Sticky vs non-sticky sessions in web application

When there is only 1 server serving all requests, a session object is created by webserver for each client and it remains in memory of the web server. Same session is used for all requests for that particular client.

However, if there are multiple servers are working behind the load balancers, it becomes the responsibility of load balancer to decide which server a particular request will be re-directed.

For example, if there are 4 webservers S1, S2, S3 and S4 behind the load balancer, it is possible that

http://javaterritory.blogspot.com/2016/03/method-overriding-in-java.html is served by server S1,

http://javaterritory.blogspot.com/2016/03/inner-class-in-java.html served from server S2,

http://javaterritory.blogspot.com/2016/04/difference-between-save-saveorupdate.html is served from server S3,

http://javaterritory.blogspot.com/2016/09/session-management.html served from server S4.

In this case, since there are 4 servers which are actually serving different requests from same client, 4 different sessions will be maintained by each server. This way of managing sessions is termed as non-sticky session. But this could lead to many potential problems as there are different sessions maintained for same client. To overcome these problems, you may have to write code to migrate session data to a common layer where each server can read. This is exactly what done by what is known as sticky-session.

You can configure load balancer to use sticky-session. With sticky session configured, all the requests will be directed to same server and it will manage the session throughout the interaction with a client.

I you are working with Apache web server, following link gives steps to configure sticky session:

https://httpd.apache.org/docs/2.4/mod/mod_proxy_balancer.html#stickyness

To summarize, in case of sticky sessions, all requests for a client will go to the same physical web server while in case of a non-sticky load-balancer may choose any webserver to serve your requests, even if they are for same client.