But this doesn‘t solve the fundamental problems 1. What does nesting mean? 2. What do syntactical variations mean? 3. What do linguistic variations mean? 4. How can we extend our knowledge?

28 Berendt: Advanced databases, 1st semester 2012/2013, What does nesting mean? Schema 1 allows for expressions like: Peter Parker...  name being an XML-element of Person means: the person HAS-A... Schema 2 allows for expressions like: Comic-book hero...  type being an XML-element of Person means: the person IS-A... Problems: a) we don‘t know what nesting means, b) even if we do know, we can‘t express this in a machine-readable way (at most build it into an application that uses these XML statements, but that would bury meaning in procedures!)

29 Berendt: Advanced databases, 1st semester 2012/2013, What do syntactical variations mean? Schema 1 allows for expressions like: Peter Parker Schema 2 allows for expressions like: Comic-book hero... Problems: a) what does it mean for some information to be an XML- element vs. an XML-attribute? b) even if we do know that they are the same, we can‘t express this in a machine-readable way, for example to combine the information from the two sources (same remark about applications as in 1.)

30 Berendt: Advanced databases, 1st semester 2012/2013, What do linguistic variations mean? Schema 1 allows for expressions like: Peter Parker... Schema 2 allows for expressions like: Peter Parker... Problems: a) we do not know whether elements from different data sources that differ by, e.g. natural, language, are the same or not b) even if we do know that they are the same, we can‘t express this in a machine-readable way, for example to combine the information from the two sources (same remark about applications as in 1.)

31 Berendt: Advanced databases, 1st semester 2012/2013, How can we extend our knowledge? Schema 1 allows for expressions like: Picture Peter Parker... Schema 2 allows for expressions like: CreativeCommons... Problems: a) we cannot refine our schema information by that provided by another source b) even if we can be sure about principal linkability (here: via the URL), we can‘t express this in a machine-readable way, for example to combine the information from the two sources (same remark about applications as in 1.)

32 Berendt: Advanced databases, 1st semester 2012/2013, 32 Summary: XML not well-suited for conceptual modelling and therefore not suited for truly semantic markup XML makes no commitment on:  Domain-specific ontological vocabulary  Ontological modeling primitives Requires pre-arranged agreement on  &  Only feasible for closed collaboration n agents in a small & stable community n pages on a small & stable intranet Not suited for sharing Web-resources

33 Berendt: Advanced databases, 1st semester 2012/2013, 33 Solution approach of the „higher levels“ of the Semantic Web 1. Break down information into atomic statements: subject-predicate-object 2. Define (in a formal-semantics way) what each component of each statement means a. Give it a URI (uniform resource identifier) to enable uniform meaning specification b. Define languages to say more about (specify) the meaning (by relating it to other units of meaning – cf. a dictionary in which each word is explained by other words) c. (exception: some components may be literals / strings – these are not defined further) 3. The languages mentioned in 2.b. each add more expressivity: 1. RDF: subject-predicate-object statements (in RDF terminology: a resource has a property with a certain value. 2. RDFS: simple ontology building blocks: class, subclass-of relation, use RDF‘s type to denote that (e.g.) an individual is a instance of a class (= make it possible to define a schema and its instances), OWL: more advanced ontology building blocks: a class (= concept) is disjoint with another one, is the same as another one; a property is functional, symmetric, the inverse of another one;...

34 Berendt: Advanced databases, 1st semester 2012/2013, 34 Semantic Web vs. Database Advantages of using RDF/RDFS/OWL to define an Ontology: n Extensible: much easier to add new properties. Contrast with a database - adding a new column may break a lot of applications n Portable: much easier to move an OWL document than to move a database. Advantages of using a Database to define an Ontology: n Mature: the database technology has been around a long time and is very mature.

35 Berendt: Advanced databases, 1st semester 2012/2013, 35 Agenda The Semantic Web: Motivation and overview Very brief recap of XML (& why it’s not semantic) RDF and RDFS OWL and ontologies Linked (Open) Data (LOD) Storing, accessing and combining SW data

36 Berendt: Advanced databases, 1st semester 2012/2013, 36 What is RDF ? RDF is a data model l the model is domain-neutral, application-neutral l the model can be viewed as directed, labeled graphs or as an object-oriented model (object/attribute/value) RDF data model is an abstract, conceptual layer independent of XML l consequently, XML is a transfer syntax for RDF, not a component of RDF l RDF data might never occur in XML form

