Semntic Web Intro Eyal Sela
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מצגת ממפגש מפתחים #1 - פיתוח אתרים ואפליקציות מבוססי טכנולוגיות סמנטיות (3 בפברואר, 2010) http://www.w3c.org.il/article/semanticweb
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Semntic Web Intro Eyal Sela
- 1. 1 www.w3c.org.il
- 2. מפגש ראשון של W3CDF 1. תג בטוויטר: #w3cdf 2. מה קורה עכשיו?? j.mp/noww3c 3. קישור זמני למצגות - 1j.mp/w3cdf 4. המפגש הבא... 5. www.w3c.org.il 2
- 3. המפגש הבא - מובייל • מתארח באוניברסיטת בר-אילן • 42 במרץ • 00:02-03:61 www.w3c.org.il 3
- 4. W3Cהשנה סידרת מפגשים למפתחים • קבוצות עניין • פיתוח אפליקציה? • סדנאות תגובה למומחים • הרצאות אורח (שלנו, שלכם) • אבל אין תמיכה בפעילות קבוצות/קהילות • הודעות שילוב מומחים בקבוצות עבודה • ) ( עוד... • www.w3c.org.il 4
- 5. ?רוצים סקירה • CSS Styling Attributes Level 1 • Selectors API Level 2 • XQuery and XPath Full Text 1.0 • SPARQL 1.1 - Property Paths • UK Government Launches Open Data Site • Programmable HTTP Caching and Serving Draft Published • New WAI Resource: Contacting Organizations about Inaccessible Websites 5 www.w3c.org.il
- 6. היום .W3C-00:71 אייל סלע, איגוד האינטרנט ו The semantic web technology stack - מבוא לווב הסמנטי .Tikal Knowledge-54:71 אודי באומן, ראש צוות פייתון ב שימוש בתשתיות קיימות לפיתוח אפליקציות סמנטיות OWL- וRDF מבוססות W3C invited expert. Former CEO UB Access ,03:81 ליסה סימן RDF in use in ARIA - How to use RDF ontologies for better Web interoperability adaptability and accessibility www.w3c.org.il 6
- 7. www.w3c.org.il 7
- 8. Semantic web intro אייל סלע מנהל פרויקטים, איגוד האינטרנט הישראלי ומשרד ה- W3Cהישראלי www.w3c.org.il 8
- 9. j.mp/w3cdoc www.w3c.org.il 9
- 10. תוכן עניינים מבוא - W3Cואיגוד האינטרנט הישראלי • אודות ה-Semantic Web • כמה דוגמאות (קונספטואליות) • הטכנולוגיות – OWL ,RDFa ,RDFS ,RDF • עוד כמה דוגמאות • www.w3c.org.il 01
- 11. אודות: איגוד האינטרנט הישראלי • שלוחת האיגוד הבינלאומי: • עמותה ללא מטרת רווח פועל לקידום האינטרנט והטמעתו בישראל כתשתית טכנולוגית, מחקרית, חינוכית, חברתית ועסקית. www.w3c.org.il 11
- 12. טכנלוגיה • תשתיות – שמות מתחם ו-IIX • קידום – - W3Cהמשרד הישראלי – - OPNמערכות פתוחות וקוד פתוח – - STSמאגר נתוני האינטרנט בישראל • נושאים נוספים www.w3c.org.il 21
- 13. W3C ארגון בינלאומי • כ-053 ארגונים • פורום ניטראלי ליצירת תקני רשת באינטרנט. • משימה: • להוביל את הרשת למיצוי הפוטנציאל על ידי פיתוח פרוטוקולים וקווים מנחים שיאפשרו את צמיחתה לטווח ארוך. www.w3c.org.il 31
- 14. Semantic Web-אודות ה www.w3c.org.il 14
- 15. Is about… machine interpretability of Web content applications can process the content of information instead of just presenting it to humans j.mp/w3cowl 15 www.w3c.org.il
- 16. תאור מידע, ברמת הקוד (כלומר, הוספת משמעות, סמנטיקה), כך שמכונות יכולות להבין את תוכנו. www.w3c.org.il
- 17. זה בעצם מנגנון מוסכם (סטנדרט) לייצוג נתונים. www.w3c.org.il
- 18. כך, הרשת הופכת מאוסף של מסמכים (שרק בני אדם מבינים את תוכנם) למאגר נתונים (שמכונות יכולות "להבין"). 81 www.w3c.org.il
- 20. Decentralization: The Future of Online Social Networking you (will) have a personal Website, with your photos, your family tree, your business details, and aggregators then (will) turn this into added value by finding the links across the whole web. ]Economist 2008[ )illustration by David Simonds( j.mp/w3cweb 20 www.w3c.org.il
- 21. j.mp/w3civn j.mp/w3cfoafe 21 www.w3c.org.il
- 22. הטכנולוגיות www.w3c.org.il 22
- 23. The semantic technology stack 24 www.w3c.org.il
- 24. XML syntax for structured documents & URIUniform resource Identifier www.w3c.org.il 25
- 25. RDF www.w3c.org.il 26
- 26. )j.mp/w3crdf -RDF primer (מבוסס בעיקר על www.w3c.org.il 27
- 27. The building block of the Semantic Web Resource Description Framework (RDF) 28 www.w3c.org.il
