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3.3 REST Integration of User Interfaces

Many of the components come with their own user interface. For example, the Silk Workbench is a user interface for the Silk linking engine. This workbench supports the creation of linking specifications, executing them and improving them using the feedback from the user on the created links. With the OntoWiki linked data browsing and authoring tool, a user can browse and update information in a knowledge base. By using both tools together, the user gains the ability to study the input sources' content structure and to create links between them.

Many stack components request similar information from the user. For example, selecting the graph of interest. To provide the end-user the feeling of a harmonized single application, we develop supportive REST-based WebAPIs. These APIs offer a common application view of the LOD2 Stack. The more tools support this API, the more harmonized and integrated the end-user experience gets. Currently, the LOD2 Stack WebAPI consists of:

• Graph management : The set of graphs is not easy to maintain. SPARQL does not support retrieval of all graphs. The only possible query which selects all graphs that have at least one triple is performance wise quite costly: SELECT DISTINCT ?g WHERE GRAPH ?g ?s ?p ?o The WebAPI also standardizes some meta information like being a system graph. When LOD2 Stack components use this common graph management WebAPI, the end-user obtains a uniform look-and-feel with respect to graph management.

• Prefix management : To make RDF resources more readable, prefixes are used to abbreviate URI namespaces. Typically, each application manages its own namespace mapping. Using this REST API, a central namespace mapping is maintained, thus producing consistency among stack components. The enduser is freed from updating the individual component mappings. Moreover, an update in one component is immediately available to another.

In addition to creating supportive REST-based APIs, the LOD2 Stack encourages component owners to open up their components using REST based WebAPIs. For example, the semantic-spatial browser, a UI tool that visualizes RDF data containing geospatial information on a map, is entirely configurable by parameters encoded within its invocation URL. Similarly other visualization and exploration widgets (such as the CubeViz statistical data visualization) can directly interact with the SPARQL endpoint (cf. Fig. 7). This makes it easy to integrate into (third party) applications into the stack.

 
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