title: Ruby on rails - flash variable
body: <p>I'm completely new to rails. 
  While learning I came across the use of the flash variable for maintaining data for the next postback when redirecting.</p>
  
  <p>My questions are</p>
  
  <ol>
  <li><p>when should this be used.</p></li>
  <li><p>how does rails maintain it for me, does it make a round trip to the user and back or is it maintained server side.</p></li>
  <li>if it is maintained server side how does rails know when to discard the variable and prevent its memory getting clogged up.</li>
  </ol>
  
  <p>Any replies would be appreciated  </p>