The Twelve-Factor App Principles

The Twelve-Factor App is a set of principles for building modern, cloud-native applications that can scale easily and remain resilient in the face of changing requirements and infrastructure. The principles were first introduced in a 2011 blog post by Heroku co-founder Adam Wiggins and have since become a popular framework for designing and building web applications.

The twelve factors are as follows:

1. Codebase: There should be only one codebase per application, which is tracked in a version control system such as Git.
2. Dependencies: Dependencies should be explicitly declared and isolated from the runtime environment.
3. Config: Configuration data should be stored in environment variables, rather than hard-coded in the application.
4. Backing services: The application should treat backing services (databases, queues, etc.) as attached resources, accessed via a URL or other runtime configuration.
5. Build, release, run: The build, release, and run stages of the application lifecycle should be separated to enable efficient testing, deployment, and rollback.
6. Processes: Applications should be designed as stateless processes that can be easily scaled horizontally.
7. Port binding: The application should export services via a port binding, which can be dynamically assigned at runtime.
8. Concurrency: The application should be able to scale horizontally by adding more instances, rather than vertically by increasing the size of individual instances.
9. Disposability: The application should be designed to start up quickly, shut down cleanly, and be able to recover from failures.
10. Dev/prod parity: The development, staging, and production environments should be as similar as possible, to avoid errors caused by differences in configuration or dependencies.
11. Logs: Applications should produce logs as event streams, which can be easily aggregated and analyzed.
12. Admin processes: Administrative tasks (e.g. database migrations) should be run as one-off processes, rather than being part of the application code.

By following these principles, developers can create applications that are highly portable, resilient, and scalable, making it easier to deploy and maintain them in the cloud.

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