- 28. The building block of the Semantic Web Resource Description Framework (RDF) שיטה לתאר דברים 29 www.w3c.org.il
- 29. בסיס רעיוני language for representing information about resources A model for data interchange on the Web. allows structured and semi-structured data to be mixed, exposed, and shared across different applications. www.w3c.org.il 30
- 30. (also) things not on the web www.w3c.org.il 31
- 31. Therefore! The ability to exchange information between different applications means that the information may be made available to applications other than those for which it was originally created. www.w3c.org.il 32
- 32. מנתונים - לאפליקציות נתונים ב-RDF XML דפי אינטרנט מאגרי נתונים j.mp/w3civan www.w3c.org.il
- 33. קצת יותר טכני 43 www.w3c.org.il
- 34. 1. With RDF we describe resources in terms of properties and property values <http://www.productivewise.com/> <CreatedBy> "Eyal Sela" . subject predicate object 35 www.w3c.org.il
- 35. 2. Which are represented as a graph of nodes and arcs http://www.productivewise.com CreatedBy Eyal sela 36 www.w3c.org.il
- 36. 3. things are identified by URI http://www.productivewise.com/#me 37 www.w3c.org.il
- 37. 1. With RDF we describe resources in terms of properties and property values 2. Which are represented as a graph of nodes and arcs 3. Things are identified by URIs 38 www.w3c.org.il
- 38. דוגמא there is a Person identified by http://www.w3.org/People/EM/contact#me, whose name is Eric Miller, whose email address is em@w3.org, and whose title is Dr. j.mp/w3crdf www.w3c.org.il 39
- 39. הכי טכני XML-based syntax www.w3c.org.il 40
- 40. www.w3c.org.il 41
- 41. www.w3c.org.il 42
- 42. RDF is: nodes-and-arcs diagrams interpreted as statements about things identified by URIrefs. www.w3c.org.il 43
- 43. רגע. כשאומרים GivenNameמתכוונים לשם של אדם או אולי משתנה בתוכנה? www.w3c.org.il 44
- 44. ...לא יודע www.w3c.org.il 45
- 45. URIs לכן משתמשים ב www.w3c.org.il 46
- 46. http://xmlns.com/foaf/0.1/givenname The given name of some person. j.mp/9tGcBh www.w3c.org.il 47
- 47. Why use URIs as properites? 1.Distinguish properties 2. Properties treated as resources (add information about them) www.w3c.org.il 48
- 48. These are known as Vocabularies set of URIs defined for some specific purpose (E.g. identify employees) www.w3c.org.il 49
- 49. dc:creator = "creator" attribute in the Dublin Core the relationship between the Web page and the creator of the page is exactly the concept identified by http://purl.org/dc/elements/1.1/creator. www.w3c.org.il 50
- 50. Another person/program familiar with the Dublin Core vocabulary will know what is meant by this relationship. www.w3c.org.il 51
- 51. ...אז בעצם From: “Kermit – the inside story” « Feminist Law Professors www.w3c.org.il 52
- 52. ...אז בעצם רק אפליקציות שיודעות לקרוא RDF מסוים יבינו הצהרותvocabulary .שמשתמשות בו RDF statements using URIs from vocabularies convey the specific meanings associated with terms only to applications written to process these vocabularies. www.w3c.org.il 53
- 53. An application cannot actually "understand" such statements. any more than a database system "understands" terms like "employee" or "salary" in processing a query like SELECT NAME FROM EMPLOYEE WHERE SALARY > 35000. www.w3c.org.il 54
- 54. It only makes it seem like it understands them. ( just as a database system and its applications can do useful work in processing employee and payroll information without understanding "employee" and "payroll". ) www.w3c.org.il 55
- 55. האם יש הבדל בין זה לזה? www.w3c.org.il 65
- 56. www.w3c.org.il 57
