Provided by: librdf-linkeddata-perl_1.02-1_all 
NAME
Plack::App::RDF::LinkedData - A Plack application for running RDF::LinkedData
VERSION
Version 1.02
SYNOPSIS
my $linkeddata = Plack::App::RDF::LinkedData->new();
$linkeddata->configure($config);
my $rdf_linkeddata = $linkeddata->to_app;
builder {
enable "Head";
enable "ContentLength";
enable "ConditionalGET";
$rdf_linkeddata;
};
DESCRIPTION
This module sets up a basic Plack application to use RDF::LinkedData to serve Linked Data,
while making sure it does follow best practices for doing so. See the README for quick
start, the gory details are here.
MAKE IT RUN
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:3000/",
"store" : {
"storetype" : "Memory",
"sources" : [ {
"file" : "/path/to/your/data.ttl",
"syntax" : "turtle"
} ]
},
"endpoint": {
"html": {
"resource_links": true
}
},
"expires" : "A86400" ,
"cors": {
"origins": "*"
},
"void": {
"pagetitle": "VoID Description for my dataset"
},
"fragments" : { "fragments_path" : "/fragments" }
}
In your shell set
export RDF_LINKEDDATA_CONFIG=/to/where/you/put/rdf_linkeddata.json
Then, figure out where your install method installed the <linked_data.psgi>, script, e.g.
by using locate. If it was installed in "/usr/local/bin", go:
plackup /usr/local/bin/linked_data.psgi --host localhost --port 3000
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.
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, 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.
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.
Details of the implementation
This server is a minimal Plack-script that should be sufficient for most linked data
usages, and serve as a an example for most others.
A minimal example of the required config file is provided above. There is are longer
examples in the distribution, which is used to run tests. In the config file, there is a
"store" parameter, which must contain the RDF::Trine::Store config hashref. It may also
have a "base_uri" URI and a "namespace" hashref which may contain prefix - URI mappings to
be used in serializations. Certain namespace, namely RDF, VoID, Hydra, DC Terms and XML
Schema are added by the module and do not need to be declared.
Note that this is a server that can only serve URIs of hosts you control, it is not a
general purpose Linked Data manipulation tool, nor is it an implementation of Linked Data
Platform or the Linked Data API.
The configuration is done using Config::ZOMG and all its features can be used.
Importantly, you can set the "RDF_LINKEDDATA_CONFIG" environment variable to point to the
config file you want to use. See also Catalyst::Plugin::ConfigLoader for more information
on how to use this config system.
Behaviour
The following documentation is adapted from RDF::LinkedData::Apache, which preceded this
module.
• "http://host.name/rdf/example"
Will return an HTTP 303 redirect based on the value of the request's Accept header. If
the Accept header contains a recognized RDF media type or there is no Accept header,
the redirect will be to "http://host.name/rdf/example/data", otherwise to
"http://host.name/rdf/example/page". If the URI has a foaf:homepage or foaf:page
predicate, the redirect will in the latter case instead use the first encountered
object URI.
• "http://host.name/rdf/example/data"
Will return a bounded description of the "http://host.name/rdf/example" resource in an
RDF serialization based on the Accept header. If the Accept header does not contain a
recognized media type, RDF/XML will be returned.
• "http://host.name/rdf/example/page"
Will return an HTML description of the "http://host.name/rdf/example" resource
including RDFa markup, or, if the URI has a foaf:homepage or foaf:page predicate, a
301 redirect to that object.
If the RDF resource for which data is requested is not the subject of any RDF triples in
the underlying triplestore, the /page and /data redirects will not take place, and a HTTP
404 (Not Found) will be returned.
The HTML description of resources will be enhanced by having metadata about the predicate
of RDF triples loaded into the same triplestore. Currently, only a "rdfs:label"-predicate
will be used for a title, as in this version, generation of HTML is done by
RDF::RDFa::Generator.
Endpoint Usage
As stated earlier, this module can set up a SPARQL Endpoint for the data using
RDF::Endpoint. Often, that's what you want, but if you don't want your users to have that
kind of power, or you're worried it may overload your system, you may turn it off by
simply having no "endpoint" section in your config. To use it, you just need to have an
"endpoint" section with something in it, it doesn't really matter what, as it will use
defaults for everything that isn't set.
RDF::Endpoint is recommended by this module, but as it is optional, you may have to
install it separately. It has many configuration options, please see its documentation for
details.
You may also need to set the "RDF_ENDPOINT_SHAREDIR" variable to wherever the endpoint
shared files are installed to. These are some CSS and Javascript files that enhance the
user experience. They are not strictly necessary, but it sure makes it pretty!
RDF::Endpoint should do the right thing, though, so it shouldn't be necessary.
Finally, note that while RDF::Endpoint can serve these files for you, this module doesn't
help you do that. That's mostly because this author thinks you should serve them using
some other parts of the deployment stack. For example, to use Apache, put this in your
Apache config in the appropriate "VirtualHost" section:
Alias /js/ /path/to/share/www/js/
Alias /favicon.ico /path/to/share/www/favicon.ico
Alias /css/ /path/to/share/www/css/
VoID Generator Usage
Like a SPARQL Endpoint, this is something most users would want. In fact, it is an even
stronger recommendation than an endpoint. To enable it, you must have RDF::Generator::Void
installed, and just anything in the config file to enable it, like in the SYNOPSIS
example.
You can set several things in the config, the property attributes of RDF::Generator::Void
can all be set there somehow. You can also set "pagetitle", which sets the title for the
RDFa page that can be generated. Moreover, you can set titles in several languages for the
dataset using "titles" as the key, pointing to an arrayref with titles, where each title
is a two element arrayref, where the first element is the title itself and the second is
the language for that title.
Please refer to the RDF::Generator::Void for more details about what can be set, and the
"rdf_linkeddata_void.json" test config in the distribution for example.
By adding an "add_void" config key, you can make pass a file to the generator so that
arbitrary RDF can be added to the VoID description. It will check the last modification
time of the file and only update the VoID description if it has been modified. This is
useful since as much of the VoID description is expensive to compute. To use it, the
configuration would in JSON look something like this:
"add_void": { "file": "/data/add.ttl", "syntax": "turtle" }
where "file" is the full path to RDF that should be added and "syntax" is needed by the
parser to parse it.
Normally, the VoID description is cached in RAM and the store ETag is checked on every
request to see if the description must be regenerated. If you use the "add_void" feature,
you can force regeneration on the next request by touching the file.
FEEDBACK WANTED
Please contact the author if this documentation is unclear. It is really very simple to
get it running, so if it appears difficult, this documentation is most likely to blame.
METHODS
You would most likely not need to call these yourself, but rather use the
"linked_data.psgi" script supplied with the distribution.
"configure"
This is the only method you would call manually, as it can be used to pass a hashref
with configuration to the application.
"prepare_app"
Will be called by Plack to set the application up.
"call"
Will be called by Plack to process the request.
AUTHOR
Kjetil Kjernsmo, "<kjetilk@cpan.org>"
COPYRIGHT & LICENSE
Copyright 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017 Kjetil Kjernsmo
This program is free software; you can redistribute it and/or modify it under the same
terms as Perl itself.