Working with container images in Go

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!

Read more

Implementing Container Runtime Shim: Interactive Containers

In the previous articles, we discussed the scope of the container runtime shim and drafted the minimum viable version. Now, it's time to move on and have some fun with more advanced scenarios! Have you ever wondered how docker run -i or kubectl run --stdin work? If so, this article is for you! We will try to replicate this piece of functionality in our experimental container manager. And as you have probably guessed, the container runtime shim will do a lot of heavy lifting here again.

conman - interactive container demo

Read more

conman - [the] Container Manager: Inception

With this article, I want to start a series about the implementation of a container manager. What the heck is a container manager? Some prominent examples would be containerd, cri-o, dockerd, and podman. People here and there keep calling them container runtimes, but I would like to reserve the term runtime for a lower-level thingy - the OCI runtime (de facto runc), and a higher-level component controlling multiple such runtime instances I'd like to call a container manager. In general, by a container manager, I mean a piece of software doing a complete container lifecycle management on a single host. In the following series, I will try to guide you myself through the challenge of the creation of yet another container manager. By no means, the implementation is going to be feature-complete, correct or safe to use. The goal is rather to prove the already proven concept. So, mostly for the sake of fun, let the show begin!

Read more