\section{Components}

With various ULO/RDF files in place we have the aim of making the
underlying data available for use with applications. For this, we
should first make out the various components that might be involved in
such a system. As a guide, figure~\ref{fig:components} illustrates the
various components and their interplay.  We will now give an overview
over all involved components. Each component will later be discussed
in more detail, this section serves only for the reader to get a
general understanding of the developed infrastructure and its
topology.


\begin{figure}[]\begin{center}
    \includegraphics{figs/components}
    \caption{Components involved in the \emph{ulo-storage} system.}\label{fig:components}
\end{center}\end{figure}

\begin{itemize}
\item ULO/RDF data is present on various locations, be it Git
  repositories, available on web servers via HTTP or on the local disk
  of a user. Regardless where this ULO/RDF data is stored, a
  \emph{Collecter} collects these {ULO/RDF}. In the easiest case, this
  involves cloning a Git repository or crawling a file system for
  matching files.

\item With streams of ULO/RDF files at the Collecter, this information
  then gets passed to the \emph{Importer}. The Importer imports
  triplets from files into some kind of permanent storage. For use in
  this project, the GraphDB~\cite{graphdb} triplet store was natural
  fit. In practice, both Collecter and Importer end up being one piece
  of software, but this does not have to be the case.

\item Finally, with all triplets stored in a database, an
  \emph{Endpoint} is where applications access the underlying
  knowledge base. This does not necessarily need to be any specific
  software, rather the programming API of the database could be
  understood as an endpoint of its own. However, some thought should
  be put into designing an Endpoint that is convenient to use.
\end{itemize}

Additionally, one could think of a \emph{Harvester} component. Before
we assumed that the ULO/RDF triplets are already available as
such. Indeed for this project this is the case as we worked on already
exported triplets from the Isabelle and Coq libraries. However, this
does not need to be the case. It might be desirable to automate the
export from third party formats to ULO/RDF and indeed this is what a
Harvester would do.  It fetches mathematical knowledge from some
remote source and then provides a volatile stream of ULO/RDF data to
the Collecter, which then passes it to the Importer and so on. The big
advantage of such an approach would be that exports from third party
libraries can always be up to date and do not have to be initiated
manually.
