Architecture design

Ensures horizontal scalability and fault tolerance by removing server-side session state that must be synchronized across nodes.

Web services: designing REST APIs that must scale across multiple servers behind a load balancer Cloud systems: building serverless functions or microservices that restart unpredictably per invocation Distributed systems: ensuring requests can be routed to any replica without session affinity

REST APIs, microservices, serverless computing (AWS Lambda, Azure Functions, Cloud Functions), cloud-native architectures, HTTP services, CDN edge logic

client sends self-contained request → server processes without prior context → response returned → no session stored

• REST API
• JWT tokens
• load balancer
• horizontal scaling
• idempotent operations
• session affinity
• sticky sessions
• serverless functions

• Stateful design with sticky sessions: simpler client logic but limits scalability and complicates failover. Database-backed sessions: persistent but adds a network round-trip per request. Client-side storage (cookies/localStorage): reduces server load but exposes data and inflates payloads.

Storing request context in instance variables: causes data leaks between concurrent users sharing a worker process Assuming local memory persists across requests: breaks behind load balancers that route to different replicas Hiding state in URL query parameters: leaks sensitive data through access logs, referrers, and browser history Using thread-locals as implicit session storage: fails when threads are recycled, scaled out, or migrated to async runtimes Caching derived state without an invalidation strategy: produces stale responses after upstream data changes

Stateful design: retains session context server-side across requests Idempotent operations: safe to retry, related but orthogonal to statelessness Eventual consistency: tolerates temporary state divergence between replicas CQRS: separates read and write models, independent of whether the service holds session state

Coming from PHP: may rely on $_SESSION superglobal — stateless design requires extracting all needed data into the request itself (headers, body, tokens) Coming from Java EE: may expect HttpSession to persist across requests — stateless services must externalize or recompute state per request, often via tokens

• REST principles
• idempotent operations
• horizontal scaling
• session management
• microservices
• serverless architecture

When NOT to use: long-running multi-step workflows requiring server-side coordination, real-time collaborative editing with conflict resolution, or systems where recomputation cost clearly exceeds state-management cost. Performance: stateless services can be aggressively cached and load-balanced, but every request re-sends context (tokens, parameters) increasing payload size and CPU spent on parsing/validation. Boundary conditions: 'stateless' is rarely absolute — databases, caches, and logs remain stateful; the principle applies specifically to request-handling logic, not the entire system.

Enables seamless horizontal scaling, zero-downtime rolling deployments, and graceful failover since any node can serve any request without warm-up, session migration, or sticky routing.

Stateless design is like a vending machine: every transaction is complete in itself, no memory of the last customer, so any machine in the row can serve you.

Event sourcing / CQRS for systems where state must be reconstructed from an event log rather than carried in each request