It’s time to begin building an app the Docker way. We start at the bottom of the hierarchy of such an app, which is a container, which we cover on this page. Above this level is a service, which defines how containers behave in production, covered in Part 3. Finally, at the top level is the stack, defining the interactions of all the services, covered in Part 5.

• Stack
• Services
• Container (you are here)

An image is a lightweight, stand-alone, executable package that includes everything needed to run a piece of software, including the code, a runtime, libraries, environment variables, and config files.

A container is a runtime instance of an image—what the image becomes in memory when actually executed. It runs completely isolated from the host environment by default, only accessing host files and ports if configured to do so.

Containers run apps natively on the host machine’s kernel. They have better performance characteristics than virtual machines that only get virtual access to host resources through a hypervisor. Containers can get native access, each one running in a discrete process, taking no more memory than any other executable.

stack. A stack is a group of interrelated services that share dependencies and can be orchestrated and scaled together. A single stack is capable of defining and coordinating the functionality of an entire application (though very complex applications may want to use multiple stacks).

Compose is a tool for defining and running multi-container Docker applications. With Compose, you use a YAML file to configure your application’s services. Then, with a single command, you create and start all the services from your configuration.