Intermediate

Stateless Services

Core Scale & Availability

Design services without server-side session state for easy scaling and resilience

Core Idea

Keep service instances free of user or request session state so they can be scaled, replaced, and rescheduled freely. Externalize all state to durable systems like databases, caches, and object storage.

When to Use

When you need horizontal scaling, rolling deployments, autoscaling, or multi-zone/region resilience and want to avoid sticky sessions and node affinity.

Recognition Cues

Indicators that this pattern might be the right solution

Sticky sessions causing uneven load
Instances cannot be drained without losing in-flight state
Scale-out stalls due to local state coupling
Blue/green or canary requires session migration

Pattern Variants & Approaches

Overview

Stateless replicas behind a load balancer with externalized session/auth and persistent state in durable stores.

Overview Architecture

When to Use This Variant

Load balancer fans out to many replicas
External session/auth store (Redis/JWT)
No node-local state or affinity

Use Case

Web/API services that need elastic scaling, rolling deploys, and resilience across zones.

Advantages

Easy scale-out and safe replacements
Zero-downtime rollouts without sticky sessions
Failure isolation per replica

Implementation Example

# Stateless service (Kubernetes sketch)
apiVersion: apps/v1
kind: Deployment
metadata:
  name: api
spec:
  replicas: 4
  template:
    spec:
      containers:
      - name: app
        image: example/api:latest
        env:
        - name: SESSION_STORE_URL
          value: redis://redis:6379
        - name: DATABASE_URL
          value: postgres://db:5432/app
        # Readiness for safe draining
        readinessProbe:
          httpGet: { path: /healthz, port: 8080 }

Tradeoffs

Pros

Simple horizontal scaling and fast replacements
Improved availability during deployments and failures
Easier blue/green and canary rollouts
Better bin-packing and autoscaling efficiency

Cons

Dependency on external stores for state
Slightly higher latency vs in-process state
More moving parts and configuration
JWT revocation and rotation complexity if used

Common Pitfalls

Storing sessions or user context in memory/disk
Local background queues or cron state coupled to pod/node
Non-idempotent handlers without safeguards
Depending on instance identity or IP
Using local filesystem for persistent artifacts

Design Considerations

Use external session store (e.g., Redis) or stateless JWTs
Design idempotent endpoints with idempotency keys
Implement readiness/liveness probes and graceful shutdown
Define consistent timeouts and retry policies
Avoid local disk; use object storage for files
Deterministic concurrency and safe parallelism

Real-World Examples

Netflix

Stateless microservices behind Zuul and Envoy enable massive scale

Millions of RPS across thousands of instances

Shopify

Stateless web pods autoscale per-merchant load

Peak flash-sale traffic globally

Heroku

12-factor apps encourage stateless dynos

Millions of containers across regions

Complexity Analysis

Scalability

Horizontal - Add instances freely

Implementation Complexity

Low to Medium - Externalize state

Cost

Low - Simpler scaling and ops

Related Patterns

Load Balancing Horizontal Scaling Caching Strategies API Gateway Idempotency Keys