Stateless web apps

[andy wix <st...@hotmail.com>]

Hi,

I am working at a place that doesn't use session state for their web apps.
The reason given is that they have 2 physical servers and 2 Oracle 
Application Servers running on each of these machines for failover.  From 
the little I know about clustering, I thought that if everything in the 
session implements Serializable then session state is tranparently valid 
accross requests - even if you end up on another jvm.

Has anyone else ever heard of this restriction - I can see problems e.g., 
can't use Struts tokens to defeat the usual refresh problems.

More immediately, I have a problem with a typical scenario.  My normal 
approach is:
I have a 'preload' action that gets an ArrayList of data from the db and 
sets this in the session.
Then forward to the jsp that displays this stuff.
This page has something like:

<c:forEach var="wrap" items="${Assignments}" varStatus="status">
<TR>
  <TD><c:out value="${status.index+1}"/></TD>
  <TD>
    <c:url var="viewApp" value="/viewNode.do">
      <c:param name="indexPos" value="${status.index}" />
    </c:url>
          <html:link href="${viewApp}">
            <c:out value="${wrap.applicationVO.applicationName}"/>
          </html:link>
  </TD>

This gives me an index of the link that was clicked and in the next action I 
simply look this value up from the ArrayList in the session.

Does anyone know how this functionality could be achieved without using the 
session?

Thanks,
Andy

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Re: Stateless web apps

[Laurie Harper <la...@holoweb.net>]

andy wix wrote:
> I am working at a place that doesn't use session state for their web apps.
> The reason given is that they have 2 physical servers and 2 Oracle 
> Application Servers running on each of these machines for failover.  
>  From the little I know about clustering, I thought that if everything 
> in the session implements Serializable then session state is 
> tranparently valid accross requests - even if you end up on another jvm.
> 
> Has anyone else ever heard of this restriction - I can see problems 
> e.g., can't use Struts tokens to defeat the usual refresh problems.

Sharing sessions between containers ('session clustering') is not a 
standard feature -- i.e. the Servlet spec neither requires this 
capability nor specifies how it should be impelemnted. However, most 
modern contains do support session clustering, in their own proprietary 
way. For example, Tomcat supports session clustering via shared disk, 
JDBC datastore, etc.

I don't know what session clustering abilities Oracle Application Server 
has but it can probably be configured to do this. Bear in mind that 
there are performance tradeoffs to consider as well as opperational 
impact -- for example, if you cluster sessions via shared disk, you now 
need to make your shared disk array redundant as well.

If you can implement your application without relying on sessions at 
all, your life will be simpler ;-) If you do need to implement session 
clustering, consider using host affinity as an alternative. With host 
affinity, the load balancer is configured to share sessions between the 
servers equally, but allways route requests for a particular session to 
the same server. That saves you the overhead of session clustering, at 
the cost that if one of the application servers goes down all users 
whose sessions were housed there will loose their session and have to 
log in again on the other server.

L.

> 
> More immediately, I have a problem with a typical scenario.  My normal 
> approach is:
> I have a 'preload' action that gets an ArrayList of data from the db and 
> sets this in the session.
> Then forward to the jsp that displays this stuff.
> This page has something like:
> 
> <c:forEach var="wrap" items="${Assignments}" varStatus="status">
> <TR>
>  <TD><c:out value="${status.index+1}"/></TD>
>  <TD>
>    <c:url var="viewApp" value="/viewNode.do">
>      <c:param name="indexPos" value="${status.index}" />
>    </c:url>
>          <html:link href="${viewApp}">
>            <c:out value="${wrap.applicationVO.applicationName}"/>
>          </html:link>
>  </TD>
> 
> This gives me an index of the link that was clicked and in the next 
> action I simply look this value up from the ArrayList in the session.
> 
> Does anyone know how this functionality could be achieved without using 
> the session?

Assuming you can rebuild the list at any time, you can use request scope 
instead of session scope and rebuild the list at the start of each action.

L.


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