Data interoperability is currently a problem that we are facing more intensely due to the appearance of elds like Big Data or IoT. Many data is persisted in information silos with neither interconnection nor format homogenisation. Our proposal to alleviate this problem is ShExML, a language based on ShEx that can map and merge heterogeneous data formats into a single RDF representation. We advocate the creation of this type of tools that can facilitate the migration of nonsemantic data to the Semantic Web.
Mapping and merging heterogeneous data sources is a task that has gained in importance throughout the last years. With the improvement of hardware support, the development of new technological areas|such as Big Data or Internet of Things (IoT)|and the deeper interconnection between heterogeneous devices, a huge amount of data is generated every second. However, this data is created in various formats and persisted using di erent technologies. Therefore, understanding and exploitation of this data becomes a hard work due to the information silos model.
One of the goals of the Semantic Web was the interconnection of data sources and the avoidance of the aforementioned information silos. Therefore, many technologies were proposed to accompany that objective. However, the migration of non-semantic data to the new semantic technologies is a hard task that many individuals and companies are not able to face due to the time or resources consumption. Migrating all databases in a company to their counterpart in Semantic Web world will carry not only the migration of the platforms, but also the data with the development of ad-hoc solutions for every dataset. Therefore, solutions that alleviate this translation can contribute to the adoption of semantic technologies or, at least, facilitate it.
We propose a language to map and merge heterogeneous data into its Resource Description Framework (RDF) counterpart. But also taking into account usability and easiness of use.
Many mapping languages and tools were proposed to perform a mapping between
a non-semantic format to its RDF counterpart. This is the case of XSPARQL
[
However, none of these works tackle the mapping and the merging of
heterogeneous datasets in the same solution. This is addressed by RML [
ShExML shares the same goal as RML and YARRRML. However, as being based on ShEx, validation of generated data can be done faster, i.e., the gap between ShExML and ShEx is small. Moreover, it is designed to keep the same simplicity and easiness of use that ShEx has. 3
ShExML3 is based on ShEx [
Listing 1.1. ShExML example for lms PREFIX : <http :// example . com /> PREFIX dbo : <http :// dbpedia . org / ontology /> PREFIX foaf : <http :// xmlns . com / foaf /0.1/ > PREFIX dbr : <http :// dbpedia . org / resource /> SOURCE films_xml < https :// example . com / films .xml > SOURCE films_json < https :// example . com / films . json > QUERY film_ids_xml <// film /@id > QUERY film_names_xml <// film / name > QUERY film_years_xml <// film / year > QUERY film_directors_xml <// film / director > QUERY film_ids_json <$. films [*]. id > QUERY film_names_json <$. films [*]. name > QUERY film_years_json <$. films [*]. year > QUERY film_directors_json <$. films [*]. director > EXPRESSION film_ids < $films_xml . film_ids_xml UNION $films_json . film_ids_json > EXPRESSION film_names < $films_xml . film_names_xml UNION $films_json . film_names_json > EXPRESSION film_years < $films_xml . film_years_xml UNION $films_json . film_years_json > EXPRESSION film_directors < $films_xml . film_directors_xml UNION $films_json . film_directors_json > : Films :[ film_ids ] { foaf : name [ film_names ] ; dbo : year dbr :[ film_years ] ; dbo : director [ film_directors ] ; }
We can see ShExML as a combination of declarations followed by a set of shapes. Being the declarations a collection of variable de nitions and the shapes the core procedure to de ne and execute the mappings. 3 ShExML on Github: https://github.com/herminiogg/ShExML
ShExML: An heterogeneous data mapping language based on ShEx
Inside the set of declarations there are pre xes, sources, queries and expressions. Pre xes work as Turtle pre xes; sources allow to de ne a URL in which the le is hosted; queries are intended to de ne reusable queries for the previously de ned sources (which normally are de ned in a query language, e.g., JSONPath or XMLPath); and expressions which are used to perform the queries over a source, make unions among queries and transform them.
Listing 1.2. JSON lms le
Listing 1.3. XML lms le { } " films ": [ { " id ": 3, " name ": " Inception ", " year ": "2010" , " director ":
" Christopher Nolan " }, { " id ": 4, " name ": " The Prestige ", " year ": "2006" , " director ":
" Christopher Nolan "
Thus, imagine that we want to make the transformation of two lists of lms: one in JSON and the other in XML (see Listings 1.2 and 1.3). We de ne a ShExML which can convert both les to RDF and merge them into a single RDF le (see Listing 1.1). This conversion has a single shape called :Films which has the main conversion for the lms. In order to construct each triple a name is de ned under the :[films ids] directive which will match with the subject of every triple generated by this shape. Then, predicates and objects are generated, based on the previous ids, using the expressions enclosed between braces. For example, foaf:name [films name] will generate a triple in the form of subject foaf:name :object. Notice that every expression enclosed between square brackets allows a pre x de nition which tells the compiler if this expression will be a node or a literal. Moreover, if a query produces a list of results, instead of a single one, the ShExML engine performs the mapping taking into account the relation of them with each entity. Hence, making it possible to merge les with various entities. Finally, the result of this example is showed in Listing 1.4. :4 dbo : director " Christopher Nolan " ; dbo : year dbr :2006 ; foaf : name " The Prestige " . dbo : director " Christopher Nolan " ; dbo : year dbr :2010 ; :2 4 foaf : name
" Inception " . dbo : director " Christopher Nolan " ; dbo : year dbr :2014 ; foaf : name " Interstellar " . dbo : director " Christopher Nolan " ; dbo : year dbr :2017 ; foaf : name " Dunkirk " .
In this work, we have presented ShExML, a language that allows to map and merge heterogeneous data into its RDF counterpart. This tool helps the migration of semi-structured data to a semantic data format, improving its interoperability and searchability. With the development of this solution, the integration of data into the Semantic Web is an easier task and it can be adapted to di erent scenarios. We are planning to include some extra features in future versions, such as: the uni cation of URIs between di erent representations, the matching between generated URIs and existing ones in the Linked Open Data cloud and the conversion of streaming sources.
Acknowledgments This work has been partially funded by the Vicerectorate for Research of the University of Oviedo under the call of "Plan de Apoyo y Promocion de la Investigacion" and by the Ministerio de Econom a, Industria y Competitividad under the call of "Programa Estatal de I+D+i Orientada a los Retos de la Sociedad" (project TIN2017-88877-R).