37 Berendt: Advanced databases, 1st semester 2012/2013, 37 RDF model RDF “statements” consist of resources (= nodes) which have properties which have values (= nodes,strings) “Ora Lassila” author = subject = predicate = object “ has the author Ora Lassila” resource value property

38 Berendt: Advanced databases, 1st semester 2012/2013, 38 RDF Model Example “Ora Lassila” dc:Creator “ ” dc:Date “W3C” dc:Publisher

39 Berendt: Advanced databases, 1st semester 2012/2013, 39 Complex values So far, values of properties have been strings A graph node (corresponding to a resource) also can be the value of a property n arbitrarily complex tree and graph structures are possible n syntactically, values can be embedded (i.e. lexically in-line) or referenced (linked) Example: “Ora Lassila” dc:Creator p: p:Name

40 Berendt: Advanced databases, 1st semester 2012/2013, 40 Complex values (continued) Corresponding triples { “ dc:Creator, x } { x, p:Name, “Ora Lassila” } { x, p: , } “Ora Lassila” dc:Creator p: p:Name

41 Berendt: Advanced databases, 1st semester 2012/2013, 41 Containers Containers are collections n they allow grouping of resources (or literal values) It is possible to make statements about the container (as a whole) or about its members individually Different types of containers exist n bag - unordered collection n seq - ordered collection (= “sequence”) n alt - represents alternatives It is also possible to create collections based on URI patterns n for example, all files in a particular web site Duplicate values are permitted n there is no mechanism to enforce unique value constraints

42 Berendt: Advanced databases, 1st semester 2012/2013, 42 Containers (continued) “Ora Lassila” rdf:_1 rdf:Seq dc:Creator rdf:Type “Ralph Swick” rdf:_2

43 Berendt: Advanced databases, 1st semester 2012/2013, 43 Higher-order statements One can make RDF statements about other RDF statements n example: “Ralph believes that the web contains one billion documents” Higher-order statements n allow us to express beliefs (and other modalities) n are important for trust models, digital signatures,etc. n also: metadata about metadata n are represented by modeling RDF in RDF itself

44 Berendt: Advanced databases, 1st semester 2012/2013, 44 Reification n RDF is not really second-order n But it does provide a built-in predicate vocabulary for reification Lassila” dc:Creator “Library of Congress” dc:Creator The dotted box corresponds to the following statements { x, rdf:predicate, “dc:creator” } { x, rdf:subject, “ } { x, rdf:object, “Ora Lassila” } { x, rdf:type, “rdf:statement” }

45 Berendt: Advanced databases, 1st semester 2012/2013, 45 Reification pers05 ISBN... Author-of NYT claims ISBN... Any statement can be an object graphs can be nested - reification

46 Berendt: Advanced databases, 1st semester 2012/2013, 46 RDF Schema Defines small vocabulary for RDF: Class, subClassOf, type Property, subPropertyOf domain, range Vocabulary can be used to define other vocabularies for your application domain Person StudentResearcher subClassOf Jeen type hasSuperVisor domain range Frank type hasSuperVisor

47 Berendt: Advanced databases, 1st semester 2012/2013, 47 RDF Schema syntax in XML

48 Berendt: Advanced databases, 1st semester 2012/2013, 48 Afterthoughts about triples I Here‘s a short text on natural languages that do not suggest subject – predicate – object structures in quite the same strong way as English (or Dutch): The question remains, of course, whether speakers of, e.g., Lisuwould still regard the RDF structure of basic statements as conceptually obvious.) Also, the entry mentions the Chinese languages as examples – an interesting contrast to what Kai-Chun said today!?

49 Berendt: Advanced databases, 1st semester 2012/2013, 49 Afterthoughts about triples II: task for the next week Representations: can you map them to all the lower ones? And why were triples chosen as the basic unit of exchange? n Graph structures n Sequential structures n Relational tables n Triples n Tuples

50 Berendt: Advanced databases, 1st semester 2012/2013, 50 Agenda The Semantic Web: Motivation and overview Very brief recap of XML (& why it’s not semantic) RDF and RDFS OWL and ontologies Linked (Open) Data (LOD) Storing, accessing and combining SW data

51 Berendt: Advanced databases, 1st semester 2012/2013, 51 Ontologies and concepts n An ontology is a conceptual model. n An Ontology is the collection of semantic definitions for a domain. n Example: an Aircraft Ontology is the set of semantic definitions for the Aircraft domain, e.g., Predator is a subClassOf Aircraft. sensorID is a FunctionalProperty. Platform is an equivalentClass to Aircraft. n Predator, Aircraft etc. are concepts.

