Apache HTTP Server Version 2.4
Apache HTTP Server Version 2.4
| Description: | Provides for content negotiation |
|---|---|
| Status: | Base |
| Module Identifier: | negotiation_module |
| Source File: | mod_negotiation.c |
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.
type-map) which explicitly lists the files
containing the variants.Multiviews
Options), where the server does
an implicit filename pattern match, and choose from amongst the
results.
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: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:en,
meaning English. If the variant contains more than one
language, they are separated by a comma.Content-Length:Content-Type:name=value. Common
parameters include:
leveltext/html this defaults to 2, otherwise
0.qsqs values are therefore specific to a given
resource.
Content-Type: image/jpeg; qs=0.8
URI:Body:
Body:----xyz----
<html>
<body>
<p>Content of the page.</p>
</body>
</html>
----xyz----
Consider, for example, a resource called
document.html which is available in English, French,
and German. The files for each of these are called
document.html.en, document.html.fr, and
document.html.de, respectively. The type map file will
be called document.html.var, and will contain the
following:
URI: document.html
Content-language: en
Content-type: text/html
URI: document.html.en
Content-language: fr
Content-type: text/html
URI: document.html.fr
Content-language: de
Content-type: text/html
URI: document.html.de
All four of these files should be placed in the same directory,
and the .var file should be associated with the
type-map handler with an AddHandler directive:
AddHandler type-map .var
A request for document.html.var in this directory will
result in choosing the variant which most closely matches the language preference
specified in the user's Accept-Language request
header.
If Multiviews is enabled, and MultiviewsMatch is set to "handlers" or "any", a request to
document.html will discover document.html.var and
continue negotiating with the explicit type map.
Other configuration directives, such as Alias can be used to map document.html to
document.html.var.
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.
The MultiviewsMatch
directive configures whether Apache will consider files
that do not have content negotiation meta-information assigned
to them when choosing files.
| Description: | Allows content-negotiated documents to be cached by proxy servers |
|---|---|
| Syntax: | CacheNegotiatedDocs On|Off |
| Default: | CacheNegotiatedDocs Off |
| Context: | server config, virtual host |
| Status: | Base |
| Module: | mod_negotiation |
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.
| Description: | Action to take if a single acceptable document is not found |
|---|---|
| Syntax: | ForceLanguagePriority None|Prefer|Fallback [Prefer|Fallback] |
| Default: | ForceLanguagePriority Prefer |
| Context: | server config, virtual host, directory, .htaccess |
| Override: | FileInfo |
| Status: | Base |
| Module: | mod_negotiation |
The ForceLanguagePriority directive uses
the given LanguagePriority to satisfy
negotiation 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.
| Description: | The precedence 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.
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. In general, the client determines the
language preference, not the server.