Software Toolkits: Practical Aspects of the Internet of Things—A Survey <s> Data Storage via RDF <s> D2R Server is a tool for publishing the content of relational databases on the Semantic Web. Database content is mapped to RDF by a declarative mapping which specifies how resources are identified and how property values are generated from database content. Based on this mapping, D2R Server allows Web agents to retrieve RDF and XHTML representations of resources and to query non-RDF databases using the SPARQL query language over the SPARQL protocol. The generated representations are richly interlinked on RDF and XHTML level in order to enable browsers and crawlers to navigate database content. <s> BIB001 </s> Software Toolkits: Practical Aspects of the Internet of Things—A Survey <s> Data Storage via RDF <s> The term “Linked Data” refers to a set of best practices for publishing and connecting structured data on the Web. These best practices have been adopted by an increasing number of data providers over the last three years, leading to the creation of a global data space containing billions of assertions— the Web of Data. In this article, the authors present the concept and technical principles of Linked Data, and situate these within the broader context of related technological developments. They describe progress to date in publishing Linked Data on the Web, review applications that have been developed to exploit the Web of Data, and map out a research agenda for the Linked Data community as it moves forward. <s> BIB002
In the IoT, as discussed above, the semantic vision is that the semantic meanings of objects are stored separately and effective tools exist for managing this information. As a key to the IoT, these data should be reconstructed using the RDF  model and published as Linked Data BIB002 on the web. Information about resources is represented using the RDF, which provides a data model that is very simple but strictly tailored toward the architecture of the web. In RDF, a description of a resource is represented as a number of triples. The three parts of each triple are its subject, predicate, and object. RDF data can be accessed by HTTP URIs. The main benefits of using the RDF data model in a Linked Data context are as follows: (e) combined with schema languages such as RDF-S or OWL, the data model allows users to use as much or as little structure as they need; that is, users can represent tightly structured and semistructured data. There are data management and storage systems that can store raw sensing data and its corresponding semantics data, managing the relations between the two via a mapping. The D2RQ Platform  BIB001 is a system for accessing relational databases as virtual read-only RDF graphs. It offers RDF-based access to the content of relational databases without having to replicate it in an RDF store. Database content is mapped to RDF by a declarative mapping that specifies how resources are identified and property values are generated from database content. Benefits of D2RQ are as follows: (a) it queries a non-RDF database using SPARQL; (b) it accesses the content of the database as Linked Data over the web; (c) it creates custom dumps of the database in RDF formats for loading into an RDF store; (d) it accesses information in a non-RDF database using the Apache Jena API; (e) it has rich interlink RDF and XHTML to enable browsers and crawlers to navigate database content. As shown in Figure 3 , the D2RQ Platform is composed of the following components: (a) the D2RQ Mapping Language, a declarative mapping language for describing relations between an ontology and a relational data model; (b) the D2RQ Engine, a plug-in for the Jena Semantic Web toolkit, which uses the mappings to rewrite Jena API calls to SQL queries against the database and then passes query results to the higher layers of the framework; (c) the D2R Server, a type of HTTP server that provides a Linked Data view, an HTML view for debugging, and a SPARQL Protocol endpoint over the database. Supported databases currently include Oracle, MySQL, PostgreSQL, SQLServer, HSQLDB, and Interbase/Firebird. There are other data storage systems worth reviewing. In  , David et al. provide an overview of available triple stores, their technical implementations, support for the SPARQL W3C recommendations, and available APIs.