As Mindas explained it before :
When you use loadbalancing it means you have several instances of tomcat and you need to divide loads.
- If you’re using session replication without sticky session : Imagine you have only one user using your web app, and you have 3
tomcat instances. This user sends several requests to your app, then
the loadbalancer will send some of these requests to the first tomcat
instance, and send some other of these requests to the secondth
instance, and other to the third. - If you’re using sticky session without replication : Imagine you have only one user using your web app, and you have 3 tomcat
instances. This user sends several requests to your app, then the
loadbalancer will send the first user request to one of the three
tomcat instances, and all the other requests that are sent by this
user during his session will be sent to the same tomcat instance.
During these requests, if you shutdown or restart this tomcat
instance (tomcat instance which is used) the loadbalancer sends the
remaining requests to one other tomcat instance that is still
running, BUT as you don’t use session replication, the instance
tomcat which receives the remaining requests doesn’t have a copy of
the user session then for this tomcat the user begin a session : the
user loose his session and is disconnected from the web app although
the web app is still running. - If you’re using sticky session WITH session replication : Imagine you have only one user using your web app, and you have 3 tomcat
instances. This user sends several requests to your app, then the
loadbalancer will send the first user request to one of the three
tomcat instances, and all the other requests that are sent by this
user during his session will be sent to the same tomcat instance.
During these requests, if you shutdown or restart this tomcat
instance (tomcat instance which is used) the loadbalancer sends the
remaining requests to one other tomcat instance that is still
running, as you use session replication, the instance tomcat which
receives the remaining requests has a copy of the user session then
the user keeps on his session : the user continue to browse your web
app without being disconnected, the shutdown of the tomcat instance
doesn’t impact the user navigation.