To avoid SPOF issue, you can have multiple web-server with load-balancer plugin. These set of web-servers need to be front-ended by hardware load-balancer. You can have multiple hardware load-balancers as well to avoid SPOF at that level as well.
Hardware load-balancer can work on simplest of algorithm and distribute incoming request to any of the web-server, say in round-robin fashion.
The load-balancer plugin installed on web-server will ensure client session stickiness is maintained to a particular back-end application server instance. This decision is based on load-balancer configuration file, and till it is same for each web-server, they will make same decision. To learn more about configuring web-server for load-balancing, please refer to document available at
http://docs.sun.com/app/docs/doc/819-3679/gchvt?l=en&a=view.
Hope this helps,
Kshitiz
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