Migrating from dockershim
This section presents information you need to know when migrating from
dockershim to other container runtimes.
Since the announcement of dockershim deprecation
in Kubernetes 1.20, there were questions on how this will affect various workloads and Kubernetes
installations. Our Dockershim Removal FAQ is there to help you
to understand the problem better.
It is recommended to migrate from dockershim to alternative container runtimes.
Check out container runtimes
section to know your options. Make sure to
report issues you encountered
with the migration. So the issue can be fixed in a timely manner and your cluster would be
ready for dockershim removal.
1 - Changing the Container Runtime on a Node from Docker Engine to containerd
This task outlines the steps needed to update your container runtime to containerd from Docker. It is applicable for cluster operators running Kubernetes 1.23 or earlier. Also this covers an example scenario for migrating from dockershim to containerd and alternative container runtimes can be picked from this page.
Before you begin
Note:
This section links to third party projects that provide functionality required by Kubernetes. The Kubernetes project authors aren't responsible for these projects, which are listed alphabetically. To add a project to this list, read the
content guide before submitting a change.
More information.
Install containerd. For more information see, containerd's installation documentation and for specific prerequisite follow this.
Drain the node
# replace <node-to-drain> with the name of your node you are draining
kubectl drain <node-to-drain> --ignore-daemonsets
Stop the Docker daemon
systemctl stop kubelet
systemctl disable docker.service --now
Install Containerd
This page contains detailed steps to install containerd.
-
Install the containerd.io package from the official Docker repositories.
Instructions for setting up the Docker repository for your respective Linux distribution and installing the containerd.io package can be found at
Install Docker Engine.
-
Configure containerd:
sudo mkdir -p /etc/containerd
containerd config default | sudo tee /etc/containerd/config.toml
-
Restart containerd:
sudo systemctl restart containerd
Start a Powershell session, set $Version to the desired version (ex: $Version="1.4.3"), and then run the following commands:
-
Download containerd:
curl.exe -L https://github.com/containerd/containerd/releases/download/v$Version/containerd-$Version-windows-amd64.tar.gz -o containerd-windows-amd64.tar.gz
tar.exe xvf .\containerd-windows-amd64.tar.gz
-
Extract and configure:
Copy-Item -Path ".\bin\" -Destination "$Env:ProgramFiles\containerd" -Recurse -Force
cd $Env:ProgramFiles\containerd\
.\containerd.exe config default | Out-File config.toml -Encoding ascii
# Review the configuration. Depending on setup you may want to adjust:
# - the sandbox_image (Kubernetes pause image)
# - cni bin_dir and conf_dir locations
Get-Content config.toml
# (Optional - but highly recommended) Exclude containerd from Windows Defender Scans
Add-MpPreference -ExclusionProcess "$Env:ProgramFiles\containerd\containerd.exe"
-
Start containerd:
.\containerd.exe --register-service
Start-Service containerd
Edit the file /var/lib/kubelet/kubeadm-flags.env and add the containerd runtime to the flags. --container-runtime=remote and --container-runtime-endpoint=unix:///run/containerd/containerd.sock"
For users using kubeadm should consider the following:
The kubeadm tool stores the CRI socket for each host as an annotation in the Node object for that host.
To change it you must do the following:
Execute kubectl edit no <NODE-NAME> on a machine that has the kubeadm /etc/kubernetes/admin.conf file.
This will start a text editor where you can edit the Node object.
To choose a text editor you can set the KUBE_EDITOR environment variable.
-
Change the value of kubeadm.alpha.kubernetes.io/cri-socket from /var/run/dockershim.sock
to the CRI socket path of your choice (for example unix:///run/containerd/containerd.sock).
Note that new CRI socket paths must be prefixed with unix:// ideally.
-
Save the changes in the text editor, which will update the Node object.
Restart the kubelet
Verify that the node is healthy
Run kubectl get nodes -o wide and containerd appears as the runtime for the node we just changed.
