Twelve Factor

12-Factor App Methodology

Guide for building scalable, maintainable, and portable cloud-native applications following the 12-Factor App principles and modern extensions.

When to Activate

Use this skill when:

• Designing or refactoring cloud-native applications
• Building applications for Kubernetes deployment
• Setting up CI/CD pipelines
• Implementing microservices architecture
• Migrating applications to containers
• Reviewing architecture for cloud readiness
• Troubleshooting deployment or scaling issues
• Working with environment configuration

The 12 Factors

I. Codebase

One codebase tracked in revision control, many deploys

• Single Git repository for the application; every environment deploys from it
• Environment-specific config lives outside the code
• Use GitOps (ArgoCD, Flux) for deployment automation

Violations: multiple repositories for one application, per-environment codebases, copying code between repositories. Fix: consolidate to one repository and move the differences into environment config.

II. Dependencies

Explicitly declare and isolate dependencies

• Declare every dependency in the package manager manifest; never rely on system-wide packages
• Commit lock files for reproducible builds (package-lock.json, mix.lock, Cargo.lock, go.sum)
• Multi-stage container builds isolate build-time dependencies from the runtime image

Worked Dockerfile and per-language manifest list: references/factor-examples.md.

III. Config

Store config in the environment

All configuration comes from environment variables — anything that varies between deploys: credentials, hostnames, ports, pool sizes, backing service URLs, model names.

# Elixir - config/runtime.exs
config :my_app, MyApp.Repo,
  hostname: System.get_env("DATABASE_HOST") || "localhost",
  password: System.fetch_env!("DATABASE_PASSWORD"),
  pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10")

What makes a violation, and the smallest correct fix:

• Hardcoded value that differs (or could differ) between deploys → read it from an environment variable, with a default only when a safe dev default exists
• Hardcoded model name, API endpoint, or service hostname → environment variable; model names are config, not code
• Config file with real values committed to version control → environment variables plus a committed .example template
• Environment-specific code path (if env == "production") → one code path driven by a config value

Litmus test from 12factor.net: the codebase could be made open source at any moment without compromising any credentials.

Kubernetes ConfigMap/Secret wiring: references/kubernetes.md.

IV. Backing Services

Treat backing services as attached resources

• Databases, queues, caches, and third-party APIs all attach via URLs in environment variables
• No distinction between local and third-party services
• Swapping a backing service is a configuration change only — never a code change

V. Build, Release, Run

Strictly separate build and run stages

1. Build: convert code to executable bundle
2. Release: combine build with config
3. Run: execute in target environment

• Releases are immutable and uniquely identified (git SHA, semver)
• Roll back by redeploying a previous release, never by modifying one
• Build artifacts stay separate from runtime config

CI/CD pipeline example: references/factor-examples.md.

VI. Processes

Execute the app as one or more stateless processes

• Processes are stateless and share-nothing; persistent state lives in backing services
• No sticky sessions, no local filesystem for persistent data
• Violation: in-memory session or state store → move it to Redis or the database

Session-store example: references/factor-examples.md. Deployment manifest: references/kubernetes.md.

VII. Port Binding

Export services via port binding

• Bind to 0.0.0.0, never localhost; take the port number from the environment
• HTTP server library embedded in the app — no reliance on runtime injection (Apache, Nginx)

VIII. Concurrency

Scale out via the process model

• Scale horizontally by adding processes, not vertically by enlarging them
• Distinct process types (web, worker, scheduler) scale independently
• The OS or platform process manager owns processes — never daemonize or write PID files

IX. Disposability

Maximize robustness with fast startup and graceful shutdown

• Minimize startup time (under 10 seconds ideal) — fast startup enables rapid scaling
• Handle SIGTERM: finish in-flight requests, close connections, then exit
• Stay robust against sudden death

Graceful-shutdown example: references/factor-examples.md.

X. Dev/Prod Parity

Keep development, staging, and production as similar as possible

• Same backing service types in dev and prod (not SQLite in dev, Postgres in prod)
• Containers ensure environment consistency; infrastructure as code for reproducibility
• Same deployment process for all environments; minimize the time gap between dev and production

Docker Compose dev-environment example: references/factor-examples.md.

