What does it mean to design a stateless service?

Stateless service doesn't store client session state between requests - each request contains all needed information. Benefits: easier horizontal scaling (any server can handle any request), simpler failover (no session to migrate), better caching. State storage: externalize to database or cache (Redis). Stateless examples: REST APIs, serverless functions. Stateful: WebSocket connections, shopping carts (externalize to Redis). Design for statelessness to maximize scalability and resilience.