How do the differences between Stateless and Stateful architectures affect the scalability of systems?

Stateless architectures do not preserve state between requests, which simplifies scaling because any server can handle any request without the need for state synchronization. This makes it easy to add new service instances and use load balancers.

Stateful architectures preserve state between requests (e.g., user sessions), which complicates scaling because access to the state (through shared cache, database, or sticky sessions) must be ensured. This can lead to bottlenecks and reduced scalability flexibility.

Thus, stateless systems are more scalable and resilient to failures, while stateful systems require additional mechanisms for synchronization and state management during scaling.