Check whether Dockershim deprecation affects you
dockershim component of Kubernetes allows to use Docker as a Kubernetes's
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
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.
- Check that scripts and apps running on nodes outside of 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
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
commands. As you cannot list containers, you cannot get logs, stop containers,
or execute something inside container using
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