You have to refer load balancing technique to share sessions between servers.
which application server you use ?
refer below URL for Tomcat.
webpage Tomcat has limitations
This is a simple implemention of a rules-based load balancer. It was not designed as a replacement for other load-balancing mechanisms used for high traffic environments. Rather, it is a simple, pure Java, easily extensible, and fast way to direct traffic among multiple servers.
I think Srinivas is referring to sharing a session across clones of the same server. (different boxes running gmail.)
Different domains sharing sessions is different. It is usually done by storing the data in a database or a cookie. You need to be on the same domain for a cookie to work. Which google definitely uses to share whether you are logged in to your google account. They also use a database for your more detailed profile information.