- 57. But RDF can’t describe vocabularies – types of things (like exterms:Person) – properties (like exterms:age and exterms:creation-date) – types of things that can serve as the subjects or objects of statements involving those properties (like specifying that the value of an exterms:age property should always be an xsd:integer). www.w3c.org.il 58
- 58. RDFS www.w3c.org.il 59
- 59. RDFS defines classes (groups of related resources) and properties (relationships between resources) that are used to describe classes, properties and other resources. j.mp/w3crdfs www.w3c.org.il 60
- 60. RDFS does NOT specify a vocabulary of descriptive properties such as "author". it specifies mechanisms to do that. www.w3c.org.il 61
- 61. www.w3c.org.il 62
- 62. www.w3c.org.il 63
- 63. Inferred properties (<http://…/isbn/000651409X> rdf:type #Fiction) www.w3c.org.il
- 64. RDFS Limits Can’t say: – “the domain of hasParent is Child when applied to Human and Calf when applied to Elephants” –“all instances of person have a mother that is also a person”, or that persons have exactly 2 parents –isAncestorOf is a transitive property –isMarriedTo or isPeeredWith is symmetrical –identification of objects with different URI-s j.mp/cO5kdK :`( www.w3c.org.il 65
- 65. OWL www.w3c.org.il 66
- 66. www.w3c.org.il 67
- 67. Term equivalences For classes: owl:equivalentClass: two classes have the same individuals owl:disjointWith: no individuals in common For properties: owl:equivalentProperty remember the a:author vs. f:auteur owl:propertyDisjointWith For individuals: owl:sameAs: two URIs refer to the same concept (“individual”) owl:differentFrom: negation of owl:sameAs www.w3c.org.il
- 68. Classes in OWL In RDFS, you can subclass existing classes… that’s all In OWL, you can construct classes from existing ones: enumerate its content through intersection, union, complement www.w3c.org.il
- 69. (RDFa) 70 www.w3c.org.il
- 70. 71 www.w3c.org.il
- 71. .זהו, בגדול 72 www.w3c.org.il
- 72. עוד כמה דוגמאות Use examples • dbrec.net • BBC - j.mp/w3cabcd • ProductiveWise in the validator: j.mp/w3ceyalsite • ProductiveWise and reach snippets: j.mp/b7cIO9 • RelFinder : j.mp/77JV63 • RDFa and Drupal RDF Sources • data.gov.uk • data.gov • About MusicBrainz - MusicBrainz Wiki • UK Retail Chain Tesco adopts RDFa www.w3c.org.il 73
- 73. Resources • Semantic web standarts wiki - j.mp/w3csemw • http://semanticweb.org/wiki/Main_Page • http://www.w3.org/TR/2004/REC-rdf-primer-20040210/ • RDFa in Javascript - j.mp/w3clet • http://www.w3.org/2001/sw/SW-FAQ • http://www.w3.org/2007/11/Talks/Semantic_Web.html • Main areas of SW Linked Data, Vocabularies, Queries, Inference, Vertical Applications • Official publications W3C SW Publications • http://semanticweb.org/wiki/Main_Page • Semantic web standarts wiki - j.mp/w3csemw • RDFa Wiki 74 www.w3c.org.il
- 74. תודה • הירשמו לידיעון • צרו קשר j.mp/w3ccon @isociltech @eyalsela www.w3c.org.il 75
Hinweis der Redaktion
- The Semantic Web is an evolution of the World Wide Web that, rather than just linking from one document to another, focuses on their meaning in relation to each other. Linked Data is a set of technologies to achieve this for data, creating a web of data.s
- In addition to the classic “Web of documents” W3C is helping to build a technology stack to support a “Web of data,” the sort of data you find in databases. The ultimate goal of the Web of data is to enable computers to do more useful work and to develop systems that can support trusted interactions over the network. The term “ Semantic Web” refers to W3C’s vision of the Web of linked data . Semantic Web technologies enable people to create data stores on the Web, build vocabularies, and write rules for handling data. Linked data are empowered by technologies such as RDF , SPARQL , OWL , and SKOS .
