Your application is experiencing session loss when deployed across multiple servers. What strategy would you implement to manage sessions effectively?

Question

Accepted Answer

Session loss across multiple servers is solved by centralizing session storage outside individual application instances, most commonly with Spring Session backed by Redis, so any server can serve any user's session.

Key Points:
• Spring Session replaces the default in-memory HttpSession with a distributed store shared by all instances.
• Redis is a common backing store due to its speed and native support for expiring keys, matching session timeout semantics.
• Adding spring-session-data-redis and a Redis connection is often all that's needed, with minimal application code changes.
• A load balancer no longer needs sticky sessions since any instance can read the shared session store.
• Session serialization format should be chosen carefully to keep session objects small and fast to (de)serialize.

Example:
A user logs in through one server instance, and a subsequent request load-balanced to a different instance still finds their session intact because both instances read from the same Redis-backed session store instead of local memory.

Code Example:
```
spring.session.store-type=redis
spring.data.redis.host=redis-host
spring.data.redis.port=6379
```

Interview Tip:
A concise interview answer is:

"I'd move to a centralized session store using Spring Session backed by Redis, so session data lives outside any single instance's memory and every server can read the same session. That removes the need for sticky sessions and fixes session loss when requests get load-balanced across servers."