The LOD2 Linked Data Stack
The LOD2 Linked Data stack is a distribution platform for software components which support one or more aspects of the Linked Data life cycle. Each package contains a pre-conﬁgured component that on installation results in a ready-touse application. The pre-conﬁguration ensures that the deployed components are able to interact with each other. The system architecture of the deployed Linked data stack components is explained in Sect. 2.1. The subsequent sections provide more details on the distribution platform and what the requirements are for software to take part of it. Finally we provide an overview of the LOD2 stack contents in Sect. 2.5.
Building a Linked Data Application
The LOD2 stack facilitates the creation of Linked Data applications according to a prototypical system architecture. The system architecture is shown below in Fig. 1. From top to bottom, one has ﬁrst the application layer with which the end-user is confronted. The applications are built with components from the component layer. These components communicate between each other and interact with the data in the data layer via the common data access layer.
The data access layer is build around the data representation framework RDF. The data is exchanged in RDF and retrieved with SPARQL queries from SPARQL end-points. All data format heterogeneity is hidden for the components by this layer. This yields an uniform data view easing the conﬁguration of the data ﬂow between the components. The RDF representation yields important
Fig. 1. Linked Data application system architecture
advantages making it suited for the role as common data representation formalism. It is a W3C standard, domain neutral, and it is web enabled: all identiﬁers are web addressable. And last but not least, data integration starts with just merging the data together in one store.
Matching the Linked Data life cycle presented in the introduction to the system architecture shows that the extraction and storage tools feature in the data layer and most of the other are part of the component layer.
An application end user will seldom be directly in touch with the underlying data layer. They are oﬀered an application interface that shows the information in a domain adapted intuitive interface. Few of the LOD2 stack browsing and exploration components have been lifted and designed to this stage. Most of the stack components are designed for the (Linked Data) data manager. The components provide user interfaces that aid the data manager in its task. For instance, the SILK workbench is a user interface for creating linking speciﬁcations. This speciﬁcation can then be used by the silk engine which might be embedded in a larger application. That is the task of the last targeted audience: the Linked Data application developer.