- RDF is a datamodel for objects (&quot;resources&quot;) and relations between them, provides a simple semantics for this datamodel, and these datamodels can be represented in an XML syntax. RDF Schema is a vocabulary for describing properties and classes of RDF resources, with a semantics for generalization-hierarchies of such properties and classes. OWL adds more vocabulary for describing properties and classes: among others, relations between classes (e.g. disjointness), cardinality (e.g. &quot;exactly one&quot;), equality, richer typing of properties, characteristics of properties (e.g. symmetry), and enumerated classes.
- XML provides a surface syntax for structured documents, but imposes no semantic constraints on the meaning of these documents. XML Schema is a language for restricting the structure of XML documents and also extends XML with datatypes. 3. … XML? When should I use RDF and when should I use XML? Both formalisms have their strengths and weaknesses; their area of usage is different. The two data models serve different constituencies and the choice really depends on the application. There is no better or worse; only different. One of XML’s strengths is its ability to describe strict hierarchies. Applications may rely on and indeed exploit the position of an element in a hierarchy: for example, most browsers provide a different rendering of HTML’s li element depending on how “deep” the enclosing list is. XML makes it easy to control the content via XML Schemas and combine XML data that abide to the same Schema or DTD. However, combining different XML hierarchies (technically, DOM trees) within the same application may become very complex. XML is not an easy tool for data integration . On the other hand, RDF consists of a very loose set of relations (triples). Due to its usage of URIs it is very easy to seamlessly merge triple sets, ie, data described in RDF within the same application; it is therefore ideal for the integration of possibly heterogenous information on the Web. But this has its price: reconstructing hierarchies from RDF may become quite complex. As an example, it would be fairly complicated (and unnecessary) to describe, eg, vector graphics, using RDF; use SVG instead! RDF based vocabularies, and the accompanying semantic formalisms like RDFS or OWL, also make it easy to define inference possibilities on RDF data. Although this could be done around XML dialects, too, it would remain application specific and not portable. Source: http://www.w3.org/2001/sw/SW-FAQ#
- information about things that can be identified on the Web ( even when they cannot be directly retrieved on the Web intended for situations in which this information needs to be processed by applications, rather than being only displayed to people unlike conventional hypertext, RDF URIs can refer to any identifiable thing, including things that may not be directly retrievable on the Web (such as the person Eric Miller). The result is that in addition to describing such things as Web pages, RDF can also describe cars, businesses, people, news events, etc RDF provides a common framework for expressing this information so it can be exchanged between applications without loss of meaning.
- Specifically, the part that identifies the thing the statement is about (the Web page in this example) is called the subject . The part that identifies the property or characteristic of the subject that the statement specifies (creator, creation-date, or language in these examples) is called the predicate , and the part that identifies the value of that property is called the object . So, taking the English statement a system of machine-processable identifiers for identifying a subject, predicate, or objesssct in a statement without any possibility of confusion with a similar-looking identifier that might be used by someone else on the Web. a machine-processable language for representing these statements and exchanging them between machines. The underlying structure of any expression in RDF is a collection of triples, each consisting of a subject, a predicate and an object. A set of such triples is called an RDF graph (defined more formally in section 6 ). This can be illustrated by a node and directed-arc diagram, in which each triple is represented as a node-arc-node link (hence the term &quot;graph&quot;). Each triple represents a statement of a relationship between the things denoted by the nodes that it links. Each triple has three parts: a subject , an object , and a predicate (also called a property ) that denotes a relationship. The direction of the arc is significant: it always points toward the object. The nodes of an RDF graph are its subjects and objects. The assertion of an RDF triple says that some relationship, indicated by the predicate, holds between the things denoted by subject and object of the triple. The assertion of an RDF graph amounts to asserting all the triples in it, so the meaning of an RDF graph is the conjunction (logical AND) of the statements corresponding to all the triples it contains. A formal account of the meaning of RDF graphs is given in [ RDF-SEMANTICS ].
