RDF::LinkedData - A Linked Data server implementation

DESCRIPTION

This module is used to create a Linked Data server that can
serve RDF data out of an RDF::Trine::Model. It will look up URIs in
the model and do the right thing (known as the 303 dance) and mint
URLs for that, as well as content negotiation. Thus, you can
concentrate on URIs for your things, you need not be concerned about
minting URLs for the pages to serve it. In addition, optional modules
can provide other important functionality: Cross-origin resource
sharing, VoID description, cache headers, SPARQL Endpoint, Triple
Pattern Fragments, etc. As such, it encompasses a fair share of
Semantic Web best practices, but possibly not in a very flexible Big
Data manner.

INSTALLATION

On Debian and derivatives, such as Ubuntu, this module can be
installed with all its dependencies using

  apt-get install librdf-linkeddata-perl

as root or using sudo. 

To install the most recent module, it is likely that you already have
the cpan tool installed. Then just run it on the command line. If you
don't have it, see http://www.cpan.org/modules/INSTALL.html

Then, in the cpan tool, type

install RDF::LinkedData

The relevant scripts and modules will be install to different paths
depending on your system. To use it, you need to find the script
linked_data.psgi, e.g. using locate.

CONFIGURATION

*Quick setup for a demo*
 
One-liner
 
It is possible to make it run with a single command line, e.g.:
 
  PERLRDF_STORE="Memory;path/to/some/data.ttl" plackup -host localhost script/linked_data.psgi
 
This will start a server with the default config on localhost on port
5000, so the URIs you're going serve from the file data.ttl will have
to have a base URI http://localhost:5000/.
 
Using perlrdf command line tool
 
A slightly longer example requires App::perlrdf, but sets up a
persistent SQLite-based triple store, parses a file and gets the
server with the default config running:
 
  export PERLRDF_STORE="DBI;mymodel;DBI:SQLite:database=rdf.db"
  perlrdf make_store
  perlrdf store_load path/to/some/data.ttl
  plackup -host localhost script/linked_data.psgi
 
*Configuration*
 
To configure the system for production use, create a configuration
file rdf_linkeddata.json that looks something like:
 
  {
        "base_uri"  : "http://localhost:5000/",
        "store" : {
                   "storetype"  : "Memory",
                   "sources" : [ {
                                "file" : "/path/to/your/data.ttl",
                                "syntax" : "turtle"
                               } ]
 
                   },
        "endpoint": {
                "html": {
                         "resource_links": true
                        }
                    },
        "cors": {
                  "origins": "*"
                },
        "void": {
                  "pagetitle": "VoID Description for my dataset"
                },
	      "expires" : "A86400" ,
        "fragments" : { 
                "fragments_path" : "/fragments" ,
                "allow_dump_dataset" : 0
        }	
  }
 
In your shell set
 
  export RDF_LINKEDDATA_CONFIG=/to/where/you/put/rdf_linkeddata.json
 
If the linked_data.psgi script was installed in /usr/local/bin, go:
 
  plackup /usr/local/bin/linked_data.psgi --host localhost --port 5000
 
The endpoint-part of the config sets up a SPARQL Endpoint. This requires
the RDF::Endpoint module, which is recommended by this module. To
use it, it needs to have some config, but will use defaults.
 
It is also possible to set an expires time. This needs
Plack::Middleware::Expires and uses Apache mod_expires syntax, in the
example above, it will set an expires header for all resources to
expire after 1 day of access. It is strongly recommended that this is
used, as it can potentially speed up access to resources that aren't
accessed frequently considerably, and take load off your server.

The cors-part of the config enables Cross-Origin Resource
Sharing, which is a W3C Recommendation for relaxing security
constraints to allow data to be shared across domains. In most cases,
this is what you want when you are serving open data, but in some
cases, notably intranets, this should be turned off by removing this
part.

The void-part generates some statistics and a description of the
dataset, using RDF::Generator::Void. It is strongly recommended to
install and run that, but it can take some time to generate, so you
may have to set the detail level.

Finally, fragments add support for Triple Pattern Fragments, a
work-in-progress, initiated by http://linkeddatafragments.org/ 
It is a more lightweight but less powerful way to query RDF data than
SPARQL. If you have this, it is recommended to have CORS enabled and
required to have at least a minimal VoID setup.

It is also worth noting that an environment variable LOG_ADAPTER can
be set to send log statements to the console. I recommend installing
Log::Any::Adapter::Screen, which can be used by setting this variable
like LOG_ADAPTER=Screen. Its documentation details more environment
variables that can be used to control log levels, etc.

This module now also contains some facilities to support read-write
hypermedia RDF, as well as the possibility to subclass RDF::LinkedData
itself. The read-write functionality is provided by a separate module,
RDF::LinkedData::RWHypermedia, but is highly experimental at this
point.

*Production server setup*

In addition to the configuration above, a production system should set
up a real Web server to run the Plack script. There are many ways to
do this (as Plack provides an elegant separation of concerns between
developers and system administrators). 

To set this up under Apache, put this in the host configuration:

  <Location />
    SetHandler perl-script
    PerlResponseHandler Plack::Handler::Apache2
    SetEnv RDF_LINKEDDATA_CONFIG /to/where/you/put/rdf_linkeddata.json
    PerlSetVar psgi_app /usr/local/bin/linked_data.psgi
  </Location>

  <Perl>
    use Plack::Handler::Apache2;
    $ENV{RDF_LINKEDDATA_CONFIG}='/to/where/you/put/rdf_linkeddata.json';
    Plack::Handler::Apache2->preload("/usr/local/bin/linked_data.psgi");
  </Perl>

  <Location ~ "^/(dumps|js/|css/|favicon.ico)">
    SetHandler default-handler
  </Location>

Note that in some environments, for example if the Plack server
is dynamically configured and/or behind a proxy server, the server
may fail to bind to the address you give it as hostname. In this case,
it is wise to allow the server to bind to any public IP address,
i.e. set the host name to 0.0.0.0.



AUTHOR
    Kjetil Kjernsmo, "<[email protected]>"

FUTURE

As the primary author of this module is now working on the Solid
Project, see https://solidproject.org/ , he isn't very interested in
further development of this module, as a Solid server would supersede
this module and allow much more advanced use cases.

BUGS
    Please report any bugs using github
    <https://github.com/perlrdf/RDF-LinkedData/issues>

SUPPORT
    You can find documentation for this module with the perldoc command.

        perldoc RDF::LinkedData

    The perlrdf IRC channel is the right place to seek help and discuss this module:

    irc://irc.perl.org/#perlrdf

ACKNOWLEDGEMENTS
    This module was started by Gregory Todd Williams "<[email protected]>"
    for RDF::LinkedData::Apache, but has been almost totally rewritten.

COPYRIGHT & LICENSE
    Copyright 2010 Gregory Todd Williams

    Copyright 2010 ABC Startsiden AS

    Copyright 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018 Kjetil Kjernsmo

    This program is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.