Remove Docker Engine
Note:
This section links to third party projects that provide functionality required by Kubernetes. The Kubernetes project authors aren't responsible for these projects, which are listed alphabetically. To add a project to this list, read the
content guide before submitting a change.
More information.
Finally if everything goes well remove docker
sudo yum remove docker-ce docker-ce-cli
sudo apt-get purge docker-ce docker-ce-cli
sudo dnf remove docker-ce docker-ce-cli
sudo apt-get purge docker-ce docker-ce-cli
2 - Find Out What Container Runtime is Used on a Node
This page outlines steps to find out what container runtime
the nodes in your cluster use.
Depending on the way you run your cluster, the container runtime for the nodes may
have been pre-configured or you need to configure it. If you're using a managed
Kubernetes service, there might be vendor-specific ways to check what container runtime is
configured for the nodes. The method described on this page should work whenever
the execution of kubectl is allowed.
Before you begin
Install and configure kubectl. See Install Tools section for details.
Find out the container runtime used on a Node
Use kubectl to fetch and show node information:
kubectl get nodes -o wide
The output is similar to the following. The column CONTAINER-RUNTIME outputs
the runtime and its version.
# For dockershim
NAME STATUS VERSION CONTAINER-RUNTIME
node-1 Ready v1.16.15 docker://19.3.1
node-2 Ready v1.16.15 docker://19.3.1
node-3 Ready v1.16.15 docker://19.3.1
# For containerd
NAME STATUS VERSION CONTAINER-RUNTIME
node-1 Ready v1.19.6 containerd://1.4.1
node-2 Ready v1.19.6 containerd://1.4.1
node-3 Ready v1.19.6 containerd://1.4.1
Find out more information about container runtimes
on Container Runtimes page.
3 - Check whether Dockershim deprecation affects you
The dockershim component of Kubernetes allows to use Docker as a Kubernetes's
container runtime.
Kubernetes' built-in dockershim component was deprecated in release v1.20.
This page explains how your cluster could be using Docker as a container runtime,
provides details on the role that dockershim plays when in use, and shows steps
you can take to check whether any workloads could be affected by dockershim deprecation.
Finding if your app has a dependencies on Docker
If you are using Docker for building your application containers, you can still
run these containers on any container runtime. This use of Docker does not count
as a dependency on Docker as a container runtime.
When alternative container runtime is used, executing Docker commands may either
not work or yield unexpected output. This is how you can find whether you have a
dependency on Docker:
- Make sure no privileged Pods execute Docker commands (like
docker ps),
restart the Docker service (commands such as systemctl restart docker.service),
or modify Docker-specific files such as /etc/docker/daemon.json.
- Check for any private registries or image mirror settings in the Docker
configuration file (like
/etc/docker/daemon.json). Those typically need to
be reconfigured for another container runtime.
- Check that scripts and apps running on nodes outside of your Kubernetes
infrastructure do not execute Docker commands. It might be:
- SSH to nodes to troubleshoot;
- Node startup scripts;
- Monitoring and security agents installed on nodes directly.
- Third-party tools that perform above mentioned privileged operations. See
Migrating telemetry and security agents from dockershim
for more information.
- Make sure there is no indirect dependencies on dockershim behavior.
This is an edge case and unlikely to affect your application. Some tooling may be configured
to react to Docker-specific behaviors, for example, raise alert on specific metrics or search for
a specific log message as part of troubleshooting instructions.
If you have such tooling configured, test the behavior on test
cluster before migration.
Dependency on Docker explained
A container runtime is software that can
execute the containers that make up a Kubernetes pod. Kubernetes is responsible for orchestration
and scheduling of Pods; on each node, the kubelet
uses the container runtime interface as an abstraction so that you can use any compatible
container runtime.
In its earliest releases, Kubernetes offered compatibility with one container runtime: Docker.
Later in the Kubernetes project's history, cluster operators wanted to adopt additional container runtimes.
The CRI was designed to allow this kind of flexibility - and the kubelet began supporting CRI. However,
because Docker existed before the CRI specification was invented, the Kubernetes project created an
adapter component, dockershim. The dockershim adapter allows the kubelet to interact with Docker as
if Docker were a CRI compatible runtime.
