== link:index.html[Index] -> link:modules.html[Modules]

Modules: Encoders
------------------

Encoders are Cherokee modules that can be used as filters. The most
common usage is to compress information allowing you to save
bandwidth. You should do that whenever you can since hardware is
mostly inexpensive while bandwidth is not. Besides, the performance
penalization suffered due to the compression process is trivial.

Of course it doesn't make sense to apply a processor intensive filter
to content that can't take advantage of it (i.e., it doesn't make
sense to try to compress an already compressed file because there is
nothing to gain).

The encoders must be enabled or disabled in a per rule basis. The
acceptable settings for an encoder are 'Leave unset', 'Allow' and
'Forbid'.

This is the list of currently provided encoders:

. link:modules_encoders_gzip.html[gzip]
. link:modules_encoders_deflate.html[deflate]

As you might have guessed from the list of acceptable settings, it is
interesting to highlight that Cherokee supports negative encoding
rules. This translates into rules that forbid an encoder to be
used. To prevent encoding files that are already compressed, a pretty
simple rule such as the following one could be included to avoid the
problem.

[options="header"]
|===================================================================
|Match                          | Final/Non-Final | Encoder setting
|Extensions (jpg,zip,gz,rar,7z) | Non-Final       | Gzip forbidden
|====================================================================

For a more complex example, imagine the following scenario:

[options="header"]
|===================================================================
|Match   | Final/Non-Final | Encoder setting
| *.php  | Non-Final       | Gzip forbidden
|a*      | Non-Final       | Gzip leave unset
|Default | Final           | Gzip allowed
|====================================================================

The rule list is evaluated from top to bottom. The 'Leave unset'
option of the encoders means that, whenever a rule is applied, it will
not change the status of the encoding setting.

Under these conditions, the following requests would be evaluated as shown:

- /foo.php - wouldn't use Gzip
- /aaa.txt - would use Gzip (a* wouldn't set it, but since it's non-final the default rule would)
- /zzz.txt - would use it (set by the default rule)
- /abc.php - it matches the first rule, so Gzip is forbidden, and
  since it is no-final the evaluation continues. It also matches the
  second rule, but since Gzip support got already forbidden nothing is
  changed. When it hits the last rule, the same thing happens: Gzip is
  already forbidden, so the value is not overwritten.