# Module mod_negotiation

This module is contained in the `mod_negotiation.c` file, and is compiled in by default. It provides for content 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 Option, where the server does an implicit filename pattern match, and choose from amongst the results.

### 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. Currently only two encodings are recognized by http; `x-compress` for compressed files, and `x-gzip` for gzipped files.
Content-Language:
  The language of the variant, as an Internet standard language code, such as `en`.
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 semi-colons. Parameter syntax is name=value; allowed parameters are:
  level
    the value is an integer, which specifies the version of the media type. For `text/html` this defaults to 2, otherwise 0.
  qs
    the value is a floating-point number with value between 0. and 1. It indications the 'quality' of this variant.
  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.

## Directives

- CacheNegotiatedDocs
- LanguagePriority

## CacheNegotiatedDocs

Syntax: CacheNegotiatedDocs
Context: server config
Status: Base
Module: mod_negotiation
Compatibility: CacheNegotiatedDocs is only available in Apache 1.1 and later.

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.

## LanguagePriority

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