Linked Open Data. University of Rome "Tor Vergata" ART: Artificial Intelligence Tor Vergata. "The Semantic Web done right" Tim Berners-Lee

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1 University of Rome "Tor Vergata" ART: Artificial Intelligence Tor Vergata Linked Open Data "The Semantic Web done right" Tim Berners-Lee Date January lastupdate: Mon, 28 Jan :00: Author Manuel Fiorelli <fiorelli@info.uniroma2.it>

2 Outline From the Web of Documents......to the Web of Data Linked Data principles Publication of Linked Data Consuming Linked Data Final remarks

3 From the Web of Documents... Global identifiers (see RFC 3986) are used to weave together documents across local hyper-texts. INTERNET

4 ...to the Web of Data Similarly, we can use web technologies to spread the data graph over remote nodes. INTERNET

5 ...using the Linked Data principles Obviously, there are many ways for wiring data together, but usually we refer to the following Linked Data principles (Tim Berners-Lee, 2006): 1. Use URIs as names for things 2. Use HTTP URIs so that people can look up those names. 3. When someone looks up a URI, provide useful information, using the standards (RDF*, SPARQL) 4. Include links to other URIs. so that they can discover more things

6 Rule #1: URIs as names for things (1) Quoting the Web Architecture specification: The World Wide Web (WWW, or simply Web) is an information space in which the items of interest, referred to as resources, are identified by global identifiers called Uniform Resource Identifiers (URI). The first step towards the Web of Data is to assign a URI to the subjects of interest. They could object that the Web is about information resources, whereas the Web of Data mostly concerns things that exist in the universe of discourse (either concrete or abstract). However, RFC 3986 clarifies that: "The URI itself only provides identification; access to the resource is neither guaranteed nor implied by the presence of a URI"

7 Rule #1: URIs as names for things (2) The RDF Abstract Syntax allowed the use of URIs as well as blank nodes (and literals). The first rule is relevant as it promotes the use of URIs over blank nodes, for the sake of global identification enabling "anyone to speak about anything anywhere". Having global scope, URIs should denote the same resource regardless of the context they appear, thus allowing people to speak about third-party resources. The creation of a URI for a resource is called minting; by social convention, only the owner of a URI or his delegates can assign it (avoiding clashes in the assignment of identifiers in a federated fashion): e.g.

8 Rule #2: HTTP URIs Once a URI has been minted, its owner has to communicate what it should mean. The second rule requires that those URIs use the HTTP scheme (e.g. so that they can be looked up for useful information about their referents. This practice was known for schemas (e.g. to retrieve the XSD definition of an XML namespace). In contrast, this practice is a novel for ground data. The RDF data model uses URIs only as logical names.

9 Rule #3: Return information using standards The third rule focuses on the use of standards to encode information about a resource. The RDF family of languages is a convenient choice the graph oriented data-model is optimized for distribution and integration; widely accepted serialization formats (e.g. RDF/XML and Turtle); self-descriptiveness through formal vocabulary definitions; open world assumption and monotonic semantic extensions are suitable for web scale usage information about a resource is not complete dealing with different levels of understanding of information

10 Rule #4: Link The last step towards the Web of Data is to wire together resources by means of links. This links are expressed through a triple whose subject and object live in different datasets. dppedia: is bound to the namespace owned by DBpedia ex:fiorelli ex:livesin dbpedia:rome Links across the boundaries of datasets support the reuse, the integration and the discovery of data.

11 Proliferation of identifiers Within the Linked Data framework URIs are overloaded being used both as identifiers to speak about something; a mechanism to retrieve the (partial) description of something. Anyone should be allowed to mint another URI for a resource; otherwise, if resources had just one URI, then there were just one place to seek for information about the resource. On the other side the proliferation of multiple (coreferencing) URIs for the same resource contrast the aggregation of the available knowledge.

12 Reuse of identifiers They should reuse terms from popular vocabularies (e.g. FOAF, Dublin Core,...) and individuals from third-party dataset (e.g. DBpedia, geonames,...). Otherwise, a new term should be linked to the rest of the Linked Data cloud (e.g. a new class extends an existing class, a new individual is in fact the same present in a popular dataset,...)

