CacheMetaLanguage » History » Revision 12
Revision 11 (davojan, 2006-06-27 08:51) → Revision 12/14 (davojan, 2012-08-11 10:42)
h1. CML aka Cache Meta Language
h2. What Is It
CML tries to move the decision about a cache-hit and cache-miss for a dynamic website
out of the dynamic application, removing the need to start the application or dynamic
language at all.
Especially PHP is known to have a huge overhead before the script is started to be executed.
h2. How To Install
The language used by CML is LUA which you can find at http://www.lua.org/
To get some background on how to write LUA code check out:
* http://lua-users.org/wiki/LuaAddons
* http://luaforge.net/
h2. Benefits
The main benefit of CML is its performance.
A very simple benchmark showed:
* about 1000 req/s for the static 'output.html' which is generated output from the PHP script
* about 600 req/s if index.cml is called (cache-hit)
* about 50 req/s if index.php is called (cache-miss)
Using CML improves the performance for the tested page by a factor of 12, getting
near enough to the possible maximum of the static file transfer.
h2. Usage Patterns
http://www.lighttpd.net/ is using CML to reduce the load (even if the load is minimal).
The layout of the front page depends on a few files:
* content-1
* content-6
* the template /main.tmpl
If any of the files are modified the cached version of the page must change as well.
<pre>
output_contenttype = "text/html"
trigger_handler = "index.php"
-- this file updated by the trigger
output_include = { "output.html" }
docroot = request["DOCUMENT_ROOT"]
cwd = request["CWD"]
-- the dependencies
files = { cwd .. "content-1", cwd .. "content-6", docroot .. "main.tmpl" }
cached_mtime = file_mtime(cwd .. "output.html")
-- if one of the source files is newer than the generated files
-- call the trigger
for i,v in ipairs(files) do
if file_mtime(v) > cached_mtime then return 1 end
end
return 0
</pre>
h2. Delaying recheck
If you are building a news aggregator it is useful to be able to delay the rebuild of the cached content for a period of time, as you can assume that the news are not changing with each request. So instead of revalidating on each request you delay the validation check.
<pre>
-- same as above
-- check again in 5 minutes
delay_recheck = 3600
if cached_mtime + delay_recheck > os.time() then return 0 end
-- we are behind the delayed recheck, check the cache as usual
for i,v in ipairs(files) do
if file_mtime(v) > cached_mtime then return 1 end
end
return 0
</pre>
And to tell the proxies inbetween not to check again in the next 5 minutes after they received this content, use the setenv module and add some cache-control or expire headers.
h2. CML and Databases
CML does not provide direct access to databases like MySQL or PostgreSQL, and probably never will.
There is a better/faster way to interface CML with Databases: MemCache
All you have to do is keep the information needed to decide whether a page has to be regenerated in a memcached storage itself. Let's say that whenever you store an entry in the database, you associate a Version-ID with it. The Version-ID is incremented as soon as you make a change to the resource.
This Version-ID is now stored in the Database and in memcache at the same time. CML can now fetch the Version-ID, check if content already has been generated for it, and generate it if necessary.
<pre>
output_contenttype = "text/html"
content_key = md5(request["PATH_INFO"])
version = memcache_get_long(content_key)
cwd = request["CWD"]
trigger_handler = "generate.php"
if version >= 0 then
output_include = { cwd .. content_key .. "-" .. version .. ".html" }
return 0
else
return 1
end
</pre>
generate.php will have to:
* get PATH_INFO
* fetch information from database about it
* generate content for the page and write it to disk
* deliver it to the client
To interface the database with the memcached you can use a UDF:
* for "MySQL":http://www.mysql.com/ can get the mysql udf at "jans mysql page":http://jan.kneschke.de/projects/mysql/udf/
* for "PostgreSQL":http://www.postgresql.org/ Sean Chittenden has written "pgmemcache":http://people.freebsd.org/~seanc/pgmemcache/
In MySQL and the UDF you just do:
<pre>
#!sql
BEGIN;
UPDATE content SET @v := (version = version + 1) WHERE id = <id>;
SELECT memcache_set("127.0.0.1:11211", <id>, @v);
COMMIT;
</pre>
To check which version is currently used by the cache:
<pre>
#!sql
SELECT memcache_get("127.0.0.1:11211", <id>);
</pre>