Type Maps

In the last section, we looked at multiviews as a way of providing language and image negotiation. The other way to achieve the same effects in the current release of Apache, as well as more lavish effects later (probably to negotiate browser plug-ins), is to use type maps, also known as *.var files. Multiviews works by scrambling together a plain vanilla type map; now you have the chance to set it up just as you want it. The Config file in .../site.typemap/conf/httpd1.conf is as follows: 

User webuser
Group webgroup
ServerName www.butterthlies.com
DocumentRoot /usr/www/APACHE3/site.typemap/htdocs

AddHandler type-map var
DirectoryIndex index.var

One should write, as seen in this file:

AddHandler type-map var

Having set that, we can sensibly say:

DirectoryIndex index.var

to set up a set of language-specific indexes.

What this means, in plainer English, is that the DirectoryIndex line overrides the default index file index.html. If you also want index.html to be used as an alternative, you would have to specify it — but you probably don’t, because you are trying to do something more elaborate here. In this case there are several versions of the index — index.en.html, index.it.html, and index.ko.html — so Apache looks for index.var for an explanation.

Look at ... /site.typemap/htdocs. We want to offer language-specific versions of the index.html file and alternatives to the generalized images bath, hen, tree, and bench, so we create two files, index.var and bench.var (we will only bother with one of the images, since the others are the same).

This is index.var : 

# It seems that this URI _must_ be the filename minus the extension...
URI: index; vary="language"
URI: index.en.html
# Seems we _must_ have the Content-type or it doesn't work...
Content-type: text/html
Content-language: en
URI: index.it.html
Content-type: text/html
Content-language: it

This is bench.var :

URI: bench; vary="type"

URI: bench.jpg
Content-type: image/jpeg; qs=0.8 level=3

URI: bench.gif
Content-type: image/gif; qs=0.5 level=1

The first line tells Apache what file is in question, here index.* or bench.* ; vary tells Apache what sort of variation we have. These are the possibilities:

  • type

  • language

  • charset

  • encoding

The name of the corresponding header, as defined in the HTTP specification, is obtained by prefixing these names with Content-. These are the headers:

  • content-type

  • content-language

  • content-charset

  • content-encoding

The qs numbers are quality scores , from 0 to 1. You decide what they are and write them in. The qs values for each type of return are multiplied to give the overall qs for each variant. For instance, if a variant has a qs of .5 for Content-type and a qs of .7 for Content-language, its overall qs is .35. The higher the result, the better. The level values are also numbers, and you decide what they are. In order for Apache to decide rationally which possibility to return, it resolves ties in the following way:

  1. Find the best (highest) qs.

  2. If there’s a tie, count the occurrences of “*” in the type and choose the one with the lowest value (i.e., the one with the least wildcarding).

  3. If there’s still a tie, choose the type with the highest language priority.

  4. If there’s still a tie, choose the type with the highest level number.

  5. If there’s still a tie, choose the highest content length.

If you can predict the outcome of all this in your head, you must qualify for some pretty classy award! Following is the full list of possible directives, given in the Apache documentation:

URI: uri [; vary= variations ]

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 the files were requested directly.

Content-type: media_type [; qs= quality [level= level ]]

Often referred to as MIME types; typical media types are image/gif, text/plain, or text/html.

Content-language: language

The language of the variant, specified as an ISO 3166 standard language code (e.g., en for English, ko for Korean).

Content-encoding: encoding

If the file is compressed or otherwise encoded, rather than containing the actual raw data, indicates how compression was done. For compressed files (the only case where this generally comes up), content encoding should be x-compress or gzip or deflate, as appropriate.

Content-length: length

The size of the file. The size of the file is used by Apache to decide which file to send; specifying a content length in the map allows the server to compare the length without checking the actual file.

To throw this into action, start Apache with ./go 1, set the language of your browser to Italian (in Netscape, choose Edit → Preferences → Netscape → Languages), and access http://www.butterthlies.com /. You should see the Italian version. MSIE seems to provide less support for some languages, including Italian. You just get the English version. When you look at Catalog-summer.html, you see only the Bench image (and that labeled as “indirect”) because we did not create var files for the other images.