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\subsection{library(semweb/rdf_cache): Cache RDF triples}

\label{sec:rdfcache}

The library \file{library(semweb/rdf_cache)} defines the caching strategy for
triples sources. When using large RDF sources, caching triples greatly
speedup loading RDF documents. The cache library implements two caching
strategies that are controlled by \predref{rdf_set_cache_options}{1}.

\textbf{Local caching} This approach applies to files only. Triples are
cached in a sub-directory of the directory holding the source. This
directory is called \verb$.cache$ (\verb$_cache$ on Windows). If the cache
option \verb$create_local_directory$ is \const{true}, a cache directory is created
if posible.

\textbf{Global caching} This approach applies to all sources, except for
unnamed streams. Triples are cached in directory defined by the cache
option \verb$global_directory$.

When loading an RDF file, the system scans the configured cache files
unless \verb$cache(false)$ is specified as option to \predref{rdf_load}{2} or caching is
disabled. If caching is enabled but no cache exists, the system will try
to create a cache file. First it will try to do this locally. On failure
it will try to configured global cache.\vspace{0.7cm}

\begin{description}
    \predicate{rdf_set_cache_options}{1}{+Options}
Change the cache policy. Provided options are:

\begin{itemize}
    \item \verb$enabled(Boolean)$
If \const{true}, caching is enabled.
    \item \verb$local_directory(Name)$.
Plain name of local directory. Default \verb$.cache$
(\verb$_cache$ on Windows).
    \item \verb$create_local_directory(Bool)$
If \const{true}, try to create local cache directories
    \item \verb$global_directory(Dir)$
Writeable directory for storing cached parsed files.
    \item \verb$create_global_directory(Bool)$
If \const{true}, try to create the global cache directory.
\end{itemize}

    \predicate[semidet]{rdf_cache_file}{3}{+URL, +ReadWrite, -File}
\arg{File} is the cache file for \arg{URL}. If \arg{ReadWrite} is \const{read}, it
returns the name of an existing file. If \const{write} it returns
where a new cache file can be overwritten or created.
\end{description}