Sticky session vs Session affinity

Sticky Session:

If you have many instances of tomcat (eg: tomcat1 & tomcat2) and you start session on tomcat1, the subsequent requests will be forwarded to tomcat1 as long as tomcat1 running an up (you never reach tomcat2). It means that if tomcat1 now is down, then your subsequent request will be forwarded to tomcat2, with a different session (a new session created at tomcat1 for you). This is not very good because if your application responding on tomcat1, then moved to tomcat2, session data will be lost. Usually Sticky session is achieved in simple load balancing of Apache server in front of tomcat servers.

Session affinity:

Session affinity, means you will have the same session id regardless of your first or subsequent requests served by tomcat1 or tomcat2. It means that you session will be duplicated on both tomcat1 and tomcat2. Sometimes you served by tomcat1, sometimes you served tomcat2, although none of them is down. Usually this is achieved when tomcat cluster is created.