Development Issues on Linked Data Weblog

Development Issues on Linked Data Weblog Enrichment Iv´ an Ruiz-Rube1 , Carlos M. Cornejo1 , Juan Manuel Dodero1 , and Vicente M. Garc´ıa2 1

Department of Computer Languages and Systems University of C´ adiz {ivan.ruiz,carlos.cornejo,juanma.dodero}@uca.es 2 Free Software Research and Development Foundation [email protected]

Abstract. In this paper, we describe the issues found during the development of LinkedBlog, a Linked Data extension for WordPress blogs. This extension enables to enrich text-based and video information contained in blog entries with RDF triples that are suitable to be stored, managed and exploited by other web-based applications. The issues have to do with the generality, usability, tracking, depth, security, trustiness and performance of the linked data enrichment process. The presented annotation approach aims at maintaining web-based contents independent from the underlying ontological model, by providing a loosely coupled RDFa-based approach in the linked data application. Finally, we detail how the performance of annotations can be improved through a semantic reasoner.

Keywords: Linked Data, annotation, RDFa, web blogging

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Introduction

The Semantic Web promised a new model in which machines were to be able to understand and process web-based information [3], so that intelligent agents can find, combine and act on the information needed by people in their everyday use of the Web. The Semantic Web hype, however, has gone flat, and today is often replaced by the term Linked Data or Web of Data, which aims at a less ambitious goal that has to do with the automated integration of information. Linked Data refers to the fact of describing and publishing the information of web pages in a structured manner that can be easily processed by software programs [4]. Semantic web techniques can be still used to integrate information from different sources and obtain value-added features. Linked Data presents a distinctive feature, i.e. its implementation is somewhat reversed to the traditional data modelling paradigm. While the classical development approach is first designing the data model and then populating it with data instances, the Linked Data approach is to provide structured data

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access after the publication of data instances. Providing a structure means enriching web pages with annotations, which are usually compliant with W3C standards such as OWL and RDF(S). The ease of enriching data held in the web to expose their underlying concepts, structure or schema depends on how the data sources are designed. For instance, there is a common ground between relational databases and ontology models. These structured models make easier to define RDF/OWL classes that hold the same base properties of a table in a relational database. Nevertheless, data on the Web are often published as regular text, which is more difficult to annotate automatically to find a correspondence with a given ontology. The issue of adding formal semantics to web contents as metadata is a major challenge for the Semantic Web community [16]. On one hand, semantic annotations are often based upon the knowledge of the actors about a specific domain while such annotations are being provided. On the other hand, Natural Language Processing (NLP) tools can provide annotation on text-based web sources, but do not have yet the required maturity to overcome this issue. Furthermore, it is increasingly common to find multimedia resources such as images and videos embedded in actual web applications. These resources implicitly contain much information, so it would also be interesting to describe and annotate the information they hold in a similar manner to the annotation of text. In either case, users can produce or consume web resources and, at the same time, can play the role of an annotator, publisher or reviewer of the contents [2]. We have built a tool for text and video annotation of blog messages based on existing, standard ontologies. The blog contents, properly annotated, can be then exploited for various purposes, such as indexing by a semantic search engine, or generating tag cloud concept representations. The Linked Data annotation approach aims at two objectives: (i) to keep independence from the web contents or applications that must be enriched; and (ii) to keep independence from the underlying ontology models used to describe annotations. These objectives deal with integration issues described elsewhere [7]. However, in the fulfillment of such goals, other development issues have emerged, such as: multiple annotation of the same concept by several users (i.e. concurrency); the need of tracking who provides each annotation (i.e. provenance); the effort required to achieve an acceptable degree of usability and performance of the annotation tool; and the exploitation of annotations by means of Semantic Web techniques in order to make users’ tasks easier. This paper describes these and other issues and illustrates them through the implementation of LinkedBlog, a Linked Data extension to WordPress3 that enables to enrich text and video contents provided by the blog users. The rest of the paper is structured as follows. Section 2 describes other works related to linked data enrichment. In section 3 we deal with the issues found in the development of the annotation approach. Section 4 presents our approach for text and video annotation and finally section 5 draws some conclusions and provides an outlook to future works. 3

