What is a twelve factor app?

 the modern era, software is commonly delivered as a service: called web apps, or software-as-a-service. The twelve-factor app is a methodology for building software-as-a-service apps that:

• Use declarative formats for setup automation, to minimize time and cost for new developers joining the project;
• Have a clean contract with the underlying operating system, offering maximum portability between execution environments;
• Are suitable for deployment on modern cloud platforms, obviating the need for servers and systems administration;
• Minimize divergence between development and production, enabling continuous deployment for maximum agility;
• And can scale up without significant changes to tooling, architecture, or development practices.

The twelve-factor methodology can be applied to apps written in any programming language, and which use any combination of backing services 

1. Codebase
   One codebase tracked in revision control, many deploys
2. Dependencies
   Explicitly declare and isolate dependencies
3. Config
   Store config in the environment
4. Backing services
   Treat backing services as attached resources
5. Build, release, run
   Strictly separate build and run stages
6. Processes
   Execute the app as one or more stateless processes
7. Port binding
   Export services via port binding
8. Concurrency
   Scale out via the process model
9. Disposability
   Maximize robustness with fast startup and graceful shutdown
10. Dev/prod parity
    Keep development, staging, and production as similar as possible
11. Logs
    Treat logs as event streams
12. Admin processes
    Run admin/management tasks as one-off processes¹