Docker is a containerisation system. Containerisation is also known as operating-system-level virtualisation. It allows multiple independent containers to run on a single host. The containers are isolated from each other and from the host.
Resource isolation features make it possible for the containers to share underlying operating system resources. Whereas more traditional virtual machines replicate an entire operating system, containerisation can provide a much more lightweight solution to virtualisation, containing only the specific stack layers required for a particular application.
Docker containers are thus smaller and consume fewer resources, avoiding the memory and CPU overheads of full virtualisation. They are faster to start up, manage and scale, and easier to move, around than full virtual machines.
Docker on Macintosh and Windows¶
Docker requires a Linux host for its containers. On Linux systems, containers will simply use the Linux operating system’s resources. Macintosh and Windows need to run a single Linux virtual machine to serve as the host.
This can be done in two ways:
On newer systems, the Alpine Linux host is provided through native operating system virtualisation.
(On Macintosh, it’s provided by through HyperKit, a lightweight virtualisation system built on top of the Hypervisor.framework (macOS 10.10 Yosemite and higher).
On Windows, it’s provided through a similar system, Microsoft’s Hyper-V (Windows 10 Professional, Enterprise and Education editions).)
On older systems, it requires a Linux virtual machine running in VirtualBox. This is managed by a tool called Docker Toolbox.
The two key components in Docker are:
Docker Engine, the underlying daemon running on the host. Docker Engine is also confusingly sometimes referred to simply as Docker (to make things worse, there is also a command-line tool named
Docker Compose (invoked as
docker-compose) is a tool for defining and managing multi-container applications; Divio projects use Docker Compose only in the local environment.
Docker terminology uses “application” in much the same way that Django uses “project”, a collection of components that together form a complete and self-contained system.
In our case, a Docker application is the collection of components that is responsible for a website and its functionality, including everything from the database to the frontend code.
A Docker application will typically include multiple containers.
A Docker container is virtualised application environment. Unlike a virtual server, it doesn’t need to provide every layer in a full working system. Instead, it encapsulates only the layers required to run an application.
An image is a template. Each container is based on an image. Once an image has been created, each container created from it will provide exactly the same environment, and the applications in it will behave identically. An image is defined by its Dockerfile.