http://wordpress.org/

Development Issues on Linked Data Weblog Enrichment

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Related work

Web-based resources must be annotated to convert them into Linked Data resources in order to be machine-understandable. Such Linked Data resources are held in semantic content management systems that often have the functionality and look-and-feel of wikis —such as DBPedia [5], Freebase [6]— or blogs —such as Zemanta [21]. These systems provide semantic Application Programming Interfaces (API) [8] in order to host and exploit linked data. On one hand, Freebase and DBPedia can extract information from the Wikipedia and publish it as RDF(S), according to their own schemas. Freebase enables to create special-purpose Linked Data applications (e.g. Thinkbase [12]) on its own hosting platform, called ACRE. Freebase can pull out Wikipedia data as well as DBPedia linked data. If there is the need of linking resource annotations to an standard ontology, the linked data schemas require an adaptation or transformation step [18]. This can be seen in Freebase’s RDF output, in which a number of OWL:SameAs RDF identifiers point to DBPedia. The Zemanta blogging assistant uses the English Wikipedia to capture knowledge of different areas of interest (e.g books, politics, sports, etc) and enrich web resources with in-text hyperlinks. At the core of the system there is a collection of semantic entities that represent concepts. However, they are not endorsed by any standard RDF schema or ontology. Instead of having the annotations centralized on a hosting platform, an approach to distributed annotation has been provided by Annotea [19]. Annotea is a Web-based shared annotation system based on a general-purpose open RDF infrastructure. Annotations are external to the documents and can be stored into special annotation servers [14]. It also provides several plugins for the client side (e.g. Annozilla4 or Amaya5 ). Related to externally stored annotations, Google SideWiki6 is a browser extension that allows web site viewers to add comments about web contents. The relevancy of these user-generated comments are based on Google’s ranking algorithms and in consequence they are not held upon the basis of an ontological model. Multimedia annotation approaches focus on acquiring metadata descriptions in order to facilitate indexing, search and retrieval. Feng et al. [9] have introduced an automatic image and video annotation technique for retrieval, based on textual queries, where the images that form the sequence are partitioned into regions. Their motivation was to improve the expensive task of manual annotations carried out by users (e.g. librarians) by proposing a statistical generative model that uses a set of annotated training images. These kind of modeling outperforms reasonable skills in closed domain collections, but the quality of the approach critically depends on the training set used. Regarding the existence of similar collaborative applications, a software tool has beed developed for the semantic annotation of video using multimedia ontologies [20]. It is based on an 4 5 6

http://annozilla.mozdev.org/ http://www.w3.org/Amaya/ http://www.google.com/sidewiki/

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educational metadata approach that adapts existing video sequences as ready-touse learning resources. They exemplify the idea of referencing domain ontology elements inside metadata elements, using Gene Ontology7 concepts to enrich multimedia resources as part of the learning activity sequence. Time aspects of such sequences are represented by using the MPEG-7 ontology [13], which classifies video as continuous annotated segments. Nevertheless, the MPEG-7 ontology is uniquely linked to a set of predefined concepts of multimedia resources (e.g. dates and time), thus losing the genericity that may offer any W3C standard ontology, such as TimeOntology8 , for instance, to describe date-time relations with the temporal dimension of elements of other domains.

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Enriching Web Contents with Linked Data

We have built an approach to ease the task of enriching web content with lined data by developing a plug-in that enables enriching text and video annotations with embedded RDFa [1] that refer to concepts and relationships provided by users. We have provided an extended interface to write/annotate texts, videos and video fragments by customizing the open source TinyMCE9 with RDFa capabilities. The reason of using RDFa is that web-based data and RDF annotations must be combined into the same web content container, because of the great diversity of data applications to be linked (i.e. wikis, blogs, etc.) The RDFa standard provides a high granularity for linked data modeling and can be seamlessly integrated into web pages through XHTML attributes. It is attainable and enables a cost-effective implementation [8]. Subsequently, Gleaning Resource Descriptions from Dialects of Languages10 (GRDDL) can be used for linking and extracting RDF data from the document. During the development of our system, a number of semantic integration issues have emerged. In the following, we first define such issues and afterwards propose an approach to overcome them. – Generality vs Usability: One important issue to consider deals with the usability of the annotation tool. Since our proposal is designed to be used with any ontological model, its user interface must be completely decoupled from that model. This feature, which a priori can be an advantage because of its generality, might not be attractive enough to the end users, due to the lack of the adaptation to a particular domain knowledge. Therefore, a usability review process would be required including the evaluation of real users (e.g. by timing the tagging process spent by the users). – Annotation tracking: The blog entries, as any other content on the web, often evolve over time, i.e. once the contents have been created, they can be updated several times or even be removed. It is common that annotations 7 8 9 10

http://www.geneontology.org/ http://www.w3.org/TR/owl-time/ http://tinymce.moxiecode.com/ http://www.w3.org/TR/grddl/


