Sessions in Rails application can be stored in a database (as well as in cookies). I used to store session data in a table in database.
Sessions table may grow very large because Rails doesn’t have a built-in mechanism to clear old session data.
So you need to remove old sessions rows from database manually.
Recently I had a strange problem when sessions didn’t work on my site on Rails in production environment. The problem was in Internet Explorer while it worked in Firefox and Chrome.