Rights management in Symfony's cache directory

[Devow]

Hi everybody !

I have installed a Symfony 2 web-application.
I saw that the cache directory need to have good rights for server writing and client reading.

I saw 2 groups which manage web directory : psaserv and psacln.

Normally, I think the server (=psaserv ?) need to have "write" permission and client need (=psacln ?) "read" permissions on the cache directory.

I don't really understand who need to have which permissions on the cache directory...that's why I don't understand why I always have 500 Forbidden errors.

I try to "chmod 777" on the directory without success.

drwxrwsrwx  19 user_which_deploy psaserv  4096 févr. 29 10:29 cache
|
|---  drwxrwsrwx  3 user psacln  4096 févr. 22 12:06 default_backend_thumb_120/
|---  drwxrwsrwx  3 user_which_deploy psaserv 4096 janv. 29 18:40 default_eta_illustration/

Anyone can explain me shortly how Plesk and its 2 groups manage a web-application like Symfony ?

Thank you so much !