13 Integration on the Web of Data (1) The integration of data is an ongoing process distributed between: data publishers who want to make their data maximally accessible data consumers who need to combine disparate dataset

14 Integration on the Web of Data (2) Recognizing that two resource are in fact the same is a difficult task, taking different names: at individual level identity resolution / objection consolidation / deduplication at conceptual level schema mapping / ontology matching The different terminology is justified by the use of different approaches and algorithms, even developed by different communities.

15 Publication

16 Publication - static documents The RDF descriptions of resources found in a dataset can be published as static documents by means of a Web Sever. The maintenance of those documents is difficult and the necessity to store a triple in multiple documents can lead to inconsistencies.

17 Publication: dynamic documents The solution is to generate dynamically the descriptions out of a triple store. Pubby: Pubby serves dereferenceable URIs by sending a DESCRIBE describe query to an endpoint SPARQL.

18 Publication: RDB2RDF (1) Most Web applications are powered by relational databases, whose content is worth to publish as Linked Data. W3C RDB2RDF Working Group1 is working on a language, called R2RML, for the specification of mappings from relational databases to RDF datasets to be served via a SPARQL endpoint or dereferenceable URIs. A similar situation happened at the origins of the Web, when the creation of the FTP scheme boosted the size of the Web through the sudden publication of those numerous FTP resources already existing in Internet. 1

19 Pubblicazione: RDB2RDF (2)

20 Publication: "embedded RDF" (1) There are various standards to embed structured data within (X)HTML pages. These embeddings reflects the semistructured data which power most of the web applications. RDFa: XHTML e HTML 5 (starting from version 1.1) microformat: (X)HTML microdata: HTML5 RDFa is the only specification directly bound to the RDF data model.

21 Publication: embedded RDF (2) An example using RDFa <p xmlns:dc=" about=" In his latest book <cite property="dc:title">wikinomics</cite>, <span property="dc:creator">don Tapscott</span> explains deep changes in technology, demographics and business. The book is due to be published in <span property="dc:date" content=" ">october 2006</span>. </p> And its Turtle : dc: < :wikinomics dc:title "Wikinomics" ; dc:creator "Don Tapscott" ; dc:date ^^< >.

22 Publication: embedded RDF (3) The embedded formats are useful when you can publish nothing but (X)HTML pages. The embedded metadata might help search engines to index a web site and, later, to produce better results for user queries. Bing, Google, Yahoo! e Yandex started the project schema.org to establish a common collection of schemas to annotate web pages.

23 Semantic Annotation of Web Sites (1) Google Rich Snippet: com/webmasters/bin/answer.py? hl=en&answer=99170&topic= &ctx=topic

24 Semantic Annotation of Web Sites (1) Google Recipes:

25 The LOD cloud diagram

26 Consuming Linked Data

27 Where to look for data? Data Hub ( a community driven directory of useful datasets Linked Open Vocabularies ( a network of vocabularies (e.g. RDF Schema and OWL Ontologies) Entity Name System An Entity Name System is an authority which assigns canonical identifiers to resources (especially ground individuals) shortly described in terms of profiles. A few information about a resource is sufficient to look up the ENS for a candidate identifier.

28 Consuming LD: Dereferencing The use of the HTTP verb GET allows to obtain the RDF description of a resource in the Linked Data cloud. Linked Data Browsers (e.g. Tabulator, Disco, Marbles,...) rely on this architecture. Moreover, this mechanism allows to answer simple queries formulated in resource centered and path oriented languages (e.g. LD Path). PRO CONS information freshness latency (for the consumer) load on the publisher architecture limited expressiveness

29 Consuming LD: Federated Sparql For the sake of efficiency a dataset should provide a SPARQL endpoint (advertised in the resource description or in the VoID description of the dataset). In SPARQL 1.1 is possible to query multiple dataset at once in a federated fashion1. PREFIX foaf: < SELECT?name FROM < WHERE { < foaf:knows?person. SERVICE < {?person foaf:name?name. } } 1 The experience in federated DBs shows that it is very difficult to optimize the query evaluation.

