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.

Anti-fabrication

This skill follows core:anti-fabrication. The 12 factors themselves are the methodology cited in sources.md (12factor.net) — a stable, versionless reference, so naming a factor correctly is low-risk. The risk sits in the worked examples: the Kubernetes manifests, runtime.exs snippets, and Docker Compose configs in references/kubernetes.md and references/factor-examples.md assert real API fields (e.g. livenessProbe, ConfigMap keys) that do change across Kubernetes versions — verify a manifest field against current API docs before asserting it applies, rather than assuming an older example still matches the current API.

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 (< 10s)
9. Immutable releases with unique IDs
10. Monitoring and telemetry via metrics and tracing

Resources

• 12factor.net: Original methodology
• Kubernetes Documentation: https://kubernetes.io/docs/
• Docker Best Practices: https://docs.docker.com/develop/dev-best-practices/
• OpenTelemetry: https://opentelemetry.io/
• Prometheus: https://prometheus.io/

Key Insights

"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)."

"The twelve-factor app never assumes that anything cached in memory or on disk will be available on a future request or job – with many processes of each type running, chances are high that a future request will be served by a different process." — Factor VI, Processes

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