In 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 (database, queue, memory cache, etc).

Resources

• The 12 Factor App
• My Review & Analysis on The DevOps Handbook
• My Review & Analysis on Accelerate
• Software Development in 3 Principles & 4 Metrics

Books

• The DevOps Handbook by Gene Kim, Jez Humble, John Willis, Patrick Debois
• Accelerate by Nicole Forsgren, Jez Humble, Gene Kim