Ansible: Up and Running, 2nd Edition

Chapter 15. Docker

The Docker project has taken the IT world by storm. I can’t think of another technology that was so quickly embraced by the community. This chapter covers how to use Ansible to create Docker images and deploy Docker containers.

What Is a Container?

A container is a form of virtualization. When you use virtualization to run processes in a guest operating system, these guest processes have no visibility into the host operating system that runs on the physical hardware. In particular, processes running in the guest are not able to directly access physical resources, even if these guest processes are provided with the illusion that they have root access.

Containers are sometimes referred to as operating system virtualization to distinguish them from hardware virtualization technologies. In hardware virtualization, a program called the hypervisor virtualizes an entire physical machine, including a virtualized CPU, memory, and devices such as disks and network interfaces. Because the entire machine is virtualized, hardware virtualization is flexible. In particular, you can run an entirely different operating system in the guest than in the host (e.g., running a Windows Server 2012 guest inside a Red Hat Enterprise Linux host), and you can suspend and resume a virtual machine just as you can a physical machine. This flexibility brings with it additional overhead needed to virtualize the hardware.

With operating system virtualization (containers), the guest processes ...