What is Twelve-Factor App?
A methodology of twelve principles for building portable, scalable SaaS applications, originally published by Heroku engineers.
Definition
The twelve-factor methodology (12factor.net, Adam Wiggins, 2011) codifies how to build apps that deploy cleanly to modern platforms: one codebase per app, explicit dependency declaration, configuration in environment variables rather than code, backing services as attached resources swappable by URL, strict build/release/run separation, stateless processes, port binding, scale-out via processes, fast startup and graceful shutdown, dev/prod parity, logs as event streams to stdout, and admin tasks as one-off processes. Written for Heroku's dyno model, the principles translated almost verbatim to Docker containers and Kubernetes, and remain the baseline vocabulary for cloud-native application design reviews.