Some applications store session data in server memory. If your second request goes to a different server, your session is lost. You get logged out.
Session persistence (sticky sessions) ensures requests from the same client always reach the same server.
Methods include:
- Cookie-based: Balancer sets a cookie with the server ID
- IP-based: Same IP always routes to same server
- Application cookie: App sets a session cookie, balancer reads it
Sticky sessions reduce load balancing effectiveness. One server might get stuck with many heavy users while others idle.