What are the main disadvantages of Stateful applications?

The main disadvantages of stateful applications are:

1. Scaling complexity — when scaling horizontally, it is necessary to synchronize state between instances, which complicates architecture.
2. Fault tolerance issues — losing state during a server failure can lead to data or session loss.
3. Load balancing difficulties — requests from the same user need to be directed to the same server (sticky sessions), reducing load balancer efficiency.
4. Update and deployment complexity — updating one instance can lead to state desynchronization.
5. Maintenance and support — storing state complicates debugging and monitoring.

Because of these reasons, for scalable and fault-tolerant systems, stateless architecture is often preferred, where state is stored outside the application (e.g., in a database or cache).