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existing at a given time cease to be valid, for example, by an error in the annotation, which might cause a later update to the post. Thus, the question about the persistence of annotations in the ontological model arises. When should annotations be persisting? Possibilities include, among others, the moment of annotation, on submitting the post, having an asynchronous update task, on user demand, etc. Annotation depth: RDFa recommendation allows to make annotations on the fly within a given HTML content, i.e. to define relationships between concepts that are not stored yet and are visible on the post. This issue can be solved by persisting annotations on the same moment of their provision. However, it might provoke an inconsistency problem in the ontological model if the post that is being annotated is not eventually saved. In consequence, another solution for this issue is adding AJAX capabilities to the editor in order to retrieve/save concepts that are not yet consolidated. Annotation security and trustiness: Our proposal is intended to develop a RDFa integrated editor that lets users annotate web contents using the existing concepts in the ontological model. This model is accesible to be queried through a SPARQL11 endpoint. Typically, content managing systems — as is our case with WordPress— provide authentication and authorization mechanisms. However, the ontological data repository has to supply its own security access mechanisms, since an authorized user can be able to modify blog contents and might not have enough privileges to work against the repository. Furthermore, one aspect to be considered occurs when two or more authorized users are annotating the same concept and contradict themselves with respect to the ontological model. This issue raises the need of an annotations review process or a publication workflow carried out by a specific domain knowledge actor. Reasoning vs performance: The content annotation process requires to identify those concepts or concept types that can be used for annotations. It also requires the establishment of relationships between them, according to the axioms contained in the ontological model. The amount and complexity of SPARQL queries necessary to support the user in this process will therefore depend on the complexity degree of the model itself. For example, in complex models with different hierarchy levels, it requires the execution of multiple SPARQL queries to collect all the data. This is originated by the fact that the mechanisms to exploit the potential benefits inherent to ontologies, such as class inheritance or inference, might be not natively provided by the annotation system. This issue can be mitigated by using semantic reasoners, which can infer logical consequences from a set of axioms and asserted facts from an ontology. Nonetheless, semantic reasoning comes at the cost of significantly decreasing performance. Multiple source annotation models: Another issue arises when two or more different ontology models are used as a reference of an annotation. For in-

http://www.w3.org/TR/rdf-sparql-protocol/

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stance, in order to carry out ontology-based annotations of the underlying information held on a video sequence, the definition of intervals should be explicitly linked with the multimedia resource. Furthermore, each of the intervals defined within the multimedia resource must have a time representation (i.e. hasBeginning, hasEnd). This approach enables multiple semantic embedded descriptions by linking each of the time intervals to concepts of the domain ontology. To overcome this issue, an ontology that models these dependencies is required, playing the role of ’semantic glue’ between multimedia resources, time intervals and user-defined concepts. – Complex ontology instances definition: When using certain ontologies, the number of RDF statements involved to define even simple concepts can complicate RDFa annotations. For instance, OWL Time12 is a standard W3C ontology for defining existing temporal concepts of Web pages and Web services. This ontology model defines relations among instants and intervals, together with information about durations and date-time information. In order to annotate videos using time intervals, we can use OWL Time models. In the listing 1.1, we can see an example of how a time interval can be described through the Time Ontology. However, the large number of RDF statements and necessary RDFa code needed to define a simple time interval can make time annotations quite complex.

