CSC/ECE 517 Fall 2009/wiki2 3 pp

Double Submit Problem

What is the requirement of having this pattern. Some examples such as Add to cart being pressed twice can result in 2 items in the cart etc. The example should provide or demonstrate how devastating this can be. Some financial example such as clicking on Pay button twice could charge credit card twice etc.

Solutions for Double Submit Problem

Can be server side or client side. Server Vs Client which one is better?

Client Side Solution - Javascript Control Disabling

Briefly describe the approach. Discuss pros and cons.

POST-REDIRECT-GET

Briefly describe the approach. Discuss pros and cons.

Synchronizer Token Pattern

How the patten works? Diagrammatic Representation? Theoretical View? Reply on session.

Using POST-REDIRECT-GET avoids accidental double submits of a single request but does not help prevent a user from completing the same business process twice. Such a business process is typically composed of multiple pages spanning several requests. Synchronizer token pattern adds additional safety on top of the POST-REDIRECT-GET idiom by preventing a possibly intentional resubmit of a page. Both the techniques should typically be combined to deliver a complete solution.

Implementations of the pattern provided by various frameworks

The basic idea of Synchronizer Token is to set a token in a session variable before returning a transactional page to the client. This page carries the token inside a hidden field. Upon submission, request processing first tests for the presence of a valid token in the request parameter by comparing it with the one registered in the session. If the token is valid, processing can continue normally, otherwise an alternate course of action is taken. After testing, the token resets to null to prevent subsequent submissions until a new token is saved in the session, which must be done at the appropriate time based on the desired application flow of control. Many web based frameworks provide built-in support for this. However some of the frameworks require serious developer attention whereas some frameworks do provide configurable automatic support. This section will describe how the built in support for synchronizer token is provided by various frameworks.

Struts

Struts provides built in mechanism for handling tokens in org.apache.struts.action.Action class using the saveToken() and isTokenValid() methods. saveToken() method creates a token (a unique string) and saves that in the user's current session, while isTokenValid() checks if the token stored in the user's current session is the same as that was passed as the request parameter.

To do this the JSP has to be loaded through an Action. Before loading the JSP call saveToken() to save the token in the user session. When the form is submitted, check the token against that in the session by calling isTokenValid(), as shown in the following code snippet:

class PurchaseOrderAction extends DispatchAction { 
      public ActionForward load(ActionMapping mapping,
                                ActionForm form,
                                HttpServletRequest request,
                                HttpServletResponse response) throws Exception {
         try {
             //save the token
             saveToken(request)
             // rest of the code for loading the form
         } catch(Exception ex){
             //exception
         }
      }

      public ActionForward submitOrder(ActionMapping mapping,
                                       ActionForm form,
                                       HttpServletRequest request,
                                       HttpServletResponse response) throws Exception {
          try {
              // check the token. Proceed only if token is valid
              if(isTokenValid(request,true)) {
                  //implement order submit functionality here
              } else {
                  return mapping.findForward("failure");
              }
          } catch(Exception ex){
              //exception
          }
     }
}

This actually what is happening behind the scene in the Action class. saveToken() has logic as below:

HttpSession session = request.getSession();
String token = generateToken(request);
if (token != null) {
   session.setAttribute(Globals.TRANSACTION_TOKEN_KEY, token);
}

The method generates a random token using session id, current time and a MessageDigest and stores it in the session using a key name org.apache.struts.action.TOKEN (This is the value of the static variable TRANSACTION_TOKEN_KEY in org.apache.struts.Globals class).

The Action class that renders the form (PurchaseOrderAction.load) invokes the saveToken() method to create a session attribute with the above name. In the JSP, the token needs to be as a hidden form field as follows:

<input type="hidden" name="<%=org.apache.struts.taglib.html.Constants.TOKEN_KEY%>" 
value="<bean:write name="<%=Globals.TRANSACTION_TOKEN_KEY%>"/>">

The embedded <bean:write> tag shown above, looks for a bean named org.apache.struts.action.TOKEN (which is the value of Globals.TRANSACTION_TOKEN_KEY) in session scope and renders its value as the value attribute of the hidden input variable. The name of the hidden input variable is org.apache.struts.taglib.html.TOKEN (This is nothing but the value of the static variable TOKEN_KEY in the class org.apache.struts.taglib.html.Constants).

When the client submits the form, the hidden field is also submitted. In the Action that handles the form submission i.e. PurchaseOrderAction.submitOrder (which most likely is different from the Action that rendered the form), the token in the form submission is compared with the token in the session by using the boolean) isTokenValid() method. The method compares the two tokens and returns a true if both are same. Be sure to pass reset=”true” in the boolean) isTokenValid() method to clear the token from session after comparison. If the two tokens are equal, the form was submitted for the first time. However, if the two tokens do not match or if there is no token in the session, then it is a duplicate submission and handle it in the manner acceptable to your users.

Although the above approach is good, it requires application developer to add the token checking method pair – saveToken() and boolean) isTokenValid() in methods rendering and submitting the sensitive forms respectively. Since the two tasks are generally performed by two different Actions, the pairs need to be identified and added manually.

If the form is spanned across the multiple pages, then every time the form is submitted before going from one page to another. You definitely want to validate token on every page submission. However you also want to allow the user to traverse back and forth using the browser back button until the point of final submission. If the token is reset on every page submission, the possibility of back and forth traversal using the browser button is ruled out. The solution is not disabling back button (using JavaScript hacks) but to handle the token intelligently. This is where the boolean) reset parameter is useful. The token is initially set before showing the first page of the form. The boolean) reset parameter is false for all the boolean) isTokenValid() invocations except in the Action for the last page. The last page uses a true value for the reset argument and hence the token is reset in the boolean) isTokenValid() method. From this point onwards you cannot use back button to traverse to the earlier form pages and successfully submit the form.

References

[1] References go here