Linked Data Tutorial

Digital Enterprise Research Institute www.deri.ie

Linked Data

A Practical Introduction by Dr. Michael Hausenblas, DI2

© Copyright 2008 Digital Enterprise Research Institute. All rights reserved.

Schedule

Linked Data Principles – 10%
 
Web of Data 101 (URI, HTTP, RDF) – 40%
 
Linking Open Data community project – 20%
 
Tools and Applications – 30%
 

2

Why?

Web of Data = linked data + vocabularies +
 
embedded metadata (RDFa, microformats, etc.)
When publishing linked data you provide a
 
standardised, uniform, and generic API for:
discovery, see also http://webofdata.wordpress.com/
 
integration/meshup
 
distributed query
 

uniform access to metadata and data
 
enable serendipity
 

See also [EXPL]
 

3

What?

In contrast to the full-fledged Semantic Web vision,
linked data is mainly about publishing structured
data in RDF using URIs rather than focusing on the
ontological level or inference. This simplification—
just as the Web simplified the established academic
approaches of Hypertext systems—lowers the entry
barrier for data provider, hence fosters a wide-
spread adoption.
[EXPL]

4

Linked Data Principles

By Tim Berners-Lee, ca. 2006 [LD]
 
Use URIs to identify things (anything, not just documents)
 
Use HTTP URIs – globally unique names, distributed
 
ownership – allows people to look up things
Provide useful information in RDF – when someone looks
 
up a URI
Include RDF links to other URIs – to enable discovery of
 
related information

5


Issues
 
These are principles, not implementation advices
 
Many things (deliberately?) kept blurry
 
Non-information resource vs. information resource debate
 
(see also [AWWSW])
CN and 303, the httpRange TAG issue [UA]
 

Formats: HTML + RDF/XML vs. RDFa
 

6


Ongoing work
 
Description/discovery
 
–  semantic sitemaps, see
http://sw.deri.org/2007/07/sitemapextension/
–  voiD, see http://semanticweb.org/wiki/VoiD
Trust (SPOT09 at ESWC09 for example)
 

Multimedia/Fragments, see
 
http://www.interlinkingmultimedia.info/
Foundational issues in TAG/AWWSW
 
Transforming the read-only Web of Data into a read/write
 
Web of Data, see for example
http://esw.w3.org/topic/PushBackDataToLegacySources

7

Schedule

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8

Web of Data 101 - URI

A Uniform Resource Identifier (URI) is a compact
 
sequence of characters that identifies an abstract
or physical resource. [RFC3986]
Syntax
 
URI = scheme quot;:quot; hier-part [ quot;?quot; query ] [ quot;#quot; fragment ]
Example
 
foo://example.com:8042/over/there?name=ferret#nose
_/ _________________/_________/ __________/ __/
| | | | |
scheme authority path query fragment

9


Don’t confuse scheme with protocol
 
Scheme: defines URI layout and (certain) semantics; go and
 
register with IANA using [RFC4395]
Protocol: defines communication means between
 
endpoints (such as HTTP, FTP, etc.)
URI resolution (as of [RFC3986])
 
STEP OUTPUT BUFFER INPUT BUFFER
1: /a/b/c/./../../g
2E: /a /b/c/./../../g
2E: /a/b /c/./../../g
2E: /a/b/c /./../../g
2B: /a/b/c /../../g
2C: /a/b /../g
2C: /a /g
2E: /a/g

10


URIrefs, URI references [RDF AS]
 
An RDF URI reference is a Unicode string does not contain
any control characters (#x00 - #x1F, #x7F-#x9F) and would
produce a valid URI character sequence representing an
absolute URI when subjected to an UTF-8 encoding along
with %-escaping non-US-ASCII octets.
QNames, Qualified Names [XML NS]
 
XML’s way to allow namespaced elements/attributes as of
QName = Prefix ‘:‘ LocalPart
CURIEs, Compact URIs [CURIE]
 
Generic, abbreviated syntax for expressing URIs, currently
 
in SPARQL, RDFa, and XHTML2 deployed

11

Web of Data 101 - HTTP

The Hypertext Transfer Protocol (HTTP) is an
 
application-level protocol for distributed,
collaborative, hypermedia information systems. It is
a generic, stateless, protocol which can be used
for many tasks beyond its use for hypertext, such
as name servers and distributed object
management systems, through extension of its
request methods, error codes and headers. A
feature of HTTP is the typing and negotiation of
data representation, allowing systems to be built
independently of the data being transferred.
[RFC2616]

12


HTTP messages consist of requests from client to
 
server and responses from server to client
Set of methods is predefined (such as GET, POST,
 
etc.), but can be expanded
Set of status codes is defined
 
Informational 1xx, provisional response, (100 Continue)
 
Successful 2xx, request successfully received, understood, and
 
accepted (201 Created)
Redirection 3xx, further action needs to be taken by user agent
 
to fulfill the request (301 Moved Permanently)
Client Error 4xx, client erred (405 Method Not Allowed)
 
Server Error 5xx, server encountered an unexpected condition
 
(501 Not Implemented)

13


GET /html/rfc2616 HTTP/1.1
REQUEST

Host: tools.ietf.org
User-Agent: Mozilla/5.0
Accept: text/html,application/xhtml
+xml,application/xml;q=0.9,*/*;q=0.8
RESPONSE

HTTP/1.x 200 OK
Date: Thu, 05 Mar 2009 08:17:33 GMT
Server: Apache/2.2.11
Content-Location: rfc2616.html
Last-Modified: Tue, 20 Jan 2009 09:16:04 GMT
Content-Type: text/html; charset=UTF-8