52 Berendt: Advanced databases, 1st semester 2012/2013, 52 Basic idea of conceptual modelling (not only in SW): The semiotic triangle

53 Berendt: Advanced databases, 1st semester 2012/2013, 53 What is an ontology? (A commonly accepted informal definition and one formal definition) An ontology is „an explicit specification of a shared conceptualisation.“ (Gruber, 1993) (Stumme, Hotho & Berendt, Semantic Web Journal 2006))

54 Berendt: Advanced databases, 1st semester 2012/2013, 54 In which semantic web languages can ontologies be formulated? n RDF Schema is sufficient to specify an ontology with the first 4 components n For the fifth component (logical axioms), need a more expressive language like OWL.

55 Berendt: Advanced databases, 1st semester 2012/2013, 55 Ontologies, decentralization, and bottom-up engineering Communities of users (application builders,...) can n Re-use existing ontologies l Established domain-specific ontologies (e.g., real-estate, medicine, bioinformatics) l „The big one“: Cyc, see l Search for ontologies –See overview at #Ontology_libraries #Ontology_libraries –Use Sindice with some tricks: dev/browse_thread/thread/831c084c3b5a0214 (or try the Advanced Search directly: ) dev/browse_thread/thread/831c084c3b5a0214http://sindice.com/search n Link to existing ontologies n Extend existing ontologies

56 Berendt: Advanced databases, 1st semester 2012/2013, 56 Ontologies as conceptual models / schemas; or: Database (knowledge base) = Ontology + Instances My Life and Times Illusions First and Last Freedom Paul McCartney Richard Bach J. Krishnamurti June, title author date BookCatalogue My Life and Times Paul McCartney June, 1998

57 Berendt: Advanced databases, 1st semester 2012/2013, 57 OWL: more details n You‘ll be working with it today n Here is another nice tutorial that takes you through OWL‘s possibilities for formulating restrictions, constructing classes, etc., starting from the Protégé interface: 06-OWL-Tutorial.ppt n And here is a version of the Tourism ontology created there in OWL (XML notation):

58 Berendt: Advanced databases, 1st semester 2012/2013, 58 Agenda The Semantic Web: Motivation and overview Very brief recap of XML (& why it’s not semantic) RDF and RDFS OWL and ontologies Linked (Open) Data (LOD) Storing, accessing and combining SW data

59 Berendt: Advanced databases, 1st semester 2012/2013, 59 What is LOD? n “A way of making the Semantic Web happen“ (it is hoped) n Key concept: leverage the existence of structured data and combine it with the languages and infrastructures of the Web and the Semantic Web n Tim Berners-Lee: four principles of Linked Data ( l Use URIs to identify things.URIs l Use HTTP URIs so that these things can be referred to and looked up ("dereferenced") by people and user agents.HTTPdereferenceduser agents l Provide useful information about the thing when its URI is dereferenced, using standard formats such as RDF/XML.RDF/XML l Include links to other, related URIs in the exposed data to improve discovery of other related information on the Web.

60 Berendt: Advanced databases, 1st semester 2012/2013, 60 Data items are identified with HTTP URIs pd:cygri Richard Cyganiak dbpedia:Berlin foaf:name foaf:based_near foaf:Person rdf:type pd:cygri = dbpedia:Berlin = From

61 Berendt: Advanced databases, 1st semester 2012/2013, 61 Resolving URIs over the Web dp:Cities_in_Germany dp:population skos:subject Richard Cyganiak dbpedia:Berlin foaf:name foaf:based_near foaf:Person rdf:type pd:cygri From

62 Berendt: Advanced databases, 1st semester 2012/2013, 62 Dereferencing URIs over the Web dp:Cities_in_Germany dp:population skos:subject Richard Cyganiak dbpedia:Berlin foaf:name foaf:based_near foaf:Person rdf:type dbpedia:Hamburg dbpedia:Muenchen skos:subject pd:cygri From

63 Berendt: Advanced databases, 1st semester 2012/2013, 63 The Linked Open Data Cloud

64 Berendt: Advanced databases, 1st semester 2012/2013, 64 Interactive visualization of the Linked Data Cloud [retrieved ] The link above is defunct – Here‘s a new visualization:

65 Berendt: Advanced databases, 1st semester 2012/2013, 65 More on LOD n W3C Linking Open Data community project: cts/LinkingOpenData cts/LinkingOpenData n A nice slideset is available at bizer.ppt bizer.ppt n and a tutorial (together with a link to a recent book) at

