Introduction
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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).