title: What are valid values for framework.session.storage_id?
body: <p>In the Symfony configuration there is an entry <code>framework.session.storage_id</code>. This setting also appears in the default config on the <a href="http://symfony.com/doc/current/reference/configuration/framework.html" rel="noreferrer">Symfony configuration documentation</a> but it is not explained. My assumption is that it defines where session data is stored on the server side.</p>
  
  <p>Values I have seen for this entry include <code>session.storage.mock_file</code>, <code>session.storage.native</code> and <code>session.storage.filesystem</code>. I am unsure of what these values exactly mean (e.g. what is the difference between a mock file and a filesystem?) and also think that this is not the complete list of possible values.</p>
  
  <p>So what exactly does this configuration key control and what values are valid?</p>