Stateless Service

Problem

A load balancer distributes requests across ten API instances. Request 1 reaches server A. Request 2 reaches server B. If A stored essential session state only in its memory, B doesn't know it.

Solution — Stateless Service

Keep necessary durable/shared state outside individual service instances.

      ┌→ API A ─┐
User ─┼→ API B ─┼→ shared state
      └→ API C ─┘

Core idea: stateless instances make requests less dependent on which particular server receives them.