You can read about it in Kubernetes Containerd integration goes GA blog post.
Switching to Containerd as a container runtime eliminates the middleman. All the
same containers can be run by container runtimes like Containerd as before. But
now, since containers schedule directly with the container runtime, they are not visible to Docker.
So any Docker tooling or fancy UI you might have used
before to check on these containers is no longer available.
You cannot get container information using docker ps or docker inspect
commands. As you cannot list containers, you cannot get logs, stop containers,
or execute something inside container using docker exec.
Note: If you're running workloads via Kubernetes, the best way to stop a container is through
the Kubernetes API rather than directly through the container runtime (this advice applies
for all container runtimes, not only Docker).
You can still pull images or build them using docker build command. But images
built or pulled by Docker would not be visible to container runtime and
Kubernetes. They needed to be pushed to some registry to allow them to be used
by Kubernetes.
4 - Migrating telemetry and security agents from dockershim
Kubernetes' support for direct integration with Docker Engine is deprecated, and will be removed. Most apps do not have a direct dependency on runtime hosting containers. However, there are still a lot of telemetry and monitoring agents that has a dependency on docker to collect containers metadata, logs and metrics. This document aggregates information on how to detect these dependencies and links on how to migrate these agents to use generic tools or alternative runtimes.
Telemetry and security agents
Within a Kubernetes cluster there are a few different ways to run telemetry or security agents.
Some agents have a direct dependency on Docker Engine when they as DaemonSets or
directly on nodes.
Why do some telemetry agents communicate with Docker Engine?
Historically, Kubernetes was written to work specifically with Docker Engine.
Kubernetes took care of networking and scheduling, relying on Docker Engine for launching
and running containers (within Pods) on a node. Some information that is relevant to telemetry,
such as a pod name, is only available from Kubernetes components. Other data, such as container
metrics, is not the responsibility of the container runtime. Early yelemetry agents needed to query the
container runtime and Kubernetes to report an accurate picture. Over time, Kubernetes gained
the ability to support multiple runtimes, and now supports any runtime that is compatible with
the container runtime interface.
Some telemetry agents rely specifically on Docker Engine tooling. For example, an agent
might run a command such as
docker ps
or docker top to list
containers and processes or docker logs
to receive streamed logs. If nodes in your existing cluster use
Docker Engine, and you switch to a different container runtime,
these commands will not work any longer.
Identify DaemonSets that depend on Docker Engine
If a pod wants to make calls to the dockerd running on the node, the pod must either:
- mount the filesystem containing the Docker daemon's privileged socket, as a
volume; or
- mount the specific path of the Docker daemon's privileged socket directly, also as a volume.
For example: on COS images, Docker exposes its Unix domain socket at
/var/run/docker.sock This means that the pod spec will include a
hostPath volume mount of /var/run/docker.sock.
Here's a sample shell script to find Pods that have a mount directly mapping the
Docker socket. This script outputs the namespace and name of the pod. You can
remove the grep '/var/run/docker.sock' to review other mounts.
kubectl get pods --all-namespaces \
-o=jsonpath='{range .items[*]}{"\n"}{.metadata.namespace}{":\t"}{.metadata.name}{":\t"}{range .spec.volumes[*]}{.hostPath.path}{", "}{end}{end}' \
| sort \
| grep '/var/run/docker.sock'
Note: There are alternative ways for a pod to access Docker on the host. For instance, the parent
directory
/var/run may be mounted instead of the full path (like in
this
example).
The script above only detects the most common uses.
Detecting Docker dependency from node agents
In case your cluster nodes are customized and install additional security and
telemetry agents on the node, make sure to check with the vendor of the agent whether it has dependency on Docker.
Telemetry and security agent vendors
We keep the work in progress version of migration instructions for various telemetry and security agent vendors
in Google doc.
Please contact the vendor to get up to date instructions for migrating from dockershim.