So far, we have talked a lot about containers but haven't mentioned Docker yet. While Docker has been emerging as the de facto standard in containerization, it is currently one of many competing technologies in this space, and what is relevant today may not be tomorrow. For this reason, we will cover a little bit of the container ecosystem so that if you see shifts occurring in this space, don't hesitate to try another solution, as picking the right tool for the job almost always beats out trying to, as the saying goes, fit a square peg in a round hole.
While most people know Docker as the Command-line Interface (CLI) tool, the Docker platform extends above and beyond that to include tooling to create and manage clusters, handle persistent storage, build and share Docker containers, and many others, but for now, we will focus on the most important part of that ecosystem: the Docker container.
Docker containers, in essence, are a grouping of a number of filesystem layers that are stacked on top of each other in a sequence to create the final layout that is then run in an isolated environment by the host machine's kernel. Each layer describes which files have been added, modified, and/or deleted relative to its previous parent layer. For example, you have a base layer with a file /foo/bar, and the next layer adds a file /foo/baz. When the container starts, it will combine the layers in order and the resulting container will have both /foo/bar and /foo/baz. This process is repeated for any new layer to end up with a fully composed filesystem to run the specified service or services.
Think of the arrangement of the filesystem layers in an image as the intricate layering of sounds in a symphony: you have the percussion instruments in the back to provide the base for the sound, wind instruments a bit closer to drive the movements, and in the front, the string instruments with the lead melody. Together, it creates a pleasing end result. In the case of Docker, you generally have the base layers set up the main OS layers and configuration, the service infrastructure layers go on top of that (interpreter installation, the compilation of helpers, and so on), and the final image that you run is finally topped with the actual service code. For now, this is all you will need to know, but we will cover this topic in much more detail in the next chapter.
In essence, Docker in its current form is a platform that allows easy and fast development of isolated (or not depending on how the service is configured) Linux and Windows services within containers that are scalable, easily interchangeable, and easily distributable.