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📄 12 Factor App

Notes from Kelsey's "12 Fractured Apps" post from 2015.

Key Points​

  1. solve app issues at the app level; don't outsource it to config management or other hacks.
  2. config files should be optional: if a load fails fallback to sane defaults instead of trying & dying e.g. config files
  3. use config vars and override where needed, for config e.g. use if !="" and then grab it from os.Getenv()
  4. create if it doesn't exist i.e. a data dir
  5. use code to retry, with limits, database connections
  6. introducing a mechanism that needs an environment to support it, increases build size e.g. bash script brings in an alpine image
  7. adding a script now adds maintenance and another artifact that can "drift" and get out of sync i.e. the script
  8. keep config out of the container image
  9. don't require app starts in specific order, make each startup step not fatal when relying on each other.

The Twelve Factors​

Source: "12factor.net"

I. Codebase​

One codebase tracked in revision control, many deploys

II. Dependencies​

Explicitly declare and isolate dependencies

III. Config​

Store config in the environment

IV. Backing services​

Treat backing services as attached resources

V. Build, release, run​

Strictly separate build and run stages

VI. Processes​

Execute the app as one or more stateless processes

VII. Port binding​

Export services via port binding

VIII. Concurrency​

Scale out via the process model

IX. Disposability​

Maximize robustness with fast startup and graceful shutdown

X. Dev/prod parity​

Keep development, staging, and production as similar as possible

XI. Logs​

Treat logs as event streams

XII. Admin processes​

Run admin/management tasks as one-off processes