Twelve-Factor App
12 principles for cloud-native applications. Adam Wiggins (Heroku co-founder), 2011. Bible for modern web app design: stateless processes, config in env vars, dev/prod parity.
The 12: Codebase, Dependencies, Config, Backing services, Build/release/run, Processes, Port binding, Concurrency, Disposability, Dev/prod parity, Logs, Admin processes. Created for the Heroku era but has survived — every container/Kubernetes best practice follows 12-factor. Modern extension: 15-factor (Pivotal/VMware, added API first, telemetry, authentication). Conflicts with modern: "stateless processes" doesn't fit WebSocket/real-time; "logs as event streams" was pre-OpenTelemetry.