Module mod_cml - CML (Cache Meta Language)


  • This module is deprecated in favor of mod_magnet which provides similar functionality.
  • you have to return 1 instead of CACHE_MISS and 0 instead of CACHE_HIT (#533)


CML (Cache Meta Language) wants to solves several problems:
  • dynamic content needs caching to perform
  • checking if the content is dirty inside of the application is usually more expensive than sending out the cached data
  • a dynamic page is usually fragmented and the fragments have different lifetimes
  • the different fragments can be cached independently

Cache Decision

A simple example should show how to a content caching the very simple way in PHP. has a very simple design:
  • the layout is taken from a template in templates/jk.tmpl
  • the menu is generated from a menu.csv file
  • the content is coming from files on the local directory named content-1, content-2 and so on

The page content is static as long non of the those tree items changes. A change in the layout is affecting all pages, a change of menu.csv too, a change of content-x file only affects the cached page itself.

If we model this in PHP we get:


## ... fetch all content-* files into $content
$cachefile = "/cache/dir/to/cached-content";

function is_cachable($content, $cachefile) {
  if (!file_exists($cachefile)) {
    return 0;
  } else {
    $cachemtime = filemtime($cachefile);

  foreach($content as $k => $v) {
    if (isset($v["file"]) && 
        filemtime($v["file"]) > $cachemtime) {
      return 0;

  if (filemtime("/menu/menu.csv") > $cachemtime) {
    return 0;
  if (filemtime("/templates/jk.tmpl") > $cachemtime) {
    return 0;

if (is_cachable(...), $cachefile) {
} else {
  # generate content and write it to $cachefile

Quite simple. No magic involved. If the one of the files is new than the cached content, the content is dirty and has to be regenerated.

Now let take a look at the numbers: * 150 req/s for a Cache-Hit * 100 req/s for a Cache-Miss

As you can see the increase is not as good as it could be. The main reason as the overhead of the PHP interpreter to start up (a byte-code cache has been used here).

Moving these decisions out of the PHP script into a server module will remove the need to start PHP for a cache-hit.

To transform this example into a CML you need 'index.cml' in the list of indexfiles and the following index.cml file:

output_contenttype = "text/html" 

b = request["DOCUMENT_ROOT"]
cwd = request["CWD"]

output_include = { b .. "_cache.html" }

trigger_handler = "index.php" 

if file_mtime(b .. "../lib/php/menu.csv") > file_mtime(cwd .. "_cache.html") or
   file_mtime(b .. "templates/jk.tmpl")   > file_mtime(cwd .. "_cache.html") or
   file_mtime(b .. "content.html")        > file_mtime(cwd .. "_cache.html") then
   return CACHE_MISS
   return CACHE_HIT
Numbers again:
  • 4900 req/s for Cache-Hit
  • 100 req/s for Cache-Miss

Content Assembling

Sometimes the different fragment are already generated externally. You have to cat them together:


We can do the same several times faster directly in the webserver.

Don't forget: Webserver are built to send out static content, that is what they can do best.

The index.cml for this looks like:

output_contenttype = "text/html" 

cwd = request["CWD"]

output_include = { cwd .. "head.html", 
                   cwd .. "menu.html",
                   cwd .. "spacer.html",
                   cwd .. "db-content.html",
                   cwd .. "spacer2.html",
                   cwd .. "news.html",
                   cwd .. "footer.html" }

return CACHE_HIT

Now we get about 10000 req/s instead of 600 req/s.

Power Magnet

Next to all the features about Cache Decisions CML can do more. Starting with lighttpd 1.4.9 a power-magnet was added which attracts each request and allows you to manipulate the request for your needs.

We want to display a maintenance page by putting a file in a specified place:

We enable the power magnet:

cml.power-magnet  = "/home/www/power-magnet.cml"

and create /home/www/power-magnet.cml with:

dr = request["DOCUMENT_ROOT"]

if file_isreg(dr .. 'maintenance.html') then
  output_include = { dr .. 'maintenance.html' }
  return CACHE_HIT


For each requested file the /home/www/power-magnet.cml is executed which checks if maintenance.html exists in the docroot and displays it instead of handling the usual request.

Another example, create thumbnail for requested image and serve it instead of sending the big image:

## image-url is /album/baltic_winter_2005.jpg
## no params -> 640x480 is served
## /album/baltic_winter_2005.jpg/orig for full size
## /album/baltic_winter_2005.jpg/thumb for thumbnail

dr = request["DOCUMENT_ROOT"]
sn = request["SCRIPT_NAME"]

## to be continued :) ... 

trigger_handler = '/gen_image.php'


You need lua and should install libmemcached and have to configure lighttpd with:

./configure ... --with-lua --with-memcached

To use the plugin you have to load it:

server.modules = ( ..., "mod_cml", ... )



the file extension that is bound to the cml-module

cml.extension = ".cml"


hosts for the memcache.* functions


(not used yet)


a cml file that is executed for each request


The language used for CML is provided by LUA

Additionally to the functions provided by lua mod_cml provides:

  • request
    • CWD
  • get
    • parameters from the query-string
  • functions
    • string md5(string)
    • number file_mtime(string)
    • string memcache_get_string(string)
    • number memcache_get_long(string)
    • boolean memcache_exists(string)

What ever your script does, it has to return either CACHE_HIT or CACHE_MISS. If an error occurs, check the error-log. The user will get a error 500. If you don't like the standard error-page use server.errorfile-prefix.

Updated by gstrauss over 7 years ago · 19 revisions