Apache Module mod_negotiation
Summary
Content negotiation, or more accurately content selection, is
the selection of the document that best matches the clients
capabilities, from one of several available documents. There
are two implementations of this.
- A type map (a file with the handler
type-map) which explicitly lists the files
containing the variants.
- A MultiViews search (enabled by the
MultiViews
Options), where the server does
an implicit filename pattern match, and choose from amongst the
results.
Directives
Topics
See also
A type map has a format similar to RFC822 mail headers. It
contains document descriptions separated by blank lines, with
lines beginning with a hash character ('#') treated as
comments. A document description consists of several header
records; records may be continued on multiple lines if the
continuation lines start with spaces. The leading space will be
deleted and the lines concatenated. A header record consists of
a keyword name, which always ends in a colon, followed by a
value. Whitespace is allowed between the header name and value,
and between the tokens of value. The headers allowed are:
Content-Encoding:- The encoding of the file. Apache only recognizes
encodings that are defined by an
AddEncoding directive.
This normally includes the encodings x-compress
for compress'd files, and x-gzip for gzip'd
files. The x- prefix is ignored for encoding
comparisons.
Content-Language:- The language(s) of the variant, as an Internet standard
language tag (RFC 1766). An example is
en,
meaning English. If the variant contains more than one
language, they are separated by a comma.
Content-Length:- The length of the file, in bytes. If this header is not
present, then the actual length of the file is used.
Content-Type:-
The MIME media type of the document, with optional
parameters. Parameters are separated from the media type
and from one another by a semi-colon, with a syntax of
name=value. Common parameters include:
level- an integer specifying the version of the media type.
For
text/html this defaults to 2, otherwise
0.
qs- a floating-point number with a value in the range 0.0
to 1.0, indicating the relative 'quality' of this variant
compared to the other available variants, independent of
the client's capabilities. For example, a jpeg file is
usually of higher source quality than an ascii file if it
is attempting to represent a photograph. However, if the
resource being represented is ascii art, then an ascii
file would have a higher source quality than a jpeg file.
All
qs values are therefore specific to a given
resource.
Example
Content-Type: image/jpeg; qs=0.8
URI:- uri of the file containing the variant (of the given
media type, encoded with the given content encoding). These
are interpreted as URLs relative to the map file; they must
be on the same server (!), and they must refer to files to
which the client would be granted access if they were to be
requested directly.
Body:- New in Apache 2.0, the actual content of the resource may
be included in the type-map file using the Body header. This
header must contain a string that designates a delimiter for
the body content. Then all following lines in the type map
file will be considered part of the resource body until the
delimiter string is found.
Example:
Body:----xyz----
<html>
<body>
<p>Content of the page.</p>
</body>
</html>
----xyz----
A MultiViews search is enabled by the MultiViews
Options. If the server receives a
request for /some/dir/foo and
/some/dir/foo does not exist, then the
server reads the directory looking for all files named
foo.*, and effectively fakes up a type map which
names all those files, assigning them the same media types and
content-encodings it would have if the client had asked for one
of them by name. It then chooses the best match to the client's
requirements, and returns that document.
If set, this directive allows content-negotiated documents
to be cached by proxy servers. This could mean that clients
behind those proxys could retrieve versions of the documents
that are not the best match for their abilities, but it will
make caching more efficient.
This directive only applies to requests which come from
HTTP/1.0 browsers. HTTP/1.1 provides much better control over
the caching of negotiated documents, and this directive has no
effect in responses to HTTP/1.1 requests.
Prior to version 2.0,
CacheNegotiatedDocs did not take an
argument; it was turned on by the presence of the directive by
itself.
The ForceLanguagePriority directive uses
the given LanguagePriority to satisfy
negotation where the server could otherwise not return a single
matching document.
ForceLanguagePriority Prefer uses
LanguagePriority to serve a one valid result, rather
than returning an HTTP result 300 (MULTIPLE CHOICES) when there
are several equally valid choices. If the directives below were
given, and the user's Accept-Language header assigned
en and de each as quality .500
(equally acceptable) then the first matching variant, en,
will be served.
LanguagePriority en fr de
ForceLanguagePriority Prefer
ForceLanguagePriority Fallback uses
LanguagePriority to
serve a valid result, rather than returning an HTTP result 406
(NOT ACCEPTABLE). If the directives below were given, and the user's
Accept-Language only permitted an es
language response, but such a variant isn't found, then the first
variant from the LanguagePriority list below will be served.
LanguagePriority en fr de
ForceLanguagePriority Fallback
Both options, Prefer and Fallback, may be
specified, so either the first matching variant from LanguagePriority will be served if
more than one variant is acceptable, or first available document will
be served if none of the variants matched the client's acceptable list
of languages.
See also
| Description: | The precendence of language variants for cases where
the client does not express a preference |
|---|
| Syntax: | LanguagePriority MIME-lang [MIME-lang]
... |
|---|
| Context: | server config, virtual host, directory, .htaccess |
|---|
| Override: | FileInfo |
|---|
| Status: | Base |
|---|
| Module: | mod_negotiation |
|---|
The LanguagePriority sets the precedence
of language variants for the case where the client does not
express a preference, when handling a MultiViews request. The list
of MIME-lang are in order of decreasing preference.
Example:
LanguagePriority en fr de
For a request for foo.html, where
foo.html.fr and foo.html.de both
existed, but the browser did not express a language preference,
then foo.html.fr would be returned.
Note that this directive only has an effect if a 'best'
language cannot be determined by any other means or the ForceLanguagePriority directive
is not None. Correctly implemented HTTP/1.1 requests
will mean this directive has no effect.
See also
