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\section{Implementation}\label{sec:implementation}
One of the two contributions of \emph{ulo-storage} is that we
implemented components for making organizational mathematical
knowledge (formulated as RDF~triplets) queryable. This section first
makes out the individual required component for this tasks and then
describes some details of the actual implementation for this project.
\subsection{Components Implemented for \emph{ulo-storage}}\label{sec:components}
With RDF files exported and available for download as Git
repositories~\cite{uloisabelle, ulocoq}, we have the goal of making
the underlying data available for use in applications. Let us first
look at a high level overview of all involved
components. Figure~\ref{fig:components} illustrates each component and
the flow of data.
\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 triplets are present in various locations, be it Git
repositories, on web servers or the local disk. 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
the file system.
\item With streams of ULO files assembled by the Collecter, this
data then gets passed to an \emph{Importer}. An Importer uploads
RDF~streams into some kind of permanent storage. As we will see,
the GraphDB~\cite{graphdb} triplet store was a natural fit.
\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 custom
software, rather the programming interface of the underlying
database itself can be understood as an endpoint of its own.
\end{itemize}
Collecter, Importer and Endpoint provide us with an easy and automated
way of making RDF files available for use within applications. We will
now take a look at the actual implementation created for
\emph{ulo-storage}, beginning with the implementation of Collecter and
Importer.
\subsection{Collecter and Importer}\label{sec:collecter}
We previously described Collecter and Importer as two distinct
components. The Collecter pulls RDF data from various sources as an
input and outputs a stream of standardized RDF data while the Importer
takes such a stream of RDF data and then dumps it to some sort of
persistent storage. In the implementation for \emph{ulo-storage},
both Collecter and Importer ended up being one piece of monolithic
software. This does not need to be the case but proved convenient
because (1)~combining Collecter and Importer forgoes the needs for an
additional IPC~mechanism and (2)~neither Collecter nor Importer are
terribly large pieces of software in themselves.
Our implementation supports two sources for RDF files, namely Git
repositories and the local file system. The file system Collecter
crawls a given directory on the local machine and looks for
RDF~XMl~files~\cite{rdfxml} while the Git Collecter first clones a Git
repository and then passes the checked out working copy to the file
system Collecter. Because it is not uncommon for RDF files to be
compressed, our Collecter supports on the fly extraction of
gzip~\cite{gzip} and xz~\cite{xz} formats which can greatly reduce the
required disk space in the collection step.
During development of the Collecter, we found that existing exports
from third party mathematical libraries contain RDF syntax errors
which were not discovered previously. In particular, both Isabelle and
Coq export contained URIs which do not fit the official syntax
specification~\cite{rfc3986} as they contained illegal
characters. Previous work~\cite{ulo} that processed Coq and Isabelle
exports used database software such as Virtuoso Open Source which do
not properly check URIs according to spec, in consequence these faults
were only discovered now. To tackle these problems, we introduced on
the fly correction steps during collection that take the broken RDF
files, fix the mentioned problems related to URIs (by escaping illegal
characters) and then continue processing. Of course this is only a
work-around. Related bug reports were filed in the respective export
projects to ensure that in the future this extra step is not
necessary.
Our Collecter takes existing RDF files, applies some on the fly
transformations (extraction of compressed files, fixing of errors),
the result is a stream of RDF data. This stream gets passed to the
Importer which imports the encoded RDF triplets into some kind of
persistent storage. The canonical choice for this task is to use a
triple store, that is a database optimized for storing RDF
triplets~\cite{triponto, tripw3c}. For our project, we used the
GraphDB~\cite{graphdb} triple store as it is easy to use an a free
version that fits our needs is available~\cite{graphdbfree}. The
import itself is straight-forward, our software only needs to upload
the RDF file stream as-is to an HTTP endpoint provided by our GraphDB
instance.
\emph{({TODO}: Write down a small comparison of different database
types, triplet stores and implementations. Honestly the main
advantage of GraphDB is that it's easy to set up and import to;
maybe I'll also write an Importer for another DB to show that the
choice of database is not that important.)}
\subsubsection{Scheduling and Version Management}
Collecter and Importer were implemented as library code that can be
called from various front ends. For this project, we provide both a
command line interface as well as a graphical web front end. While the
command line interface is only useful for manually starting single
jobs, the web interface allows scheduling of jobs. In particular, it
allows the user to automate import jobs. For example, it is possible
to schedule an import of a given Git repository every seven days to a
given GraphDB instance.
Automated job control that regularly imports data from the same
sources leads us to the problem of versioning. ULO
exports~$\mathcal{E}$ depend on an original third party
library~$\mathcal{L}$. Running~$\mathcal{E}$ through the workflow of
Collecter and Importer, we get some database
representation~$\mathcal{D}$. We see that data flows
\begin{align*}
\mathcal{L} \rightarrow \mathcal{E} \rightarrow \mathcal{D}
\end{align*}
which means that if records in~$\mathcal{L}$ change, this will
probably result in different triplets~$\mathcal{E}$ which in turn
results in a need to update~$\mathcal{D}$. This is difficult. As it
stands, \emph{ulo-storage} only knows about what is in~$\mathcal{E}$.
