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Container runtime

In this part of our tutorial we will focus of the container runtime.

Firt of all, container runtime is a tool which can be used by other kubernetes components (kubelet) to manage containers. In case if we have two parts of the system which communicate - we need to have some specification. Nad tehre is the cpecification - 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.

In this tutorial we will use containerd as tool for managing the containers on the node.

On other hand there is a project under the Linux Foundation - OCI.

The OCI is a project under the Linux Foundation is aims to develop open industry standards for container formats and runtimes. The primary goal of OCI is to ensure container portability and interoperability across different platforms and container runtime implementations. The OCI has two main specifications, Runtime Specification (runtime-spec) and Image Specification (image-spec).

In this tutorial we will use runc as tool for running containers.

Now, we can start with the configuration.

runc

Lets download runc binaries

wget -q --show-progress --https-only --timestamping \
  https://github.com/opencontainers/runc/releases/download/v1.0.0-rc93/runc.amd64

As download process complete, we need to move runc binaries to bin folder

{
    sudo mv runc.amd64 runc
    chmod +x runc 
    sudo mv runc /usr/local/bin/
}

Now, as we have runc installed, we can run busybox container

{
mkdir -p ~/busybox-container/rootfs/bin
cd ~/busybox-container/rootfs/bin
wget https://www.busybox.net/downloads/binaries/1.31.0-defconfig-multiarch-musl/busybox-x86_64
chmod +x busybox-x86_64
./busybox-x86_64 --install .
cd ~/busybox-container
runc spec
sed -i 's/"sh"/"echo","Hello from container runned by runc!"/' config.json
}

Now, we created all proper files, required by runc to run the container (including container confguration and files which will be accesible from container).

runc run busybox

Output:

Hello from container runned by runc!

Great, everything works, now we need to clean up our workspace

{
cd ~
rm -r busybox-container
}

containerd

As already mentioned, Container Runtime: The software responsible for running and managing containers on the worker nodes. The container runtime is responsible for pulling images, creating containers, and managing container lifecycles

Now, let's download containerd.

wget -q --show-progress --https-only --timestamping \
  https://github.com/containerd/containerd/releases/download/v1.4.4/containerd-1.4.4-linux-amd64.tar.gz

Unzip containerd binaries to the bin directory

{
    mkdir containerd
    tar -xvf containerd-1.4.4-linux-amd64.tar.gz -C containerd
    sudo mv containerd/bin/* /bin/
}

In comparison to the runc, containerd is a service which can be called by someone to run container.

Before we will run containerd service, we need to configure it.

{
sudo mkdir -p /etc/containerd/

cat << EOF | sudo tee /etc/containerd/config.toml
[plugins]
  [plugins.cri.containerd]
    snapshotter = "overlayfs"
    [plugins.cri.containerd.default_runtime]
      runtime_type = "io.containerd.runtime.v1.linux"
      runtime_engine = "/usr/local/bin/runc"
      runtime_root = ""
EOF
}

As we can see, we configured containerd to use runc (we installed before) to run containers.

Now we can configure contanerd service

cat <<EOF | sudo tee /etc/systemd/system/containerd.service
[Unit]
Description=containerd container runtime
Documentation=https://containerd.io
After=network.target

[Service]
ExecStartPre=/sbin/modprobe overlay
ExecStart=/bin/containerd
Restart=always
RestartSec=5
Delegate=yes
KillMode=process
OOMScoreAdjust=-999
LimitNOFILE=1048576
LimitNPROC=infinity
LimitCORE=infinity

[Install]
WantedBy=multi-user.target
EOF

Run service

{
  sudo systemctl daemon-reload
  sudo systemctl enable containerd
  sudo systemctl start containerd
}

Ensure if service is in running state

sudo systemctl status containerd

We should see the output like this

● containerd.service - containerd container runtime
     Loaded: loaded (/etc/systemd/system/containerd.service; enabled; vendor preset: enabled)
     Active: active (running) since Sat 2023-04-15 21:04:43 UTC; 59s ago
       Docs: https://containerd.io
    Process: 1018 ExecStartPre=/sbin/modprobe overlay (code=exited, status=0/SUCCESS)
   Main PID: 1031 (containerd)
      Tasks: 9 (limit: 2275)
     Memory: 22.0M
     CGroup: /system.slice/containerd.service
             └─1031 /bin/containerd
...

As we have running containerd service, we can run some containers.

To do that, we need the tool called ctr, it is distributed as part of containerd.

Lets pull busbox image

sudo ctr images pull docker.io/library/busybox:latest

And check if it is presented on our server

ctr images ls

Output:

REF                              TYPE                                                      DIGEST                                                                  SIZE    PLATFORMS                                                                                                                          LABELS
docker.io/library/busybox:latest application/vnd.docker.distribution.manifest.list.v2+json sha256:b5d6fe0712636ceb7430189de28819e195e8966372edfc2d9409d79402a0dc16 2.5 MiB linux/386,linux/amd64,linux/arm/v5,linux/arm/v6,linux/arm/v7,linux/arm64/v8,linux/mips64le,linux/ppc64le,linux/riscv64,linux/s390x -

Now, lets start our container.

ctr run --rm --detach docker.io/library/busybox:latest busybox-container sh -c 'echo "Hello from container runned by containerd!"'

Output:

Hello from container runned by containerd!

Now, lets clean-up.

ctr task rm busybox-container

Next: Kubelet

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Container runtime

runc

containerd

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