Nov 26, 2009

Static gzip is your best friend

After we touched several aspects of caching let's return to gzip and review very simple and powerful technique 'Static gzip'.

What is Static Gzip?

Static Gzip is a way to serve compressed content w/o its actual compression 'on fly' (here is a blog post about gzip). Very roughly we have gzipped files and serve them instead of common ones. How can we do this?

General algorithm

  • First of all we need to have gzipped versions of initial files. Usually they are named with a postfix .gz, i.e. main.css.gz. As far as these files are static we can have them compressed at maximum. With Linux you can do the following
    gzip -c -n -9 main.css > main.css.gz
    to get the smallest compressed file from the initial one.
  • Secondly we must have any way to route HTTP requests to take compressed version of the file. Via Apache and mod_rewrite we can add the following rules to .htaccess file or Apache configuration.
    <IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteCond %{HTTP:Accept-encoding} gzip
    RewriteCond %{HTTP_USER_AGENT} !Konqueror
    RewriteCond %{REQUEST_FILENAME}.gz -f
    RewriteRule ^(.*)\.css$ $1.css.gz [QSA,L]
    <FilesMatch \.css\.gz$>
    ForceType text/css
    <IfModule mod_mime.c>
    AddEncoding gzip .gz

Rip off the veil of the mystery

What does this set of rules actually do?

  • First of all we enable RewriteEngine (it may have been already enabled in your configuration).
  • Then
    RewriteCond %{HTTP:Accept-encoding} gzip
    rule selects all HTTP requests with
    Accept-Encoding: gzip
    header and allows Apache to perform other rules.
  • Then we skip Konqueror browser
    RewriteCond %{HTTP_USER_AGENT} !Konqueror
    because it seems it doesn't understand compressed content for CSS / JavaScript files.
  • Then we check if there is physical file with postfix .gz
    RewriteCond %{REQUEST_FILENAME}.gz -f
  • And only after all these checks we perform actual internal rewrite rule.
    RewriteRule ^(.*)\.css$ $1.css.gz [QSA,L]
    We redirect all requests to .css files to .css.gz files.
  • After this redirect (which is the last one — modificator [L]) we force Content-Type of such files to be text/css. Most servers send .gz with default archive encoding, and browsers can't properly detect this content.
  • Finally with AddEncoding gzip .gz rule sets proper Content-Encoding header for compressed content.

Of course this logic can be applied not only to CSS files but to all files which can be efficiently compressed — JavaScript, fonts, favicon.ico, etc.

That's all?

Graceful Degradation

What if Apache doesn't have mod_rewrite module? Or we need to serve CSS content via PHP? Or there is environment which doesn't support such logic (CGI / IIS)?

Well we can perform the whole algorithm actually via PHP too. All what we need here is the following.

  • Form compressed file name (usually just with a postfix .gz). And check for this file's existence.
  • If no such file exists, create compressed version of the file (i.e. via gzcompress function).
  • Then write this compressed content to a new file (with already defined 'gzipped' file name).
  • And set for this file time of change (mtime) to the value equals to initial (non-compressed) one. Why it's required? Because we can with check for existence also check if gzipped version of the current file has the same mtime and skip its re-creation in case of equivalence. If the current file and its compressed version have different time of change — it seems we need to re-create latter.

So with this logic we just check for file's existence and perform 2 checks for mtime (all such checks can be cached on file system level, or cache folder can be mapped to shared memory) and serve gzipped version. CPU is saved (with 80-85% of transferred content size)!

So Web Optimizer has all such approaches integrated and with version 0.6.7+ allows you to create .gz versions of CSS / JS files within any folder on your website.


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