On my recent project I used cloudhosting www.cloudcontrol.com for easy scaling. Unfortunately they don’t support synchronized filestorage, so when you store a session on one machine, on the next request it is gone 
There is a quite simple workaround, just store your sessions in the database:
http://symfony.com/doc/current/cookbook/configuration/pdo_session_storage.html
The drawback is a (possibly) pretty huge database, so make sure you have some kind of garbage collector that cleans the entries from time to time.
About Christian Baer
Hi, I'm a Berlin-based developer and working on webstuff since 2000. My focus lies on PHP, TYPO3 and Symfony. 2009 I became a certified TYPO3-Integrator. I worked on projects for clients like Paypal, Union Investment, Commerzbank or Warsteiner.
Go check out my page http://www.christian-baer.com