- 1identifying things using Web identifiers (called Uniform Resource Identifiers , or URIs ) Figure 1 illustrates that RDF uses URIs to identify: individuals, e.g., Eric Miller, identified by http://www.w3.org/People/EM/contact#me kinds of things, e.g., Person, identified byhttp://www.w3.org/2000/10/swap/pim/contact#Person properties of those things, e.g., mailbox, identified byhttp://www.w3.org/2000/10/swap/pim/contact#mailbox values of those properties, e.g. mailto:em@w3.org as the value of the mailbox property (RDF also uses character strings such as &quot;Eric Miller&quot;, and values from other datatypes such as integers and dates, as the values of properties)
- . In addition, RDF properties themselves have URIs, to precisely identify the relationships that exist between the linked items.
- Let's say I want to express that the website found at http://www.google.com was created by Larry Page. In this case we have the following 3 things that comprise our assertion: The subject: http://www.google.com The predicate or property: creator The object or value: Larry Page Now to put this into RDF syntax, we do the following: <?xml version=&quot;1.0&quot;?> <rdf:RDF xmlns:rdf=&quot;http://www.w3.org/1999/02/22-rdf-syntax-ns#&quot; xmlns:dc=&quot;http://purl.org/dc/elements/1.1/&quot;> <rdf:Description rdf:about=&quot;http://www.google.com&quot; > < dc:creator > Larry Page </ dc:creator > </rdf:Description> </rdf:RDF>Note that in the above example, the schema used to define the property 'creator' is the Dublin Core.
- Distinguish properties one person may use from different properties someone else may use that would otherwise be identified by the same character string. in Figure 4 , example.org uses &quot;name&quot; to mean someone's full name written out as a character string literal (e.g., &quot;John Smith&quot;), but someone else may intend &quot;name&quot; to mean something different (e.g., the name of a variable in a piece of program text). A program encountering &quot;name&quot; as a property identifier on the Web (or merging data from multiple sources) would not necessarily be able to distinguish these uses. However, if example.org writes http://www.example.org/terms/name for its &quot;name&quot; property, and the other person writes http://www.domain2.example.org/genealogy/terms/name for hers, it is clear that there are distinct properties involved (even if a program cannot automatically determine the distinct meanings) using URIrefs to identify properties enables the properties to be treated as resources themselves. Since properties are resources, additional information can be recorded about them (e.g., the English description of what example.org means by &quot;name&quot;), simply by adding additional RDF statements with the property's URIref as the subject.
- Distinguish properties one person may use from different properties someone else may use that would otherwise be identified by the same character string. in Figure 4 , example.org uses &quot;name&quot; to mean someone's full name written out as a character string literal (e.g., &quot;John Smith&quot;), but someone else may intend &quot;name&quot; to mean something different (e.g., the name of a variable in a piece of program text). A program encountering &quot;name&quot; as a property identifier on the Web (or merging data from multiple sources) would not necessarily be able to distinguish these uses. However, if example.org writes http://www.example.org/terms/name for its &quot;name&quot; property, and the other person writes http://www.domain2.example.org/genealogy/terms/name for hers, it is clear that there are distinct properties involved (even if a program cannot automatically determine the distinct meanings) using URIrefs to identify properties enables the properties to be treated as resources themselves. Since properties are resources, additional information can be recorded about them (e.g., the English description of what example.org means by &quot;name&quot;), simply by adding additional RDF statements with the property's URIref as the subject.
- In the rest of the Primer, the term vocabulary will be used when referring to a set of URIrefs defined for some specific purpose, such as the set of URIrefs defined by RDF for its own use, or the set of URIrefs defined by example.org to identify its employees. ex:index.html dc:creator exstaff:85740 . the predicate dc:creator, when fully expanded as a URIref, is an unambiguous reference to the &quot;creator&quot; attribute in the Dublin Core metadata attribute set (discussed further in Section 6.1 ), a widely-used set of attributes (properties) for describing information of all kinds. The writer of this triple is effectively saying that the relationship between the Web page (identified byhttp://www.example.org/index.html ) and the creator of the page (a distinct person, identified byhttp://www.example.org/staffid/85740 ) is exactly the concept identified byhttp://purl.org/dc/elements/1.1/creator. Another person familiar with the Dublin Core vocabulary, or who finds out what dc:creator means (say by looking up its definition on the Web) will know what is meant by this relationship. In addition, based on this understanding, people can write programs to behave in accordance with that meaning when processing triples containing the predicate dc:creator. Of course, this depends on increasing the general use of URIrefs to refer to things instead of using literals; e.g., using URIrefs likeexstaff:85740 and dc:creator instead of character string literals like John Smith and creator. Even then, RDF's use of URIrefs does not solve all identification problems because, for example, people can still use different URIrefs to refer to the same thing. For this reason, it is a good idea to try to use terms from existing vocabularies (such as the Dublin Core) where possible, rather than making up new terms that might overlap with those of some other vocabulary. Appropriate vocabularies for use in specific application areas are being developed all the time, as illustrated by the applications described in Section 6 . However, even when synonyms are created, the fact that these different URIrefs are used in the commonly-accessible &quot;Web space&quot; provides the opportunity both to identify equivalences among these different references, and to migrate toward the use of common references.
