All articles tagged #container-image

From time to time I use kind as a local Kubernetes playground. It's super-handy, real quick, and 100% disposable.

Up until recently, all the scenarios I've tested with kind were using public container images. However, a few days ago, I found myself in a situation where I needed to run a pod using an image that I had just built on my laptop.

One way of doing it would be pushing the image to a local or remote registry accessible from inside the kind Kubernetes cluster. However, kind still doesn't spin up a local registry out of the box (you can vote for the GitHub issue here) and I'm not a fan of sending stuff over the Internet without very good reasons.

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You need containers to build images. Yes, you've heard it right. Not another way around.

For people who found their way to containers through Docker (well, most of us I believe) it may seem like images are of somewhat primary nature. We've been taught to start from a Dockerfile, build an image using that file, and only then run a container from that image. Alternatively, we could run a container specifying an image from a registry, yet the main idea remains - an image comes first, and only then the container.

But what if I tell you that the actual workflow is reverse? Even when you are building your very first image using Docker, podman, or buildah, you are already, albeit implicitly, running containers under the hood!

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As we already know, containers are just isolated and restricted Linux processes. We also learned that it's fairly simple to create a container with a single executable file inside starting from scratch image (i.e. without putting a full Linux distribution in there). This time we will go even further and demonstrate that containers don't require images at all. And after that, we will try to justify the actual need for images and their place in the containerverse.

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Not every container has an operating system inside, but every one of them needs your Linux kernel.

Before going any further it's important to understand the difference between a kernel, an operating system, and a distribution.

• Linux kernel is the core part of the Linux operating system. It's what originally Linus wrote.
• Linux OS is a combination of the kernel and a user-land (libraries, GNU utilities, config files, etc).
• Linux distribution is a particular version of the Linux operating system like Debian or CentOS.

To be technically accurate, the title of this article should have sounded something like Does container image have a whole Linux distribution inside? But I find this wording a bit boring for a title 🤪

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I've been working on adding basic images support to my experimental container manager and to my surprise, the task turned to be more complex than I initially expected. I spent some time looking for ways to manage container images directly from my application code. There is plenty of tools out there (docker, containerd, podman, buildah, cri-o, etc) providing image management capabilities. However, if you don't want to have a dependency on an external daemon running in your system, or you don't feel like shelling out for exec-ing a command-line tool from the code, the options are at best limited.

I've reviewed a bunch of the said tools focusing on the underlying means they use to deal with images and at last, I found two appealing libraries. The first one is github.com/containers/image library "[...] aimed at working in various way with containers' images and container image registries". The second one is github.com/containers/storage "[...] which aims to provide methods for storing filesystem layers, container images, and containers". The libraries are meant to be used in conjunction and form a very powerful image management tandem. But unfortunately, I could not find a sufficient amount of documentation, especially how to get started kind of it.

Without the docs the only way to learn how to use the libraries for me was to analyze the code of their dependants (most prominently - buildah and cri-o). It took me a while to forge a working example which is capable of:

• pulling images from remote repositories;
• storing images locally;
• creating and mounting containers (i.e. writable instances of images).

In the rest of the article, I'll try to show how to use the libraries to perform the said task and highlight the most interesting parts of this journey.

Disclaimer: This is by no means an attempt to fully or even partially document the libraries!

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