Listing 1.1. Sample Interval in OWL functional-style syntax −− Main I n t e r v a l d e f i n i t i o n [10 −55] : myInterval ) ClassAssertion ( t i m e : P r o p e r I n t e r v a l −− I n i t i a l i n s t a n t d e f i n i t i o n and d e s c r i p t i o n ObjectPropertyAssertion ( t i m e : hasBeginning : m y I n t e r v a l ObjectPropertyAssertion ( t i m e : inDateTime : i n s t a n t S t a r t

: instantStart ) : dateTimeDescriptionStart )

−− I n i t i a l i n s t a n t t e m p o r a l i t y ObjectPropertyAssertion ( t i m e : u n i t T y p e : d a t e T i m e D e s c r i p t i o n S t a r t t i m e : unitSecond ) DataPropertyAssertion ( t i m e : second : d a t e T i m e D e s c r i p t i o n S t a r t ” 1 0 ” ˆ ˆ xsd : decimal ) −− F i n a l i n s t a n t d e f i n i t i o n and d e s c r i p t i o n ObjectPropertyAssertion ( t i m e : hasEnd : m y I n t e r v a l : i n s t a n t E n d ) ObjectPropertyAssertion ( t i m e : inDateTime : i n s t a n t E n d : dateTimeDescriptionEnd ) −− I n i t i a l i n s t a n t t e m p o r a l i t y ObjectPropertyAssertion ( t i m e : u n i t T y p e : dateTimeDescriptionEnd t i m e : unitSecond ) DataPropertyAssertion ( t i m e : second : dateTimeDescriptionEnd ” 5 5 ” ˆ ˆ xsd : decimal )

To avoid the fact of having a complex amount of RDFa code generated by the annotation tool, we have used a simplified way of defining OWL Time intervals. With this solution, each multimedia resource is annotated with simple temporal properties (e.g. start and end) and a RDF pre-harvesting process is done before adding the RDF statements to the repository. This process is responsible for translating the simple temporal properties into the proper OWL Time syntax. 12

http://www.w3.org/TR/owl-time/


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Linked Blog application

The objective of LinkedBlog is to foster and enhance users’ collaboration tasks while examining and annotating the information held in a blog. Our proposal to metadata annotation would be helpful for users by granting them the possibility of Linked Data editing. One objective of the Linked Data annotation approach is to keep it independent from the web applications that are semantically enriched. In order to field-test this goal, we have extended the Wordpress blog engine with an add-on used to enrich posts on the basis of concepts and relationships defined by a standard ontology. Blog users can provide text and video annotations as embedded RDFa. The second objective is to keep annotations independent from the underlying ontology models used to describe web resources. For that reason, the add-on can work with any ontological model available through a query end-point compatible with the SPARQL protocol for RDF. There is only a dependence of our approach with a specific ontology, but this is the Time Ontology, that is general enough and serves to define time intervals in video annotations. 4.1

Text and video annotations

LinkedBlog has been first used13 in the context of the eCultura project [7], which aims at providing a complete set of services and applications to access and integrate diverse web-based contents of the cultural domain. These services and applications use a data repository mainly fed with the CIDOC CRM ontology14 to depict cultural information, as well as with other ontologies, such as the Music Ontology15 , used to describe information about musical artists and works. Text annotation The text annotating procedure is similar to, for instance, how the user transforms a selected text into a hyperlink. Firstly, the user must select a portion of text to process and then selects the concept. In the concept selector pop-up window (see figure 1) the user can assign, change or delete a concept (i.e. individual) and/or associate a concept type (i.e. class) to the enclosed text. Once the user has selected the right concept or concept type, he/she is able to insert or edit annotations. In the window that pops up (figure 2) the user can indicate whether he/she wants to define a direct relationship (i.e. an RDF property having the concept as its domain) or a reverse relationship (i.e. an RDF property having the concept as its range). Depending on the applicable range of the relationship selected, the user can specify a literal content, the data type or a related concept. The listing 1.2 shows the HTML-embedded RDFa code that is generated by the add-on after annotating a post about Chano Lobato flamenco singer. In the example, you can see how his name is annotated with foaf:firstName (i.e. Sebasti´ an) and foaf:surname (i.e. Ram´ırez Sarabia) datatype properties, and his discography using the mo:discography object property. 13 14 15

http://www.ecultura.org/blog http://cidoc.ics.forth.gr/ http://musicontology.com/

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Fig. 1. Concept Selector

Fig. 2. Insert/Edit Annotation


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Listing 1.2. RDFa generated code example for text annotation Juan Miguel Ramirez Sarabia( Chano Lobato ) was born i n t h e B a r r i o de Santa Maria ( Cadiz ) , began i n t h e flamenco stages o f h i s hometown , i n t h e Venta La Palma , w i t h A u r e l i o S e l l e s , Servando Roa and A n t o n i o E l H e r r e r o . Then he moved t o Madrid t o debut p r o f e s s i o n a l l y , becoming p a r t o f t h e b a l l e t o f A l e j a n d r o Vega . Chano Lobato i n h i s d i s c o g r a p h y has more than t e n albums .