- RDF defines only the graph syntax of subject, predicate, and object triples, certain meanings associated with URIrefs in the rdf: vocabulary, and certain other concepts RDF does not define the meanings of terms from other vocabularies, such as dc:creator, that might be used in RDF statements . Specific vocabularies will be created, with specific meanings assigned to the URIrefs defined in them RDF statements using URIrefs from these vocabularies may convey the specific meanings associated with those terms to people familiar with these vocabularies, or to RDF applications written to process these vocabularies, without conveying any of these meanings to an arbitrary RDF application not specifically written to process these vocabularies.
- The result of all this is that RDF provides a way to make statements that applications can more easily process. An application cannot actually &quot;understand&quot; such statements, as noted already, any more than a database system &quot;understands&quot; terms like &quot;employee&quot; or &quot;salary&quot; in processing a query likeSELECT NAME FROM EMPLOYEE WHERE SALARY > 35000. However, if an application is appropriately written, it can deal with RDF statements in a way that makes it seem like it does understand them, just as a database system and its applications can do useful work in processing employee and payroll information without understanding &quot;employee&quot; and &quot;payroll&quot;. For example, a user could search the Web for all book reviews and create an average rating for each book. Then, the user could put that information back on the Web. Another Web site could take that list of book rating averages and create a &quot;Top Ten Highest Rated Books&quot; page. Here, the availability and use of a shared vocabulary about ratings, and a shared group of URIrefs identifying the books they apply to, allows individuals to build a mutually-understood and increasingly-powerful (as additional contributions are made) &quot;information base&quot; about books on the Web. The same principle applies to the vast amounts of information that people create about thousands of subjects every day on the Web.
- ssas far as any built-in meaning is concerned. Similarly, any natural language text describing the meaning of dc:creator that might be found on the Web provides no additional meaning that an arbitrary RDF application can directly use. Of course, URIrefs from a particular vocabulary can be used in RDF statements even though a given application may not be able to associate any special meanings with them. For example, generic RDF software would recognize that the above expression is an RDF statement, that ed:dsfbups is the predicate, and so on. It will simply not associate with the triple any special meaning that the vocabulary developer might have associated with a URIref like ed:dsfbups. Moreover, based on their understanding of a given vocabulary, people can write RDF applications to behave in accordance with the special meanings assigned to URIrefs from that vocabulary, even though that meaning will not be accessible to RDF applications not written in that way.
- RDF Schema defines classes and properties that may be used to describe classes, properties and other resources. RDFS does not specify a vocabulary of descriptive properties such as &quot;author&quot;. Instead it specifies mechanisms that may be used to name and describe properties and the classes of resource they describe. RDF's vocabulary description language, RDF Schema, is a semantic extension of RDF. mechanisms for describing groups of related resources and the relationships between these resources.
- RDFS Limits may not have enough detail for your modeling No localized range and domain constraints Can’t say that “the domain of hasParent is Child when applied to Human and Calf when applied to Elephants” No existence/cardinality constraints Can’t say that “all instances of person have a mother that is also a person”, or that persons have exactly 2 parents No transitive, inverse or symmetrical properties Can’t say that isAncestorOf is a transitive property Can’t say that bundles is the inverse of isBundledBy Can’t say that isMarriedTo or isPeeredWith is symmetrical
- RDF is a datamodel for objects (&quot;resources&quot;) and relations between them, provides a simple semantics for this datamodel, and these datamodels can be represented in an XML syntax. RDF Schema is a vocabulary for describing properties and classes of RDF resources, with a semantics for generalization-hierarchies of such properties and classes. OWL adds more vocabulary for describing properties and classes: among others, relations between classes (e.g. disjointness), cardinality (e.g. &quot;exactly one&quot;), equality, richer typing of properties, characteristics of properties (e.g. symmetry), and enumerated classes.