NAME

       HTTP::LRDD - link-based resource descriptor discovery

SYNOPSIS

        use HTTP::LRDD;

        my $lrdd        = HTTP::LRDD->new;
        my @descriptors = $lrdd->discover($resource);
        foreach my $descriptor (@descriptors)
        {
          my $description = $lrdd->parse($descriptor);
          # $description is an RDF::Trine::Model
        }

DESCRIPTION

       Note: the LRDD specification has ceased to be, with some parts being merged into the host-
       meta Internet Draft. This CPAN module will go in its own direction, bundling up best-
       practice techniques for discovering links and descriptors for a given URI.

   Import Routine
       "use HTTP::LRDD (@predicates)"
           When importing HTTP::LRDD, you can optionally provide a list of predicate URIs (i.e.
           the URIs which rel values expand to). This may also include IANA-registered link
           types, which are short tokens rather than full URIs. If you do not provide a list of
           predicate URIs, then a sensible default set is used.

           Because this configuration is global in nature, it is not recommended.  It is better
           to supply a list of predicates to the constructor instead, or rely on the defaults.
           This feature should be regarded as deprecated.

   Constructors
       "HTTP::LRDD->new(@predicates)"
           Create a new LRDD discovery object using the given predicate URIs.  If @predicates is
           omitted, then the predicates passed to the import routine are used instead.

       "HTTP::LRDD->new_strict"
           Create a new LRDD discovery object using the 'describedby' and 'lrdd' IANA-registered
           predicates.

       "HTTP::LRDD->new_default"
           Create a new LRDD discovery object using the default set of predicates ('describedby',
           'lrdd', 'wdrs:describedby', 'xhv:meta' and 'rdfs:seeAlso').

   Public Methods
       "$lrdd->discover($resource_uri)"
           Discovers a descriptor for the given resource; or if called in a list context, a list
           of descriptors.

           A descriptor is a resource that provides a description for something.  So, if the
           given resource URI was the web address for an image, then the descriptor might be the
           web address for a metadata file about the image. If the given URI was an e-mail
           address, then the descriptor might be a profile document for the person to whom the
           address belongs.

           The following sources are checked (in order) to find links to descriptors.

           •   HTTP response headers ("Link" header; "303 See Other" status)

           •   HTTP response message (RDF or RDFa)

           •   https://HOSTNAME/.well-known/host-meta

           •   http://HOSTNAME/.well-known/host-meta

           If none of the above is able to yield a link to a descriptor, then the resource URI
           itself may be returned if it is in a self-describing format (e.g. RDF).

           There is no guaranteed file format for the descriptor, but it is usually RDF, POWDER
           XML or XRD.

           This method can also be called without an object (as a class method) in which case, a
           temporary object is created automatically using "new".

       "$lrdd->parse($descriptor_uri)"
           Parses a descriptor in XRD or RDF (RDF/XML, RDFa, Turtle, etc).

           Returns an RDF::Trine::Model or undef if unable to process.

           This method can also be called without an object (as a class method) in which case, a
           temporary object is created automatically using "new".

       "$lrdd->process($resource_uri)"
           Performs the equivalent of "discover" and "parse" in one easy step.

           Calls "discover" in a non-list context, so only the first descriptor is used.

       "$lrdd->process_all($resource_uri)"
           Performs the equivalent of "discover" and "parse" in one easy step.

           Calls "discover" in a list context, so multiple descriptors are combined into the
           resulting graph.

EXAMPLES

       Discover the hub address (PubSubHubub) for a feed:

        my $lrdd = HTTP::LRDD->new('hub');
        my $hub  = $lrdd->discover('http://example.net/feed.atom');

       Discover an author link (rel="author") from an HTML page:

        my $lrdd   = HTTP::LRDD->new('author');
        my $author = $lrdd->discover('http://example.com/page.html');

       (For RDF people, you should note that rel="author" is not semantically equivalent to the
       "foaf:maker" property but closer to the "foaf:maker/foaf:homepage" SPARQL 1.1 property
       path - i.e. the rel="author" link destination is not a URI for the author themselves, but
       a page about the author.)

       If that author resource is in a machine-readable format (e.g. RDF), then parse the data:

        my $author_data = $lrdd->parse($author);

       Or, you can combine "discover" and "parse":

        my $lrdd        = HTTP::LRDD->new('author');
        my $author_data = $lrdd->process('http://example.com/page.html');

       Get metadata for an image:

        my $lrdd = HTTP::LRDD->new;
        my $data = $lrdd->process_all('http://example.org/flower.jpeg');

       As we're not passing any arguments to the constructor, we can use a shortcut:

        my $data = HTTP::LRDD->process_all('http://example.org/flower.jpeg');

       Find the title of the image:

        use RDF::QueryX::Lazy;

        my $image   = q<http://example.org/flower.jpeg>;
        my $results = RDF::QueryX::Lazy
          -> new("SELECT * WHERE { <$image> dc:title ?t }")
          -> execute( HTTP::LRDD->process_all($image) );

BUGS

       Please report any bugs to <http://rt.cpan.org/>.

       Note: many problems can stem from servers that send incorrect "Content-Type" headers. If
       you send an XRD file as "text/html", then this module will not guess what you're doing -
       it will assume the file is really HTML, and inspect it for RDFa. For host-meta files, this
       module is slightly more relaxed, as there's a strong assumption that they are XRD... but
       YOU SHOULD NOT RELY ON THIS.  If you're running a server, use the correct media type.

SEE ALSO

       HTTP::Link::Parser, XRD::Parser, XML::Atom::OWL WWW::Finger.

       <http://www.perlrdf.org/>.

AUTHOR

       Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE

       Copyright 2010-2012 Toby Inkster

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

DISCLAIMER OF WARRANTIES

       THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING,
       WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR
       PURPOSE.