While it should be possible to find out the difference between a new
version of~$\mathcal{E}$ and the current version of~$\mathcal{D}$ and
compute the changes necessary to be applied to~$\mathcal{D}$, the big
number of triplets makes this appear unfeasible. So far, our only
suggestion to solve the problem of changing third party libraries is
to regularly re-create the full data set~$\mathcal{D}$ from scratch,
say every seven days. This circumvents all problems related to
updating existing data sets, but it does mean additional computation
requirements. It also means that changes in~$\mathcal{L}$ takes some
to propagate to~$\mathcal{D}$. If the number of triplets raises
by orders of magnitude, this approach will eventually not be scalable
anymore.
\subsection{Endpoints}\label{sec:endpoints}
With ULO triplets imported into the GraphDB triplet store by Collecter
and Importer, we now have all data available necessary for querying.
As discussed before, querying from applications happens through an
Endpoint that exposes some kind of {API}. The interesting question
here is probably not so much the implementation of the endpoint itself,
rather it is the choice of API than can make or break such a project.
There are multiple approaches to querying the GraphDB triplet store,
one based around the standardized SPARQL query language and the other
on the RDF4J Java library. Both approaches have unique advantages.
\begin{itemize}
\item SPARQL is a standardized query language for RDF triplet
data~\cite{sparql}. The specification includes not just syntax
and semantics of the language itself, but also a standardized
REST interface~\cite{rest} for querying database servers.
\textbf{Syntax} SPARQL is inspired by SQL and as such the
\texttt{SELECT} \texttt{WHERE} syntax should be familiar to many
software developers. A simple query that returns all triplets
in the store looks like
\begin{lstlisting}
SELECT * WHERE { ?s ?p ?o }
\end{lstlisting}
where \texttt{?s}, \texttt{?p} and \texttt{?o} are query
variables. The result of any query are valid substitutions for
the query variables. In this particular case, the database would
return a table of all triplets in the store sorted by
subject~\texttt{?o}, predicate~\texttt{?p} and
object~\texttt{?o}.
\textbf{Advantage} Probably the biggest advantage is that SPARQL
is ubiquitous. As it is the de facto standard for querying
triplet stores, lots of implementations and documentation are
available~\cite{sparqlbook, sparqlimpls, gosparql}.
\item RDF4J is a Java API for interacting with triplet stores,
implemented based on a superset of the {SPARQL} REST
interface~\cite{rdf4j}. GraphDB is one of the database
servers that supports RDF4J, in fact it is the recommended way
of interacting with GraphDB repositories~\cite{graphdbapi}.
\textbf{Syntax} Instead of formulating textual queries, RDF4J
allows developers to query a repository by calling Java API
methods. Previous query that requests all triplets in the store
looks like
\begin{lstlisting}
connection.getStatements(null, null, null);
\end{lstlisting}
in RDF4J. \texttt{getStatements(s, p, o)} returns all triplets
that have matching subject~\texttt{s}, predicate~\texttt{p} and
object~\texttt{o}. Any argument that is \texttt{null} can be
replace with any value, i.e.\ it is a query variable to be
filled by the call to \texttt{getStatements}.
\textbf{Advantage} Using RDF4J does introduce a dependency on
the JVM and its languages. But in practice, we found RDF4J to be
quite convenient, especially for simple queries, as it allows us
to formulate everything in a single programming language rather
than mixing programming language with awkward query strings.
We also found it quite helpful to generate Java classes from
OWL~ontologies that contain all definitions of the
ontology~\cite{rdf4jgen}. This provides us with powerful IDE
auto completion features during development of ULO applications.
We see that both SPARQL and RDF4J have unique advantages. While SPARQL
is an official W3C standard and implemented by more database systems,
RDF4J can be more convenient when dealing with JVM-based code bases.
For \emph{ulo-storage}, we played around with both interfaces and
chose whatever seemed more convenient at the moment. We recommend any
implementors to do the same.
\subsection{Deployment and Availability}
\def\gorepo{https://gitlab.cs.fau.de/kissen/ulo-storage-collect}
\def\composerepo{https://gl.kwarc.info/supervision/schaertl_andreas/-/tree/master/experimental/compose}
Software not only needs to get developed, but also deployed. To deploy
the combination of Collecter, Importer and Endpoint, we provide a
single Docker Compose file which starts three containers, namely
(1)~the Collecter/Importer web interface, (2)~a database server for
that web interface such that it can persist import jobs and finally
(3)~a GraphDB instance which provides us with the required
Endpoint. All code for Collecter and Importer is available in the
\texttt{ulo-storage-collect} Git repository\footnote{\url{\gorepo}}
Additional deployment files, that is Docker Compose and additional
Dockerfiles are stored in a separate
repository\footnote{\url{\composerepo}}.
This concludes our discussion of the implementation developed for the
\emph{ulo-storage} project. We designed a system based around (1)~a
Collecter which collects RDF triplets from third party sources, (2)~an
Importer which imports these triplets into a GraphDB database and
(3)~looked at different ways of querying a GraphDB Endpoint. All of
this is easy to deploy using a single Docker Compose file. With this
stack ready for use, we will continue with a look at some interesting
applications and queries built on top of this interface.