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NAME
HTTP::Negotiate - choose a variant to serve
SYNOPSIS
use HTTP::Negotiate qw(choose);
# ID QS Content-Type Encoding Char-Set Lang Size
$variants =
[['var1', 1.000, 'text/html', undef, 'iso-8859-1', 'en', 3000],
['var2', 0.950, 'text/plain', 'gzip', 'us-ascii', 'no', 400],
['var3', 0.3, 'image/gif', undef, undef, undef, 43555],
];
@preferred = choose($variants, $request_headers);
$the_one = choose($variants);
DESCRIPTION
This module provides a complete implementation of the HTTP content
negotiation algorithm specified in draft-ietf-http-v11-spec-00.ps
chapter 12. Content negotiation allows for the selection of a preferred
content representation based upon attributes of the negotiable variants
and the value of the various Accept* header fields in the request.
The variants are ordered by preference by calling the function choose().
The first parameter is reference to an array of the variants to choose
among. Each element in this array is an array with the values [$id, $qs,
$content_type, $content_encoding, $charset, $content_language,
$content_length] whose meanings are described below. The
$content_encoding and $content_language can be either a single scalar
value or an array reference if there are several values.
The second optional parameter is either a HTTP::Headers or a
HTTP::Request object which is searched for "Accept*" headers. If this
parameter is missing, then the accept specification is initialized from
the CGI environment variables HTTP_ACCEPT, HTTP_ACCEPT_CHARSET,
HTTP_ACCEPT_ENCODING and HTTP_ACCEPT_LANGUAGE.
In an array context, choose() returns a list of [variant identifier,
calculated quality, size] tuples. The values are sorted by quality,
highest quality first. If the calculated quality is the same for two
variants, then they are sorted by size (smallest first). *E.g.*:
(['var1', 1, 2000], ['var2', 0.3, 512], ['var3', 0.3, 1024]);
Note that also zero quality variants are included in the return list
even if these should never be served to the client.
In a scalar context, it returns the identifier of the variant with the
highest score or `undef' if none have non-zero quality.
If the $HTTP::Negotiate::DEBUG variable is set to TRUE, then a lot of
noise is generated on STDOUT during evaluation of choose().
VARIANTS
A variant is described by a list of the following values. If the
attribute does not make sense or is unknown for a variant, then use
`undef' instead.
identifier
This is a string that you use as the name for the variant. This
identifier for the preferred variants returned by choose().
qs This is a number between 0.000 and 1.000 that describes the "source
quality". This is what draft-ietf-http-v11-spec-00.ps says about this
value:
Source quality is measured by the content provider as representing
the amount of degradation from the original source. For example, a
picture in JPEG form would have a lower qs when translated to the XBM
format, and much lower qs when translated to an ASCII-art
representation. Note, however, that this is a function of the source
- an original piece of ASCII-art may degrade in quality if it is
captured in JPEG form. The qs values should be assigned to each
variant by the content provider; if no qs value has been assigned,
the default is generally "qs=1".
content-type
This is the media type of the variant. The media type does not
include a charset attribute, but might contain other parameters.
Examples are:
text/html
text/html;version=2.0
text/plain
image/gif
image/jpg
content-encoding
This is one or more content encodings that has been applied to the
variant. The content encoding is generally used as a modifier to the
content media type. The most common content encodings are:
gzip
compress
content-charset
This is the character set used when the variant contains text. The
charset value should generally be `undef' or one of these:
us-ascii
iso-8859-1 ... iso-8859-9
iso-2022-jp
iso-2022-jp-2
iso-2022-kr
unicode-1-1
unicode-1-1-utf-7
unicode-1-1-utf-8
content-language
This describes one or more languages that are used in the variant.
Language is described like this in draft-ietf-http-v11-spec-00.ps: A
language is in this context a natural language spoken, written, or
otherwise conveyed by human beings for communication of information
to other human beings. Computer languages are explicitly excluded.
The language tags are defined by RFC 3066. Examples are:
no Norwegian
en International English
en-US US English
en-cockney
content-length
This is the number of bytes used to represent the content.
ACCEPT HEADERS
The following Accept* headers can be used for describing content
preferences in a request (This description is an edited extract from
draft-ietf-http-v11-spec-00.ps):
Accept
This header can be used to indicate a list of media ranges which are
acceptable as a response to the request. The "*" character is used to
group media types into ranges, with "*/*" indicating all media types
and "type/*" indicating all subtypes of that type.
The parameter q is used to indicate the quality factor, which
represents the user's preference for that range of media types. The
parameter mbx gives the maximum acceptable size of the response
content. The default values are: q=1 and mbx=infinity. If no Accept
header is present, then the client accepts all media types with q=1.
For example:
Accept: audio/*;q=0.2;mbx=200000, audio/basic
would mean: "I prefer audio/basic (of any size), but send me any
audio type if it is the best available after an 80% mark-down in
quality and its size is less than 200000 bytes"
Accept-Charset
Used to indicate what character sets are acceptable for the response.
The "us-ascii" character set is assumed to be acceptable for all user
agents. If no Accept-Charset field is given, the default is that any
charset is acceptable. Example:
Accept-Charset: iso-8859-1, unicode-1-1
Accept-Encoding
Restricts the Content-Encoding values which are acceptable in the
response. If no Accept-Encoding field is present, the server may
assume that the client will accept any content encoding. An empty
Accept-Encoding means that no content encoding is acceptable.
Example:
Accept-Encoding: compress, gzip
Accept-Language
This field is similar to Accept, but restricts the set of natural
languages that are preferred in a response. Each language may be
given an associated quality value which represents an estimate of the
user's comprehension of that language. For example:
Accept-Language: no, en-gb;q=0.8, de;q=0.55
would mean: "I prefer Norwegian, but will accept British English
(with 80% comprehension) or German (with 55% comprehension).
COPYRIGHT
Copyright 1996,2001 Gisle Aas.
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
AUTHOR
Gisle Aas <gisle@aas.no>