Video annotation LinkedBlog enables to annotate videos that are included in a blog entry. The annotations can be done to videos of the Youtube platform, because it is the most popular video provider that is integrated with Wordpress. The procedure to describe the information held in the clip is as follows: First, the user adds the video to the post. Once the video is added, the user must select it (by mouse clicking) and then select the insert/edit video annotations option.

Fig. 3. Insert/Edit Video Annotations

For describing the information contained in the video, the user must define time intervals along it. For each interval, the user can specify the proper concept and/or concept type, as well as the required annotations. To achieve this, the user can set direct/indirect relationships with literals or other concepts. Listing 1.3 shows the HTML-embedded RDFa code generated by the add-on after annotating a blog post including a video about Chano Lobato and Juan Carmona (also known as Juan Habichuela) flamenco artists, performing a Sola (i.e. a kind of flamenco style). In this example, we have defined a single sample interval, which describes the musical performance concept (mo:Performance).

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There are also two mo:performed inverse relationships with Juan Carmona and Chano Lobato artists (mo: MusicArtist). The RDFa snippet code includes both concepts from the specified domain (in our case, MusicOntology) as well as the needed concepts of the ontology used to specify time intervals. Listing 1.3. RDFa generated code example for video annotation

4.2

Improving Annotation through Semantic Reasoning

Web semantics provide reasoning capabilities that can help when querying on complex domain models and inferring from a set of asserted class descriptions. Using a semantic reasoner such as Pellet [17] enables to obtain all the properties that can be applicable to a class (e.g. up to 86 properties formo:SoloMusicArtist instances) by running a single SPARQL query, because the reasoner is able to infer on the class hierarchy. Therefore, our add-on provides a specific endpoint interface for semantic reasoning enhanced queries. This implementation drastically reduces the number and complexity of SPARQL queries. Our add-on performs different SPARQL queries against the ontological repository. For example, when the insert/edit annotation window is shown, the repository will be asked by those object- or data-properties whose domain matches to the current concept type or any of its supertypes. In our case of study we use a repository based on MusicOntology (see Figure 4). When finding the applicable properties to mo:SoloMusicArtist, it is required a pre-processing step that collects the set of nearby classes (i.e. foaf:Person and mo:MusicArtist) and more distant superclasses (i.e. foaf:Agent). Subsequently, the applicable properties to that set of classes are consulted.

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Conclusion and future work

In this paper, we described a set of issues that arise when developing a utility to annotate text and videos in web contents. The main annotation issues that must be addressed are the genericity vs usability, tracking, depth, security and trustiness, reasoning vs performance, multiple source annotation models, and complex ontology instances definition. The LinkedBlog system enables to formally describe web contents using Linked Data technologies, with independence of the underlying conceptual model


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Fig. 4. Excerpt from Music Ontology

and loosely coupled with the application where it is hosted. The developed software is an add-on that has been deployed in the Wordpress blog engine, but can be easily integrated in other kind of content management systems. In order to verify the strengths and the weaknesses of the add-on in a real environment, we will do an implementation and a monitoring on the web portal of the second edition of the university course for beginners about Flamenco (www.flamencoenred.tv). In this domain, a specific enhancement is planned as a future work to integrate Linked Data benefits with music recognition capabilities, either melodic [10] or rythmic [11], in order to further automate the annotation process of flamenco music themes. Furthermore, it is argued that Humanities cannot perform their research without ontology toolkits that handle ontological plurality and contexts, provide argumentation and question corpora management systems, as well as support adequate representational expressivity [15]. Therefore, the main evolution point of our annotation approach is using the RDF reification mechanism. Thus, before submitting any proposed semantic annotation to a server for persisting metadata, it can be contextualized for the corresponding user, post, and date-time.

Acknowledgments

This work has been sponsored by grants from the eCultura project (TSI-0205012008-53) of the Spanish Avanza R+D programme of the Ministry of Industry, Tourism and Trade.

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