choose - choose a variant of a document to serve
choose - choose a variant of a document to serve (HTTP content negotiation)
# ID QS Content-Type Encoding Char-Set Lang Size
[['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],
@prefered = choose($variants, $request_headers);
$the_one = choose($variants);
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
The first parameter is reference to an array of the variants to
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 pairs. 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], ['var2', 0.3], ['var3' => 0]);
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().
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
This is a string that you use as the name for the variant. This
identifier for the preferred variants returned by choose().
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
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''.
This is the media type of the variant. The media type does not
include a charset attribute, but might contain other parameters.
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:
This is the character set used when the variant contains text.
The charset value should generally be
undef or one of these:
iso-8859-1 ... iso-8859-9
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-1766. Examples
en International English
en-US US English
This is the number of bytes used to represent the content.
The following Accept* headers can be used for describing content
preferences in a request (This description is an edited extract from
This header can be used to indicate a list of media ranges which are
acceptable as a reponse 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.
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''
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
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
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 1996, Gisle Aas.
This library is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.
Gisle Aas <firstname.lastname@example.org>
choose - choose a variant of a document to serve