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28 Containers For Dummies, HPE and Docker Special Edition These materials are © 2017 John Wiley & Sons, Inc. Any dissemination, distribution, or unauthorized use is strictly prohibited. The Docker Engine The Docker Engine is the workhorse of the solution and consists of three main elements: » Docker host: This is the server that runs the Docker engine as an operating system (OS) daemon or process that lives forever. » Application programming interface (API): The Docker engine's REST API dictates how the program can interact with the daemon. » Command line interface (CLI): The Docker client's CLI is exactly what it sounds like. It's a command line that allows an administrator to interact with the API, which interacts with the server. Dockerfile Everything starts with a Dockerfile. This is a list of the exact OS commands that ultimately generate a Docker image. Images A Docker image is the result of running the docker build com- mand with the Dockerfile as the input. An image is just a static (read-only), nonrunning representation of all the various compo- nents that define the workload. Think of an image as a workload template that sits on storage, awaiting the day when it's put into service. An image can consist of multiple layers, as shown in Figure 4-1. Each layer is a set of file system changes imposed by the Docker- file. I explain a bit more about how images are assembled later in this chapter. Containers When you start up an image on a Docker host, you have a running container. You can have a lot of running containers that are based on a single image. But a running container is more than just a running image. It's an image to which a read/write layer has been attached atop all the read-only information that's present in the container.