I was reading a book on Java Servlets where I came across HTTPSessionActivationListener. It was specified that in a clustered environment , there can be only one HTTPSession object containing a specific session id. Assume there are 2 nodes A and B in a cluster -
first request goes to node A. Here a HTTPSession S1 is created along with session attributes and response goes back to the client. Same client sends the subsequent request. This request goes to node B. Now the session object S1 is moved from node A to node B (activated in Node B and passivated in node A).
In this case should the session object along with the attributes be serializable? What happens if it is not serializable?
In order to count the number of active sessions , should the sessions in both nodes be added up to get the actual value? How is this usually done?
Also I guess ServletContext is unique for each JVM. Are the attributes set as part of servletcontext copied to servlet context in all nodes of the cluster?
Usually I've seen people use sticky sessions (provided usually by the load balancer, for example ec2 ELB has this feature: http://shlomoswidler.com/2010/04/elastic-load-balancing-with-sticky-sessions.html), OR the session data is stored in a shared repository, such as a database or NoSQL store.