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What is deflatemod?

Deflatemod is a module for Ocsigen that allows you to deflate content, using HTTP/1.1 mechanism. It supports gzip as well as deflate.

What do I need to use deflatemod?

You need the camlzip library by Xavier Leroy and a working Ocsigen installation.

How do I use deflatemod?

First, load the extension in the configuration file:

<extension findlib-package="ocsigenserver.ext.deflatemod" />

Alternatively, if you do not want to use findlib, you need to:

  • load camlzip
<extension module="/path/to/camlzip.cma" />
  • then load deflatemod
<extension module="/path/to/deflatemod.cmo" />

How do I configure deflatemod?

Again, the options are in ocsigen.conf. You can set a few options globally, in the extension tag:

  • the compress level: from 0 (no compression) to 9. The higher, the better compression rate you achieve. Conversely, the lower, the faster it is. Default is 6.
  • the buffer size: default is 1024. You shouldn't need to change this one, but you can still try a few other values (any feedback welcomed).

For example:

<extension module="/home/gabriel/ocsigen/extensions/deflatemod.cmo">
<compress level="6" />
<buffer size="1024" />

Next, you '''have to''' specify which pages you whish to deflate, in which host/site (otherwise, deflatemod won't compress anything). See next question to do that.

I want to deflate only some files. Is it possible?

To some extent, yes. You can '''exclude''' some files and deflate every other, according to their mime-type or extension (''ie.'' the end of the '''url''' in fact, not really the file name on the disk).

Here is an example:

<!-- ... -->

<deflate compress="allbut">
<!-- deflate everything but *.tar.gz files and image/* files -->

As you can see, you must put the <deflate> tag at the very end of the <host> element. Indeed, Ocsigen deals with extensions sequentially, so if you put <site> or other elements '''after''' <deflatemod>, they won't be compressed.

Conversely, you can choose to deflate '''only''' some files, based on their mime-types or extension:

<!-- ... -->

<deflate compress="only">
<!-- deflate only text/html files -->

Warning: you can use only one contenttype tag! So you must choose, either the "only" or the "allbut" way.

Limitations: you cannot filter by directory or by filename extension (remember extension is the url's extension). If you really need it, please fill a bug report or send a patch.

Technical questions

How can I choose the content-encoding (gzip or deflate)?

You can't. Deflatemod follows the RFC and chooses automatically the right encoding, based upon Accept-encoding header sent by the client.

Does deflatemod support transfert-encoding?

No, it doesn't. Content-encoding is definitely a better solution, providing end to end compression at the content-level.

How does deflatemod deal with the content-length header?

As it cannot compute the new content-length, deflatemod uses chunked-encoding. This is the recommended way to go.

Page written by Gabriel Kerneis