oracular (3) Plack::App::RDF::LinkedData.3pm.gz
NAME
Plack::App::RDF::LinkedData - A Plack application for running RDF::LinkedData
VERSION
Version 1.940
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/".
There is also a "LOG_ADAPTER" that can be set to any of Log::Any::Adapter to send logging to the console.
If used with Log::Any::Adapter::Screen, several other environment variables can be used to further
control it.
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.
Read-write support
Some recent effort has gone into experimental write support, which for this module has the implications
that a boolean option "writes_enabled" that configures the application for writes. This is also meant as
security, unless set to true, writes will never be performed. To support writes, a "class" option can be
set with a class name, which can be instantiated to replace RDF::LinkedData. See
RDF::LinkedData::RWHypermedia for more on this.
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.
"auth_required ( $request )"
A method that returns true if the current request will require authorization.
"check_read_operation ( $request )"
A method that will return true if the current request is a pure read operation.
AUTHOR
Kjetil Kjernsmo, "<kjetilk@cpan.org>"
COPYRIGHT & LICENSE
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.