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From Justin Ruthenbeck <just...@nextengine.com>
Subject RE: How to synchronize based on session? (Prevent multiple submissions of forms)
Date Tue, 01 Jul 2003 22:48:15 GMT

 > // Member variable
 > protected Object m_synchObj = new Object();

Yes, your service() method will be run multi-threaded, but all of those 
threads share the same *servlet* member variables (declared outside your 
service() method).  They do not, as you asked, each have their own copy of 
that variable.  If you synch on an object created there, all request 
threads running your servlet will wait on a lock for that object.

It's very possible that Tomcat may already synch on session creation, I 
don't know (and I can't look at the source from where I'm at now) ... 
perhaps someone else out there knows or can check?

As I said, if I wanted to make ABSOLUTELY sure that no two threads had your 
problem, I would feel comfortable synching like this unless the number of 
concurrent requests was very, very, high (which would be very rare).  If 
Tomcat already synchs, then I would look into using an HttpSessionListener 
(Disclaimer: I haven't fully thought through that option yet).

justin


At 03:35 PM 7/1/2003, you wrote:
>Hello Justin,
>
>Thanks for your response.  My understanding is that there will be
>multiple instances of a servlet depending on what the container thinks
>is necessary.  This will most likely happen, because this is a high-volume
>site.  If that's the case, won't there be multiple instances of the
>servlet, each with its own member variable?  I believe this would lead
>for varying results as a particular session could access any of these
>instances on a given request.
>
>Now granted, what you've mentioned below will make the race condition even
>harder to meet, since we've now introduced a level of synchronization
>based on the servlets themselves, but I believe it still has the potential
>to be a problem... since two different instances of the servlet could both
>attempt to add the session attribute at exactly the same time... even
>though each has its own respective lock.
>
>As you've probably guessed, we've been avoiding the single-threaded
>servlet model, because we do have such a large volume of
>simultaneous accesses on these pages.
>
>Thanks again,
>-Raiden
>
>
>On Tue, 1 Jul 2003, Justin Ruthenbeck wrote:
>
> >
> > If your code worked like this:
> >
> > 1:   if (session.getAttribute(LOCKED_ATTR) == LOCKED)
> > 2:      return user a waiting page
> > 3:   else
> > 4:   {
> > 5:      session.setAttribute(LOCKED_ATTR, LOCKED)
> > 6:      work with submitted data
> > 7:   }
> >
> > then you're right -- it's possible for one thread to get interrupted after
> > evaluating line 1 to "false".  Another thread could come in, evaluate line
> > 1 to "false" again and you'd end up with two threads doing the "work with
> > submitted data" code (a double submission).
> >
> > I don't see any way around this save for synchronizing on a lock in your
> > servlet.  Don't worry about setting anything at Session creation time ...
> > make your logic do something like the following:
> >
> > // Member variable
> > protected Object m_synchObj = new Object();
> >
> > // In your service() method
> > boolean processData = false;
> > synchronized (m_synchObj)
> > {
> >     if (session.getAttribute(LOCKED_ATTR) != LOCKED)
> >     {
> >        session.setAttribute(LOCKED_ATTR, LOCKED);
> >        processData = true;
> >     }
> > }
> > if (processData)
> > {
> >     [Process the Data]
> > }
> > else
> > {
> >     [Send user to a "waiting" page]
> > }
> >
> > True, you've now got a potential bottleneck in your servlet since every
> > thread for that servlet waits (a small amount of time) on a single
> > object.  Unless you've got *LOTS* of concurrent servlet threads, I don't
> > think this would affect you.  I haven't given due diligence to the thought
> > of synching on an Object in the session, but perhaps that's a better idea
> > -- seems like you could still have a race condition if two thread both
> > tried to create a session at the same time (perhaps Tomcat guarantees this
> > won't happen by synching in it's session creation code?).  You could
> > probably work something out with an HttpSessionListener.  Me?  I'd synch on
> > the servlet in favor of simplifying my code.
> >
> > justin
> >
> >
> > At 02:56 PM 7/1/2003, you wrote:
> > >Hi Stefan,
> > >
> > >Thanks for the explanation.  That makes sense, except how can you make
> > >sure that the object is added to the session only once, when the session
> > >is created?
> > >
> > >It would seem like you would need to synchronize on something else for
> > >adding the object to the session, because there might be a race
> > >condition (unless there is a special bit of code that is only ever called
> > >once, when the session is created).  Otherwise, two different threads
> > >might each have their own instance of the lock object.
> > >
> > >How do I guarantee that the lock object is only created once, at the time
> > >the session is created?
> > >
> > >Thanks,
> > >Raiden
> > >
> > >
> > >On Tue, 1 Jul 2003, Stefan Radzom wrote:
> > >
> > > > For security reasons your are not handed the internal
> > > > org.apache.catalina.session.StandardSession . Instead, the implicit 
> session
> > > > you are trying to synchronize holds a reference to
> > > > org.apache.catalina.session.StandardSessionFacade which is backed 
> by the
> > > > actual StandardSession to which all method calls are delegated. The 
> facade
> > > > object can therefore change between different requests, thus making 
> it not
> > > > appropriate for synchronisation.
> > > >
> > > > Try putting a dummy object into the session when creating a new 
> session:
> > > > session.setAttribute("lock", new Object()) . On subsequent requests 
> you can
> > > > get the attribute and obtain a lock on this object.
> > > >
> > > > Object lock = session.getAttribute("lock");
> > > > synchronized (lock) { ; }
> > > >
> > > > -Stefan
> > > >
> > > >
> > > > > -----Original Message-----
> > > > > From: Raiden [mailto:raiden@wonko.inow.com]
> > > > > Sent: Tuesday, July 01, 2003 8:19 PM
> > > > > To: tomcat-user@jakarta.apache.org
> > > > > Subject: How to synchronize based on session? (Prevent
> > > > > multiple submissions of forms)
> > > > >
> > > > >
> > > > > Hello,
> > > > >
> > > > > We are trying to prevent the "multiple submission of a form problem",
> > > > > that can result when a user double-clicks the submit button.
> > > > > We have a
> > > > > process in place, but we have been unable to get the session based
> > > > > synchronization to work correctly.
> > > > >
> > > > > Our order page submits to a "meta-refresh" page that checks to 
> see the
> > > > > current status of the order processing each time it loads.
> > > > > If there is
> > > > > an error, it sends the user back to the order page.  If the order
was
> > > > > successful, the user is sent to the success page.  If the
> > > > > order is still
> > > > > processing, it refreshes in 5 seconds (and meanwhile, the user has
a
> > > > > pretty 5 second progress bar to watch).  However, The
> > > > > relevant snippet of
> > > > > code is below:
> > > > >
> > > > > String nextUrl = null;
> > > > > synchronized (session) {
> > > > >   BigDecimal status =
> > > > >     (BigDecimal)session.getAttribute(SiteProps.PROCESS_STATUS);
> > > > >
> > > > >   if (status != null) {
> > > > >     System.out.println("  Processing has already begun.  Checking
> > > > > status.");
> > > > >
> > > > >     if (status.equals(SiteProps.PROCESS_SUCCESS)) {
> > > > >       System.out.println("  We already know it was approved.
> > > > > Just showing
> > > > > them the success page.");
> > > > >       nextUrl = "/success.jsp";
> > > > >       response.sendRedirect(response.encodeRedirectUrl(nextUrl));
> > > > >       return;  // will remove status object on next page
> > > > >     } else if (status.equals(SiteProps.PROCESS_ERROR)) {
> > > > >       System.out.println("  We know the credit card charge
> > > > > failed.  Send
> > > > > them back to the order form.");
> > > > >       nextUrl = "/order.jsp";
> > > > >       response.sendRedirect(response.encodeRedirectUrl(nextUrl));
> > > > >       return;  // will remove status object on next page
> > > > >     } else if (status.equals(SiteProps.PROCESS_UNKNOWN)) {
> > > > >       System.out.println("  Still processing.  Not sure what
> > > > > the result
> > > > > will be.  This page will refresh in 5 seconds.");
> > > > >       nextUrl = "/order_check.jsp"; // refresh back to this same
page
> > > > >     }
> > > > >   } else {
> > > > >     // we don't have any record of a previous attempt to process
> > > > >     System.out.println("  First time trying to process order page.
> > > > > Starting processing, scheduling a refresh.");
> > > > >     session.setAttribute(SiteProps.PROCESS_STATUS,
> > > > >       SiteProps.PROCESS_UNKNOWN);
> > > > >   }
> > > > >   %>
> > > > >   <html>
> > > > >     <!-- progress bar animated gif html code here -->
> > > > >   </html>
> > > > >   <%
> > > > >   if (nextUrl == null) {
> > > > >     // first time to page
> > > > >
> > > > >     // order processing code here
> > > > >   }
> > > > > }
> > > > >
> > > > > The problem is, that there isn't any real synchronization
> > > > > here.  This is
> > > > > because the "session" object is a different object each time this

