**Finally decided to put this up after I writing it up a year ago. Sure hope someone finds it useful though slightly outdated**
It is amazing to see how quickly Docker has picked up pace in the tech industry with a lot of momentum in its sails for years to come. It is early and a great time to get on board. There are so many resources out there describing what Docker is, but so few articulate it as well as Nigel Poulton on Pluralsight (Docker deep dive tutorial). In fact I credit all descriptions here to him. I will not be one bit offended if you stop reading here to check out his tutorial and never come back here.
My aim here is to try to describe 4 key components that are core to Docker : The Docker Engine, Docker Images, Docker Containers and Docker Repos/Registry. This is part of my attempt to wrap my mind around this technology.
If you have perused the web on what docker is you probably may have come across the dockyard analogy. After a while it gets kinda boring to hear just one analogy, but it’s amazing to see how perfectly it describes it. The descriptions here do tag along this analogy, but also attempts to break each component down using simple words.
The Docker Engine
This is also referred to as the docker daemon. In the shipping analogy, think of the docker engine as the shipping yard. Essentially, it is a program that provides standardized app infrastructure and runtime dependencies needed to run containers. So if you think about this in the context of the analogy. The shipping yard has standard cranes, paths and machines which hardly ever vary from dock to dock. So a worker who goes to another shipping yard even in a different country should easily be able to operate the machinery upon arrival because the infrastructure is standardized and optimized to ship containers. This is how docker provides portability.
The Docker Image
This is the manifest in the shipping analogy. Essentially, it is a static or frozen version of a container that contains data and metadata needed to fire up a container. Typically a manifest describes what is being shipped and probably comes with instructions on how to handle the shipment.
The Dock Container
Well this is where the goods go in. Simply put, it is the running instance of an image. It is a light weight instance of linux
Docker Repositories and Registries
The best way to think about this really is the Apple app store or Google play store. Think of each app as a repo and the app store as the registry. Better yet, you can also think of this in terms of Github as the registry and the repos, well as repos. The thing to keep in mind here is that some images are verified and not all images can be trusted. Images which don’t require user prefix (like Ubuntu, Nginx, Mongodb etc ) have usually been verified by the docker team. Said more formally repos are where images are pulled from and registry is where images live (docker hub being the default)
I know this was not deep and comprehensive. I can share more on what I have grasped so far, but I’m not expert by any means. I do hope you are one step closer to nailing down this technology. Now go check out Nigel’s tutorial.