apache2 reverse proxy & mod_mem_cache, not working

Hi,

everything is in the subject..

using apache 2.0.53-5 on debian testing
i've setup some apache2 http reverse proxies to act as a frontend for some
backend IIS6 servers, everything works fine so far.

i'm using some basic ProxyPass/ProxypassReverse setup for some of them and
some more "advanced" RewriteRules for others.
Now i'd like to use apache2 caching modules to let apache cache some pages
and files and serve them directly to the client..

My "advanced" rproxy conf looks like that (virtual host conf)

-------------------

ProxyRequests Off
ProxyPreserveHost On

<Proxy *>
Order deny,allow
Allow from all
</Proxy>

RewriteEngine On
RewriteLog /var/log/apache2/sub.domain.tld/rewrite.log
RewriteLogLevel 3

RewriteRule ^/?(str1|str2)/?(.*)? http://backendserver/$2
[E=EnvLang:$1,P,L]
RewriteRule ^/?(.*) http://backendserver/$1 [P,L]

# Add HTTP Header required for language env
RequestHeader add AX-Rewrite-URL %{EnvLang}e

# Reverse proxy
<Location />
ProxyPassReverse /
<LimitExcept GET POST HEAD>
Order allow,deny
Deny from all
</LimitExcept>
</Location>

---------------

the basic conf would simply use

ProxyPass / http://backendserver/

instead of all the Rewrite crap

----------------

don't worry about the RequestHeader thing it's a trick i have to use in
order to let the backend server initiate the proper env langage for the
requested url
One other thing, the mod_proxy.conf file has an additional conf parameter
which is "ProxyVia On"

now i've loaded mod_mem_cache and mod_cache and i've added the following
peace of conf

<IfModule mod_cache.c>
# <IfModule mod_disk_cache.c>
# CacheRoot /tmp/apachecache
# CacheSize 5000000
# CacheEnable disk /fr
# CacheDisable /fr/images
# CacheDisable /en
# </IfModule>

<IfModule mod_mem_cache.c>
CacheEnable mem /
CacheDefaultExpire 1800
CacheDisable /images
CacheIgnoreCacheControl On
CacheIgnoreNoLastMod On
CacheMaxExpire 3600
MCacheSize 8192
MCacheMaxObjectCount 100
MCacheMinObjectSize 1
MCacheMaxObjectSize 204800
</IfModule>
</IfModule>

All the dynamic documents are located in the root and all the other files
are pictures in /images so i've disabled caching for pictures

now for a reason i don't really understand caching doesn't work.. i simply
check that on a test backend server by requesting an url and monitoring the
backend server cpu load.. if the dynamic file was cached and served by
apache the backend server wouldn't have anything to do besides sending
pictures and i wouldn't see any cpu load peak..
I know that debug method is lame but i haven't found any ways to debug
apache caching yet and as it doesn't seem to work i have nothing in the
access/error logs that would say otherwise..

So if anyone has an idea about what's wrong with my proxy/mem_cache conf i'd
really appreciate it.. I was wondering if i had anything special to do
about expiry headers/conf on the backend server documents or on apache thru
maybe another module like mod_expire ?

I'd also really like to have a good way to debug/trace apache caching..
haven't found anything to monitor apache caching..
Thx for your time

Marc