15/02/10

Suppose you have several variants of a resource, e.g.:
- MIME type: the same image as image/jpg & image/gif
- Encoding: the same page compressed and not
- Charset: the same page in UTF-8 & in ISO-8859-1
- Language: the same page in English, French, ...

We could leave it to the user to choose the right URL

Or we could use Content Negotiation

This uses certain HTTP request headers:
- Accept:
- Accept-Encoding:
- Accept-Charset:
- Accept-Language:
They allow the client to express preferences

In these headers, order is not significant
Preferences are indicated by quality factors, 
e.g. q=1.0 (default), q=0.5

Configuring the server for content negotiation
- two ways: MultiViews or Type Maps
- we look at Type Maps

Modules: mod_negotiation (and mod_mime)

To httpd./conf, insert:
  AddHandler type-map .var

Then create a type map, which is a file whose extension
is usually .var

In this file, put one record per variant of the 
resource, e.g.

  URI: welcome.en.html
  Content-Type: text/html;qs=1.0
  Content-Language: en
  Description: "English original"

  URI: welcome.fr.html
  Content-Type: text/html;qs=0.5
  Content-Language: fr
  Description: "French translation"

Apache's negotiation algorithm will then decide
which version to send to the client by matching
client preferences with server versions.

The client now has to request, e.g. welcome.html.var

But we can tidy this up by using an Alias

Multilingual error messages can be done in this way.