Introduction to CRI

Introduction to CRI

Let's learn about what CRI actually means


  • Docker
  • Kubernetes

What is CRI?

The CRI is a plugin interface which enables the kubelet to use a wide variety of container runtimes, without having a need to recompile the cluster components.

What is the Open container initiative (OCI)?

The Open Container Initiative is an open governance structure for the express purpose of creating open industry standards around container formats and runtimes.


It is a CNCF Graduated project which is OCI Standard. Its is a container manager tool. All other container management tools make use of containerd to run containers Like Kubernetes, Docker, Nerdctl Image

Why learn it?

However, since it's the closest thing to the actual containerd API, it can serve as a great exploration means - by examining the available commands, you can get a rough idea of what containerd can and cannot do. You can also replace the other tools with containerd + nerdctl in your local environment. It is also used in kubernetes as default container runtime for debugging

Nerdctl, Crictl, ctr

These are CLI tools for running and managing the containers being run by containerd Among these ctr is pre shipped when containerd is installed, and you’ve to install crictl and nerdctl manually.

Local Setup


One of the best hassle-free way to install containerd along with nerdctl is to use arkade. If you’re hearing about arkade for the first time, checkout the blog here

arkade system install containerd 
arkade get nerdctl 
arkade system install cni -p /usr/libexec/cni

Package managers

The packages in DEB and RPM formats are distributed by Docker (not by the containerd project). See the Docker documentation for how to set up apt-get or dnf to install packages:

CentOS, Debian, Fedora, Ubuntu The package contains runc too, but does not contain CNI plugins.

sudo yum install -y
Sudo apt install -y

We need CNI (Container networking interface) Installing CNI plugins Download the cni-plugins---.tgz archive from Tar File to download, verify its sha256sum, and extract it under /opt/cni/bin

$ mkdir -p /opt/cni/bin
$ tar Cxzvf /opt/cni/bin cni-plugins-linux-amd64-<Version>.tgz

Install Nerdctl via brew

$ brew install nerdctl
# do this when you need access to nerdctl in root user (Root install)
$ sudo cp -v /home/linuxbrew/.linuxbrew/bin/nerdctl /usr/local/bin/
$ sudo nerdctl version

What are the options and why in ctr

ctr run <image-ref> <container-id>

So all setup is done, let's go through some concepts

What is namespace, tasks in containerd terms

Task is the runtime state of the container.

Namespace in the context of containerd is the logical separation between tools which use containerd like docker has namespace of moby and nerdctl uses default.

NOTE: Instead of building images with ctr, you can import existing images built with docker build or other OCI-compatible software. Surprisingly, containerd doesn't provide out-of-the-box image building support. However, containerd itself is often used to build images by higher-level tools.

Start and stop container

# When pulling images, the fully-qualified reference seems to be required, so you cannot omit the registry or the tag part
$ ctr images pull
$ ctr run hello

Hello from Docker!

$ ctr -namespace default container ls

$ ctr container rm hello

When you do ctr create container, then task are not automatically created

Screenshot from 2022-09-04 09-03-26.png

When using ctr run task is created

Screenshot from 2022-09-04 09-07-03.png

Create Namespace

$ ctr ns create <name>

$ctr ns rm <name>

ctr run command is actually a shortcut for ctr container create + ctr task start

Lets exec into a container which is running in detached mode

Screenshot from 2022-09-04 09-15-12.png

Now to stop the container

Screenshot from 2022-09-04 09-21-30.png

Got error

# first stop the task
ctr task kill nginx
ctr task ls
# will see that  the taks is now stopper state

ctr c rm nginx

Now you understand that it's not very practical to use ctr in day to day use only for debugging, so there is another alternative to docker cli which is nerdctl which is almost compatible with most of the docker commands and is easy to use

Translate docker learnings to nerdctl

Almost all the docker commands are there in nerdctl (Very flat learning curve) with some additional as namespace create and delete ….

If you want, you can also set an alias using the command alias docker=nerdctl and keep using the docker command, and it will use nerdctl under the hood.

Playground Link

Some more resource Live stream on containerd and nerdctl


  • Anurag
  • Dipankar

Like what you read? Follow Kubesimplify on Hashnode, Twitter and LinkedIn. Join our Discord server to learn with us.

Did you find this article valuable?

Support Kubesimplify by becoming a sponsor. Any amount is appreciated!