Product Documentation

Use Case: Limiting the Number of Sessions

Jun 25, 2015

In this use case, the requirement is to limit the number of active backend sessions. In the deployment, each session login has login in the URL and each session logout has logout in the URL. On successful login, the backend sets a sessionid cookie with a unique 10 character value.

To achieve this use case, perform the following operations:

  1. Create a map variable that can store each active session. The key of the map is the sessionid. The expiry time for the variable is set to 600 seconds (10 minutes).
    > add ns variable session_map -type map(text(10),ulong,100) -expires 600
  2. Create the following assignments for the map variable:
    • Create an entry for the sessionid and set that value to 1 (this value is not actually used).
      > add ns assignment add_session -variable '$session_map[http.req.cookie.value("sessionid")]' -set 1
    • Deallocate the entry for a session ID, which implicitly decrements the value count for session_map.
      > add ns assignment delete_session -variable '$session_map[http.req.cookie.value("sessionid")]' -clear
  3. Create responder policies for the following:
    • To check if a map entry exists for that sessionid in the HTTP request. The add_session assignment is executed if the map entry does not exist.
      > add responder policy add_session_pol 'http.req.url.contains("twbkwbis.P_SabanciLogin") || $session_map.valueExists(http.req.cookie.value("netsuis"))' add_session
      Note: The valueExists() function in the add_session_pol policy counts as a reference to the session's map entry, so each request resets the expiration timeout for its session. If no requests for a session are received after 10 minutes, the session's entry will be deallocated.
    • To check when the session is logged out. The delete_session assignment is executed.
      > add responder policy delete_session_pol "http.req.url.contains(\"Logout\")" delete_session
    • To check for login requests and if the number of active sessions exceed 100. If these conditions are satisfied, in order to limit the number of sessions, the user is redirected to a page that indicates that the server is busy.
      > add responder action redirect_too_busy redirect "/too_busy.html" 
      > add responder policy check_login_pol "http.req.url.contains(\"twbkwbis.P_SabanciLogin\") && $session_map.valueCount > 100" redirect_too_busy
  4. Bind the responder policies globally.
    > bind responder global add_session_pol 30 next 
    > bind responder global delete_session_pol 10 
    > bind responder global check_login_pol 20