What are sticky sessions and why are they needed for a connection to fall on one instance?

Sticky sessions are a load balancing mechanism where all requests from the same client are directed to the same server (instance). This is necessary when session state is stored locally on the server rather than in a shared storage (e.g., database or cache).

Without sticky sessions, requests from the same client may be routed to different instances, and if the session is stored locally, the server will not be able to correctly process the request as it won't find the necessary data.

Example: you have 3 application instances, and a user logs in on instance 1. If the next request goes to instance 2, there will be no session information there, and the user will be perceived as unauthenticated.

Sticky sessions are usually implemented at the load balancer level (e.g., Nginx, HAProxy) using cookies or IP hashing.