Twelve-factor app

A set of conventions for applications that are going to be run as disposable processes by a platform: config in the environment, backing services as attached resources, stateless processes, logs as a stream. It predates containers and describes them almost exactly, which is why it is still the fastest way to explain why an application resists being containerised. The failures it names are concrete — a config file baked into the image, a process that writes logs to a path, a session held in local memory.