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From "Akins, Brian" <>
Subject Re: Adding purge/invalidation to mod_cache
Date Fri, 30 May 2008 11:59:32 GMT
How we handle purge:

-Our cache is only disk based, so it is somewhat easier.
-We have a hook that gets called after each cache store - ie, after we write
a vary meta file, after we write a "real" meta file, after a datafile write
(on close).
-We also have a cache_get_info function that is exported (think
curl_get_info) so that other modules can get info about cache objects
(filenames, uri, sizes, etc.)
-Our default "cache store" registers this post-store hook.  It just writes
the data out to a pipe - uri filename size expires\n
-This piped program streams this info into a database (MySQL usually, as we
had issues with SQLite)
-Our cache manager is really just a fastcgi that uses the database to expire
and purge cache objects.

In a vary situation, we will get 3 post-store's for the initial cache:
-first is the "main" key for "main" meta file
-second is the varied key for the "real" meta file (the one that has
headers, etc.)
-third is the varied key/file for the data

So a simple sql query for "select file from my_table where url='/my/url'"
will return all 3. (LIKE and regex's work as well.)  We also can easily keep
track of cache size (select sum(object_size) from my_table), oldest entries,

Subsequent requests for same uri with different vary will add 2 entries to
the database - varied meta and data.

The fastcgi and pipe are because we prototyped all of this in perl and never
rewrote it because it all works fairly well.  Also, we had a few issues with
mysql client directly in Apache and the pipe was just easier...

Brian Akins
Chief Operations Engineer
Turner Digital Media Technologies

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