Comment by Jd

Abbreviated version:

(1) One codebase tracked in revision control, many deploys (running instances, typically including a production and one or more staging sites)

(2) Explicitly declare and isolate dependencies (e.g. GEMFILE for ruby)

(3) Store config in the environment NOT code, "A litmus test for whether an app has all config correctly factored out of the code is whether the codebase could be made open source at any moment, without compromising any credentials."

(4) Treat any service the app consumes over the network as attached resources, a 12-factor app "should be able to swap out a local MySQL database with one managed by a third party (such as Amazon RDS) without any changes to the app’s code."

(5) Strictly separate build, release, and run stages.

(6) Execute the app as one or more stateless processes. "The twelve-factor app never assumes that anything cached in memory or on disk will be available on a future request or job."

(7) Export services via port binding. "The twelve-factor app is completely self-contained and does not rely on runtime injection of a webserver into the execution environment to create a web-facing service. The web app exports HTTP as a service by binding to a port, and listening to requests coming in on that port."

(8) Scale out via the process model. "processes are a first class citizen... the share-nothing, horizontally partitionable nature of twelve-factor app processes means that adding more concurrency is a simple and reliable operation."

(9) Maximize robustness with fast startup and graceful shutdown. "Twelve-factor app’s processes are disposable... should strive to minimize startup time...shut down gracefully when they receive a SIGTERM signal from the process manager...should also be robust against sudden death"

(10) Keep development, staging, and production as similar as possible. "Twelve-factor app is designed for continuous deployment by keeping the gap between development and production small... resist[ing] the urge to use different backing services between development and production"

(11) Treat logs as event streams. Twelve-factor app "never concerns itself with routing or storage of its output stream"

(12) Run admin/management tasks as one-off processes.