Not sure what you mean? How does using EJB help in front-end session level
fail-over? Most EJB containers that I have seen don't support statful
fail-over. If you move statefullness to the EJB, you still need a way to
loadbalance between at least two servers AND have ejb statefull fail-over as
well..so your in the same dilema, only as I said, most EJB servers don't
support fail-over..at least in terms of a session fail-over.
Seems to me servlet container session fail-over is easier to implement as
your only worried about the HttpSession and possibly the web-app servlet
context. In EJB, you have a lot more to contend with.
> -----Original Message-----
> From: Shawn Zhu [mailto:[EMAIL PROTECTED]]
> Sent: Tuesday, August 21, 2001 2:06 PM
> To: [EMAIL PROTECTED]
> Subject: Re: Developing for scalability..HttpSession fail-over...
>
>
> That's one reason to use EJB. =)
>
> -----Original Message-----
> From: Kevin Duffey [mailto:[EMAIL PROTECTED]]
> Sent: Tuesday, August 21, 2001 9:50 AM
> To: [EMAIL PROTECTED]
> Subject: Developing for scalability..HttpSession fail-over...
>
>
> Hi all,
>
> Just wanted to see if anyone has developed an application
> that works and is
> HttpSession fail-over safe. In other words, your web.xml has the
> <distributable/> tag, and you can successfully deploy the
> application into a
> container and handle session level fail over so that if one
> of two (or more)
> servers craps out, the user(s) on that box are still able to
> have their
> session in existence and are routed to another server that
> has it alive.
>
> Now, the reason I am asking is that there is one problem I
> noticed when
> testing my application for this. Something I think alot of
> people don't
> realize that in order to be ready to scale, you have to do this.
>
> If you use JavaBeans at the session scope level, you may be
> used to doing
> something like this in say, a servlet:
>
> MyBean myBean = (MyBean) session.getAttribute("MyBean");
>
> myBean.setFirstName("SomeName");
> myBean.setLastName("AnotherName");
>
> RequestDispatcher rd =
> request.getRequestDispatcher("/somepage.jsp").forward(
> request,response );
>
> -----
>
> somepage.jsp:
>
> <jsp:useBean id="MyBean" scope="session" class="somePackage.MyBean"/>
>
> <html>
> <head>
> <title>Demonstation</title>
> </head>
> <body>
> Welcome <%= MyBean.getFirstName() %> <%= MyBean.getLastName() %>
> </body>
> </html>
>
>
> This works just fine. You set the bean first and last name,
> forwarded to a
> JSP page and can display it. The problem is..since you are
> setting object
> references (and even values) on a HttpSession object, these
> changes are NOT
> reflected on another server in a cluster unless you ALWAYS
> reset the object
> back into the session. So, every time you get a bean out, you
> would have to
> add this at the end of any changes you make:
>
> session.setAttribute("MyBean", myBean);
>
> From what I have read, the only way a server (that supports
> session-level
> failover) replicates HttpSession data is when the setAttribute() or
> putValue() (deprecated Servlet 2.1 call) is called upon! If
> you just change
> various references of objects in the HttpSession, this data
> will never get
> copied over to another server.
>
> Now, my point in all this is that I am wondering if anyone
> working with say
> WebLogic, WebSphere, and so on has had to program in this
> manner for session
> replication, or if those servers just do it for you? If they do..how?
> Because the only other way I can think of is not exactly
> accurate: after a
> set interval, copy the HttpSession to any other server in the
> cluster! This
> would suck though..because if that set interval was every 1
> second, it would
> seriously degrade your system. If it was every say 10 to 15
> seconds, a few
> things could change in that time in one session..and if the
> server crapped
> out, the users session on another server would not be
> accurate. But on the
> other hand, if the session is replicated every time any one
> user stores an
> object in the session, it is possible that 100 users do this
> every second
> (on a high-traffic site) and thus replication might be evern WORSE on
> performance at this pace, than just doing it every 1 second.
> Ofcourse..the
> last part of this...it would be worse in this case only if the entire
> HttpSession was replicated. If it was on a per object per user type of
> replication, then it would be much more effecient.
>
> So..which is it? Do servlet containers only replicate session
> data when code
> sets the object back into the HttpSession (which makes sense
> if it can do
> "smart" object replication and not serialize the entire
> HttpSession every
> time)? Do the containers serialize the entire HttpSession
> every time? What
> happens if you have 500MB of HttpSession objects in memory?
> Does it really
> try to send 500MB every time the setAttribute() is called?
>
> Love to hear some answers on this topic.
>
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>
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Some relevant FAQs on JSP/Servlets can be found at:
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http://www.jguru.com/jguru/faq/faqpage.jsp?name=JSP
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