XI. Logs

Treat logs as event streams

• Write unbuffered to stdout/stderr; never manage or rotate log files in the app
• Use structured logging (JSON) with correlation IDs for tracing
• The platform routes logs (Fluentd, Logstash) — never send directly to an aggregation service from app code

XII. Admin Processes

Run admin/management tasks as one-off processes

• Migrations, consoles, and one-time scripts use the same codebase, config, and environment as regular processes
• Run against a release, not development code
• Use the platform scheduler for recurring tasks; ship admin code with application code

Kubernetes Job/CronJob examples: references/kubernetes.md.

Modern Extensions (Beyond 12)

Three extensions round out contemporary cloud-native practice — full detail and examples in references/modern-extensions.md:

• XIII. API First — design and document APIs (OpenAPI) before implementation; contract-first development
• XIV. Telemetry — /metrics endpoint, distributed tracing (OpenTelemetry), health check endpoints
• XV. Security — OAuth 2.0/OIDC, RBAC, secrets in environment never in code, TLS everywhere, security scanning in CI/CD

Common Patterns

Configuration Validation

Validate required configuration at startup — fail fast, naming the missing variables:

function validateConfig() {
  const required = ['DATABASE_URL', 'JWT_SECRET', 'REDIS_URL'];
  const missing = required.filter(key => !process.env[key]);

  if (missing.length > 0) {
    throw new Error(`Missing required environment variables: ${missing.join(', ')}`);
  }
}

// Call before starting server
validateConfig();

Health checks, graceful degradation, and the troubleshooting guide: references/operations.md.

Anti-Patterns to Avoid

❌ Environment-Specific Code Paths

// DON'T: branch on environment
if (process.env.NODE_ENV === 'production') { /* different behavior */ }

// DO: one code path, configured value
const timeout = parseInt(process.env.TIMEOUT || '5000');

❌ Local File Storage

// DON'T: write persistent data to local filesystem
fs.writeFile('/tmp/uploads/' + filename, data);

// DO: use object storage
await s3.putObject({ Bucket: process.env.S3_BUCKET, Key: filename, Body: data });

❌ In-Memory State

// DON'T: store state in memory
const sessions = new Map();

// DO: use external store
const session = await redis.get(`session:${sessionId}`);

❌ Hardcoded Dependencies

// DON'T: hardcode service locations
const db = connect('localhost:5432');

// DO: use environment variables
const db = connect(process.env.DATABASE_URL);

Best Practices Summary

1. Environment variables for all configuration
2. Stateless processes that can scale horizontally
3. Structured logging to stdout
4. Containers for development parity
5. Automated CI/CD pipelines
6. Health checks for orchestration
7. Graceful shutdown handling
8. **Fast startup times ( "The twelve-factor methodology can be applied to apps written in any programming language, and which use any combination of backing services (database, queue, memory cache, etc)."

> "A twelve-factor app never relies on implicit existence of state on the filesystem. Even if a process has written something to disk, it must assume that file won't be available on the next request."

Design applications from day one to be cloud-native, scalable, and maintainable.

References

• references/factor-examples.md — per-factor worked code examples (Dockerfile, runtime config, session storage, graceful shutdown, Docker Compose, structured logging, CI/CD pipeline)
• references/kubernetes.md — Kubernetes manifests per factor (ConfigMaps, Secrets, Deployments, HPA, Jobs, probes) and Kubernetes-specific best practices
• references/modern-extensions.md — API First, Telemetry, and Security extensions with examples
• references/operations.md — health check endpoints, graceful degradation, troubleshooting guide
• references/infrastructure-conventions.md — NGINX upstream over community ingress; Kustomize + Helm layout; single universal secret store; IaC over manual UI; bind all interfaces in prod

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: vinnie357
• Source: vinnie357/claude-skills
• License: MIT

Install and usage instructions live in the source repository linked above.