Stateless services - the simplest cluster architecture

A stateless service holds no session state in memory. Every request is independent. This is the simplest cluster architecture and the foundation of scalable web services.

What is stateless

A stateless service:

• Holds no client session in memory.
• Stores no state in local files.
• Each request is independent; processing one does not require state from a previous one.

Examples:

• nginx (serves static files; no per-request state).
• Stateless API (no session in memory; state in DB).
• CDN (serves cached content; origin is the source of truth).

Why stateless is the foundation

• Easiest to scale: add instances, distribute load.
• Easiest to fail over: any instance can handle any request.
• Easiest to test: no state to set up; just request the endpoint.
• Easiest to operate: no stateful logic to debug.

Stateless is the “easy mode” of cluster architecture. The discipline is to keep it stateless where possible.

Properties of a stateless service

• No local state: no sessions, no caches (or only read-only caches).
• All persistent state in a database or external store.
• Any instance can handle any request.
• No sticky sessions required (though they may still be used).

When stateless is not possible

Some applications have stateful logic by nature:

• Database (state is the data).
• Long-running job queue (state is the queue).
• WebSocket server (state is the open connection).
• Game server (state is the game world).

For these, stateless is not possible. The state lives somewhere (DB, queue, in-memory). The cluster must manage that state.

Design patterns

Stateless API

• All state in the database.
• Any instance can handle any request.
• Session token in a cookie or header (stateless JWT).
• The DB is the single source of truth.

CDN

• Static content cached at edge.
• Origin is the source of truth.
• Cache invalidation on origin update.

Worker pool

• Jobs in a queue (SQS, Kafka, RabbitMQ).
• Workers pull jobs; no shared state.
• Multiple workers, no coordination needed.

Knowledge check