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).