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")