14


Content Negotiation (CN, conneg) is the process of
 
selecting the best representation for a given
response when there are multiple representations
available
Three types of CN: server-driven, agent-driven CN,
 
transparent CN
Example
 
curl -I -H quot;Accept: application/rdf+xmlquot; http://dbpedia.org/resource/Galway
HTTP/1.1 303 See Other
Content-Type: application/rdf+xml
Location: http://dbpedia.org/data/Galway.rdf

15


Caching (see Cache–Control header field) is
 
essential for scalability
HTTPbis [HTTPbis], IETF WG chaired by Mark
 
Nottingham, mainly about: patches, clarifications,
deprecate non-used features, documentation of
security properties

16


Representational State Transfer [REST]
 
resource the intended conceptual target of a hypertext
reference
resource identifier URL, URN
representation HTML document, JPEG image
representation media type, last-modified time
metadata
resource source link, alternates, vary
metadata
control data if-modified-since, cache-control

17

Web of Data 101 - RDF

As of [RDF AS] a data model: a directed, labeled
 
graph based on URIs
Triple: (subject predicate object)
 
subject … URIref or bNode
 
predicate … URIref
 
object … URIref or bNode or literal
 

18

Web of Data 101 - RDF

19

Web of Data 101 - Overview

Web's Standard Retrieval Algorithm as of [SDD]:
1.  parse URI and find HTTP protocol
2.  look up DNS name to determine the
associated IP address
3.  open a TCP stream to port 80 at the IP
address determined above
4.  format an HTTP GET request for resource
and sends that to the server
5.  read response from the server
6.  from the status code (200) determine that a
representation of the resource is available
7.  inspect the returned Content-Type
8.  pass the entity-body to its HTML rendering
engine

20


21


22


23

Schedule

 
 
 
 

24

Linking Open Data Project

Community project with W3C support started in
 
early 2007 [LOD]
Idea: take existing (open) data sets and make
 
them available on the Web in RDF
Interlink them with other data sets
 

Kudos to Tom Heath and Richard Cyganiak;
the material in this section is heavily based
on their work.

25


May 2007

26


Feb 2009

27


DBpedia

28


Geonames

29

Schedule

 
 
 
 

30

Tools and Applications

Linking Open Data homepage [LOD] has
 
Browsing with Tabulator, VisiNav, DBpedia Mobile, etc.
 
Searching with Sindice, SWSE, Falcons, etc.
 
Mashups, e.g. Revyu, BBC Music, DERI Pipes
 

See further
 
http://esw.w3.org/topic/SweoIG/TaskForces/
CommunityProjects/LinkingOpenData/Applications

31


DBpedia Mobile

32


BBC music beta

33


Virtuoso including RDF triple store, SPARQL access
 
to data, Open source edition
http://virtuoso.openlinksw.com/
Talis Platform, SaaS, Cloud-based storage for RDF
 
data and binary objects, SPARQL access, REST APIs
http://www.talis.com/platform
ARC (PHP) http://arc.semsol.org/
 
Jena (Java) http://jena.sourceforge.net/
 
Summary see
 
http://www.semanticscripting.org/SFSW2005/SFSW-
Toolkits.pdf

34


Frameworks
 
LOOMP http://www.loomp.org/
 
Silk http://www4.wiwiss.fu-berlin.de/bizer/silk/
 
SQUIN http://squin.sourceforge.net/
 
Paget http://code.google.com/p/paget
 

Other Tools (debug, etc.)
 
curl
 
Live HTTP headers (FF plug in)
 
http://sparql.org/sparql.html
 
https://wiki.mozilla.org/Labs/Ubiquity/
 
Commands_In_The_Wild (Web of Data, etc.)

35


Further resources regarding the publishing process
 
http://linkeddata.org/docs/how-to-publish
 
http://events.linkeddata.org/iswc2008tutorial/
 
http://videolectures.net/iswc08_heath_hpldw/
 
http://www.w3.org/TR/swbp-vocab-pub/
 

http://vapour.sourceforge.net/
 

36

References

[EXPL] … ‘Exploiting Linked Data For Building Web Applications’, Hausenblas,
 
2009, accepted for publication in IEEE IC
(pre-print version: http://sw-app.org/pub/exploit-lod-webapps-IEEEIC-preprint.pdf)
[LD] … http://www.w3.org/DesignIssues/LinkedData.html
 
[UA] … http://esw.w3.org/topic/FindingResourceDescriptions
 
[RFC3986] … http://www.ietf.org/rfc/rfc3986.txt
 
[RDF AS] … http://www.w3.org/TR/rdf-concepts/#section-Graph-URIref
 
[XML NS] … http://www.w3.org/TR/xml-names/#ns-qualnames
 
[CURIE] … http://www.w3.org/TR/curie/
 
[RFC4395] … http://tools.ietf.org/html/rfc4395
 
[RFC2616] … http://www.ietf.org/rfc/rfc2616.txt
 
[HTTPbis] … http://tools.ietf.org/wg/httpbis/
 
[REST] … ‘Principled design of the modern Web architecture’, Fielding and Taylor,
 
2002, http://portal.acm.org/citation.cfm?doid=514183.514185
[SDD] … http://www.w3.org/2001/tag/doc/selfDescribingDocuments
 
[AWWSW] … http://esw.w3.org/topic/AwwswHome
 
[LOD] …
 
http://esw.w3.org/topic/SweoIG/TaskForces/CommunityProjects/LinkingOpenData

37

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