> page
> > > > > loads.  In fact, printing out the "session" object each time the
page
> > > > > loads results in:
> > > > >
> > > > > org.apache.catalina.session.StandardSessionFacade@24ea85
> > > > > org.apache.catalina.session.StandardSessionFacade@10275fa
> > > > > org.apache.catalina.session.StandardSessionFacade@18706f6
> > > > > org.apache.catalina.session.StandardSessionFacade@f1f34a
> > > > > org.apache.catalina.session.StandardSessionFacade@fb6763
> > > > >
> > > > > Since the session object keeps changing, what is the proper way to
> > > > > synchronize based on the user's session, to make this
> > > > > anti-double-process
> > > > > code work properly?
> > > > >
> > > > > This middle page with the progress bar helps to remove most of our
> > > > > double-click problems, but occassionally a user is able to
> > > > > get multiple
> > > > > copies of this "progress page" opened, and then if the timing
> > > > > is right,
> > > > > they both enter that snippet of code above at the same time,
> > > > > both think
> > > > > it's the first time this page is being processed, and both
> > > > > run the order
> > > > > processing code.
> > > > >
> > > > > Thank you,
> > > > > Raiden
> > > > >
> > > > >
> > > > > ---------------------------------------------------------------------
> > > > > To unsubscribe, e-mail: tomcat-user-unsubscribe@jakarta.apache.org
> > > > > For additional commands, e-mail: tomcat-user-help@jakarta.apache.org
> > > > >
> > > > >
> > > >
> > > >
> > > >
> > > > ---------------------------------------------------------------------
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> > > > For additional commands, e-mail: tomcat-user-help@jakarta.apache.org
> > > >
> > >
> > >---------------------------------------------------------------------
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> >
> >
> > ____________________________________
> > Justin Ruthenbeck
> > Software Engineer, NextEngine Inc.
> > justinr - AT - nextengine DOT com
> > Confidential
> >     See http://www.nextengine.com/confidentiality.php
> > ____________________________________
> >
> >
> > ---------------------------------------------------------------------
> > To unsubscribe, e-mail: tomcat-user-unsubscribe@jakarta.apache.org
> > For additional commands, e-mail: tomcat-user-help@jakarta.apache.org
> >
>
>---------------------------------------------------------------------
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____________________________________
Justin Ruthenbeck
Software Engineer, NextEngine Inc.
justinr - AT - nextengine DOT com
Confidential
    See http://www.nextengine.com/confidentiality.php
____________________________________


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