# Module mod_negotiation

This module provides for content negotiation.

Status: Base
Source File: mod_negotiation.c
Module Identifier: negotiation_module

## 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 Option, where the server does an implicit filename pattern match, and choose from amongst the results.

## Directives

- CacheNegotiatedDocs
- LanguagePriority

See also: DefaultLanguage, AddEncoding, AddLanguage, AddType, and Options.

## Type maps

A type map has the same format as 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 of the variant, as an Internet standard language tag (RFC 1766). An example is `en`, meaning English.
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:
  The path to the file containing this variant, relative to the map file.

## MultiViews

A MultiViews search is enabled by the MultiViews Option. 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.

## CacheNegotiatedDocs directive

Syntax: CacheNegotiatedDocs on|off
Default: `CacheNegotiatedDocs off`
Context: server config
Status: Base
Module: mod_negotiation
Compatibility: CacheNegotiatedDocs is only available in Apache 1.1 and later. The syntax changed in version 2.0.

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.

## LanguagePriority directive

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. Correctly implemented HTTP/1.1 requests will mean this directive has no effect.

See also: DefaultLanguage and AddLanguage