66 Berendt: Advanced databases, 1st semester 2012/2013, 66 Agenda The Semantic Web: Motivation and overview Very brief recap of XML (& why it’s not semantic) RDF and RDFS OWL and ontologies Linked (Open) Data (LOD) Storing, accessing and combining SW data

67 Berendt: Advanced databases, 1st semester 2012/2013, 67 How is this data stored? (1) In „Semantic Web / LOD databases“: triplestores n A triplestore is a purpose-built database for the storage and retrieval of Resource Description Framework (RDF) metadata.databaseResource Description Framework metadata n A triplestore can store many (up to billions) of RDF triples n For a list of implementations, see

68 Berendt: Advanced databases, 1st semester 2012/2013, 68 How is this data stored? (2) Embedded in Web pages n RDFa l a W3C Recommendation: l e.g. for people: embed FOAF n Microformats l e.g. for people: hcard + XHTML Friends Network n Microdata l e.g. n What are the differences? Which one(s) are “truly Semantic Web“? l Just one out of many sample blogs comparing the three:

69 Berendt: Advanced databases, 1st semester 2012/2013, 69 How is this data accessed? n By search engines that can extract the markup from Web pages l e.g., Google n By search engines that directly access triplestores l e.g. Sindice n By your own applications that directly access triplestores l e.g. your homeworks 2, 3 Obviously, data can then also be transformed into RDF (e.g. RDFa) or into human-readable web pages, see the following for an example 

70 Berendt: Advanced databases, 1st semester 2012/2013, 70 LOD on people (1)

71 Berendt: Advanced databases, 1st semester 2012/2013, 71 LOD on people (2)

72 Berendt: Advanced databases, 1st semester 2012/2013, 72 What do the LOD on people actually say? (1)

73 Berendt: Advanced databases, 1st semester 2012/2013, 73 What do the LOD on people actually say? (2)

74 Berendt: Advanced databases, 1st semester 2012/2013, 74 What does the combination/integration of this information require? n “Linkability“ at the technical level: see Linked Data principles n “Linkability“ at the semantic level of identity: sameAs n “Linkability“ at the semantic level of more complex relationships: schema / ontology matching l e.g. your homeworks 2, 3

75 Berendt: Advanced databases, 1st semester 2012/2013, 75 PS: What are „semantic technologies“? n encode meanings separately from data and content files, and separately from application code n Often uses elements (e.g. the OWL language) of the Semantic Web n But not necessarily open data n Thus, increasingly popular for example for within-company solutions

76 Berendt: Advanced databases, 1st semester 2012/2013, 76 Task for the next week Given n your knowledge of logic n and the presence of interconnected data sources such as those shown on p. 63: What inferences would you want to draw (that go beyond data retrieval)? Name 3 examples. Be as concrete or as abstract as you want. (Recall: retrieval is just getting the data – a standard SELECT statement with some selections and projections would be a typical example of retrieval in the SQL world Inference is concluding more from this – it starts with operators such as COUNT or MAX in SELECT statements and then goes as far as logic or other ways of data processing take you)

77 Berendt: Advanced databases, 1st semester 2012/2013, 77 Outlook The Semantic Web: Motivation and overview Very brief recap of XML (& why it’s not semantic) RDF and RDFS OWL and ontologies Linked (Open) Data (LOD) Storing, accessing and combining SW data Inference on the Semantic Web

78 Berendt: Advanced databases, 1st semester 2012/2013, 78 Used sources (From or based on): p. 6: pp , 49: Costello, R.L. & Jacobs, D.B. (2003). A Two Minute Intro to XML. p. 33, pp : Unnamed (no date). RDF and XML tutorial. p. 35, 53: Costello, R.L. & Jacobs, D.B. (2003). OWL Web Ontology Language. pp : pp : Dodds, L. (2004). An Introduction to FOAF. Picture credits: see PPT „comments“ field

79 Berendt: Advanced databases, 1st semester 2012/2013, 79 Further references, background reading; acknowledgements J. C. Paolillo, S. Mercure, and E. Wright. (2005). The social semantics of Livejournal FOAF: Structure and change from 2004 to In G. Stumme, B. Hoser, C. Schmitz, and H. Alani, editors, Proceedings of the 1st Workshop on Semantic Network Analysis at the ISWC 2005 Conference, pages 69 – Specifications: RDF: OWL: OWL2: FOAF: