Container Runtime

In summary;

• containerd provides a container runtime
• ctr is a low-level debugging, mainly for debugging contained
• nerdctl is a general purpose, Docker-like CLI (alternative to Docker) to containerd
• crictl interacts with any CRI runtime, mainly for inspecting and debugging the container runtimes, it's good for Kubernetes users.

Why Kubernetes no need to install Docker?

In summary, Kubernetes supports other runtime engines that adhere to the OCI standards, like containerd or Rocket, so it is not necessary to install Docker before installing Kubernetes.

Kubernetes only needs the Container Runtime Interface (CRI) to work as a container runtime to perform the container operations as long as they adhere to the OCI standards. Therefore, Kubernetes is compatible with any runtime engines via CRI, such as Rocket, containerd, etc.

containerD

If you don't need Docker's other features, you can just install containerD without installing Docker. When you install containerD, it will auto install it's command line called "ctr". This tool is solely made for debugging containerd and it's not user-friendly, but it gives you a great way to understand how containers work under the hood.

You can refer this link ctr (opens in a new tab) to understand more ctr CLI options.

ctr --help
ctr images pull docker.io/library/redis:alpine
ctr run docker.io/library/redis:alpine redis

Therefore, there is an alternative tool called nerdctl (opens in a new tab) that provides a stable, human-friendly user experience, and it is mainly used for general purposes. It's very similar to Docker command.

• Same UI/UX (Docker-like CLI) as docker
• Supports Docker Compose nerdctl compose up
• Supports rootless mode, without slirp overhead
• Supports lazy-pulling
• Supports encrypted container images
• Supports P2P image distribution
• Supports container image image signing and verifying
• Apply Kubernetes namespaces for containers, etc

nerdctl
nerdctl run -it ubuntu
nerdctl run -p 8080:8080 -d webapplication
nerdctl compose -f ./docker-compose.yaml up

There is another tool called crictl (opens in a new tab), which provides a CLI for CRI-compatible container runtimes like "containerd".

• This tool need to install separately
• It can interact with any CRI runtime, so the developers can debug the runtime without having to install Kubernetes components
• It's used to inspect and debug container runtimes and it is ideal not to create containers

crictl help
crictl pull busybox
crictl images
crictl ps -a
crictl pods

If you try to use crictl to create containers on the Kubernetes environment, kubelet will be going to delete them as kubelet ensures a specific number of pods are available on a single node, that means containers created outside Kubelet's environment are unknown to it. So this tool is mainly used for debugging purposes (exec into container).