\section{Components}\label{sec:components}

With ULO/RDF files in place and available for download on
MathHub~\cite{uloisabelle, ulocoq} as Git repositories, we have the
aim of making the underlying data available for use in applications.
It makes sense to first identify the various components that might
be involved in such a system. They are illustrated in
figure~\ref{fig:components}.

\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 data is present on various locations, be it Git
  repositories, available on web servers or on the local disk.
  Regardless where this ULO data is stored, it is the job of a
  \emph{Collecter} to assemble these {RDF}~files and forward them
  for further processing. This may involve cloning a Git repository or
  crawling a file system.

\item With streams of ULO files assembled by the Collecter, this
  data then gets passed to an \emph{Importer}. An Importer uploads
  received RDF~streams into some kind of permanent storage. For
  use in this project, the GraphDB~\cite{graphdb} triplet store was
  natural fit.

  In this project, both Collecter and Importer ended 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 interface of the underlying
  database itself could be understood as an endpoint of its
  own.

  Regardless, some thought can be put into designing an Endpoint as a
  layer that lives between application and database that is more
  convenient to use than the one provided by the database.
\end{itemize}

These are the components realized for \emph{ulo-storage}. Additionally
to these components, one could think of a \emph{Harvester} component.
We assumed that the ULO triplets are already available in RDF~format.
Indeed for this project this was the case as we work with already
exported triplets from the Isabelle and Coq libraries. However, this
is not necessarily true.

It might be desirable to automate the export from third party formats
to ULO and we think this is what a Harvester should do.  It fetches
mathematical knowledge from some remote source and then provides a
volatile stream of ULO 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. We did not implement a Harvester
for \emph{ulo-storage} but we suggest that it is an idea to keep in mind.

Each component we did implement (Collecter, Importer, Endpoint) will
now be discussed in a separate section. Here we only wanted to give
the reader a general understanding in the infrastructure that makes
up \emph{ulo-storage}.