30 Consuming LD: Heuristic Sparql The biggest limitation of Federated SPARQL is the necessity to mention explicitly the datasets to be used. Unfortunately, the Linked Data principles promotes the interlinking across disparate datasets so that it is very difficult to foresee the required endpoints. An hybrid approach allows the evaluation of a plain SPARQL query relying on heuristics to determine incrementally the required data based on the inspection of the query and the partial data during the evaluation. This approach is unable to guarantee the conformance to the SPARQL semantics.

31 Consuming LD: Crawler (1) An LD crawler is a software agent (like search engine bots) which surfs the Linked Data cloud to gather information. The local copy can be processed for schema mapping object consolidation quality management The output is a local Knowledge Base which can be managed using traditional methodologies. The use of a cached fragment of the Linked Data cloud reduce the load on data publishers and allows complex reasoning to be performed effectively.

32 Consuming LD: Crawler (2)

33 Final remarks

34 Beyond the RDF semantics The RDF formal semantics assumes that triples are accurate. Clearly, this assumption does not hold in the context of Linked Data when navigating across disparate datasets. Hot topics include: tracking provenance; judging trustworthiness; evaluating data quality. These problems are hard to solve in general dealing with the Linked Data cloud as a whole, while they are easy for applications dealing with a few (controlled) datasets.

35 Why are Linked Data so relevant? Big providers are happy to provide developers with APIs to invoke their services, but are afraid of sharing their databases (the so called "database hugging"). At the hearth of Linked Data there is a movement for the liberation of data in their raw form as the basis for innovative usage scenarios.

36 Open Data and Open Government According to Open Data is defined as follows "A piece of data is open if anyone is free to use, reuse, and redistribute it subject only, at most, to the requirement to attribute and/or share-alike" The Open Government movement asks Public Administrations to open their datasets: which have been supported by tax payers; for the sake of transparency. data.gov (United States of America) data.gov.uk (United Kingdom) dati.gov.it (Italy) and much more...

37 Open Data vs Linked Data Open Data and Linked Data are orthogonal concepts. LOD Open data licensing dimension Closed data "Unlinked" Data Linked Data publishing dimension Linked Data guarantees the technical means for the reuse of Open Data Open Data guarantees the critical mass for the success of the phenomenon Linked Open Data (LOD) = Linked Data + Open Data

38 Tim Berners-Lee on the next TED

39 References (1) 1. Tim Berners-Lee. Design Issues: Linked Data, 27 July Talk at TED : "Tim Berners-Lee on the next Web", February Tom Heath and Christian Bizer (2011) Linked Data: Evolving the Web into a Global Data Space (1st edition). Synthesis Lectures on the Semantic Web: Theory and Technology, 1:1, Morgan & Claypool. 4. Tom Heath. Linked Data? Web of Data? Semantic Web? WTF, March Tim Berners-Kee and Mark Fischetti. Weaving the Web: The original design and ultimate destiny of the World Wide Web by its inventor, 2000, HarperInformation. 6. Ian Jacobs and Norman Walsh.Architecture of the World Wide Web, Volume One, 15 December 2004.

40 References (2) 7. Sebastian Schaffert, Christoph Bauer, Thomas Kurz, Fabian Dorschel, Dietmar Glachs, Manuel Fernandez. The linked media framework: integrating and interlinking enterprise media content and data. Proceedings of 8th 8th International Conference on Semantic Systems, ISEMANTICS 2012, Graz, Austria, 5-7 September, Olaf Hartig, Christian Bizer and Freytag Johann-Christoph. Executing SPARQL Queries over the Web of Linked Data. ISWC 2009, Springer Berlin Heidelberg, Robert Isele, J urgen Umbrich, Christian Bizer, and Andreas Harth. LDSpider: An open-source crawling framework for the Web of Linked Data. ISWC Proceedings of 9th International Semantic Web Conference (ISWC 2010) Posters and Demos, 2010.