This document goes over some frequently asked questions regarding the Dockershim
deprecation announced as a part of the Kubernetes v1.20 release. For more detail
on the deprecation of Docker as a container runtime for Kubernetes kubelets, and
what that means, check out the blog post
Don’t Panic: Kubernetes and Docker.

Why is dockershim being deprecated?

Maintaining dockershim has become a heavy burden on the Kubernetes maintainers.
The CRI standard was created to reduce this burden and allow smooth interoperability
of different container runtimes. Docker itself doesn’t currently implement CRI,
thus the problem.

Dockershim was always intended to be a temporary solution (hence the name: shim).
You can read more about the community discussion and planning in the
Dockershim Removal Kubernetes Enhancement Proposal.

Additionally, features that were largely incompatible with the dockershim, such
as cgroups v2 and user namespaces are being implemented in these newer CRI
runtimes. Removing support for the dockershim will allow further development in
those areas.

Can I still use Docker in Kubernetes 1.20?

Yes, the only thing changing in 1.20 is a single warning log printed at kubelet
startup if using Docker as the runtime.

When will dockershim be removed?

Given the impact of this change, we are using an extended deprecation timeline.
It will not be removed before Kubernetes 1.22, meaning the earliest release without
dockershim would be 1.23 in late 2021. We will be working closely with vendors
and other ecosystem groups to ensure a smooth transition and will evaluate things
as the situation evolves.

Will my existing Docker images still work?

Yes, the images produced from docker build will work with all CRI implementations.
All your existing images will still work exactly the same.

What about private images?

Yes. All CRI runtimes support the same pull secrets configuration used in
Kubernetes, either via the PodSpec or ServiceAccount.

Are Docker and containers the same thing?

Docker popularized the Linux containers pattern and has been instrumental in
developing the underlying technology, however containers in Linux have existed
for a long time. The container ecosystem has grown to be much broader than just
Docker. Standards like OCI and CRI have helped many tools grow and thrive in our
ecosystem, some replacing aspects of Docker while others enhance existing
functionality.

Are there examples of folks using other runtimes in production today?

All Kubernetes project produced artifacts (Kubernetes binaries) are validated
with each release.

Additionally, the kind project has been using containerd for some time and has
seen an improvement in stability for its use case. Kind and containerd are leveraged
multiple times every day to validate any changes to the Kubernetes codebase. Other
related projects follow a similar pattern as well, demonstrating the stability and
usability of other container runtimes. As an example, OpenShift 4.x has been
using the CRI-O runtime in production since June 2019.

For other examples and references you can look at the adopters of containerd and
CRI-O, two container runtimes under the Cloud Native Computing Foundation (CNCF).

People keep referencing OCI, what is that?

OCI stands for the Open Container Initiative, which standardized many of the
interfaces between container tools and technologies. They maintain a standard
specification for packaging container images (OCI image-spec) and running containers
(OCI runtime-spec). They also maintain an actual implementation of the runtime-spec
in the form of runc, which is the underlying default runtime for both
containerd and CRI-O. The CRI builds on these low-level specifications to
provide an end-to-end standard for managing containers.

Which CRI implementation should I use?

That’s a complex question and it depends on a lot of factors. If Docker is
working for you, moving to containerd should be a relatively easy swap and
will have strictly better performance and less overhead. However, we encourage you
to explore all the options from the CNCF landscape in case another would be an
even better fit for your environment.

What should I look out for when changing CRI implementations?

While the underlying containerization code is the same between Docker and most
CRIs (including containerd), there are a few differences around the edges. Some
common things to consider when migrating are:

  • Logging configuration
  • Runtime resource limitations
  • Node provisioning scripts that call docker or use docker via it’s control socket
  • Kubectl plugins that require docker CLI or the control socket
  • Kubernetes tools that require direct access to Docker (e.g. kube-imagepuller)
  • Configuration of functionality like registry-mirrors and insecure registries
  • Other support scripts or daemons that expect Docker to be available and are run
    outside of Kubernetes (e.g. monitoring or security agents)
  • GPUs or special hardware and how they integrate with your runtime and Kubernetes

If you use Kubernetes resource requests/limits or file-based log collection
DaemonSets then they will continue to work the same, but if you’ve customized
your dockerd configuration, you’ll need to adapt that for your new container
runtime where possible.

Another thing to look out for is anything expecting to run for system maintenance
or nested inside a container when building images will no longer work. For the
former, you can use the crictl tool as a drop-in replacement (see mapping from docker cli to crictl) and for the
latter you can use newer container build options like img, buildah,
kaniko, or buildkit-cli-for-kubectl that don’t require Docker.

For containerd, you can start with their documentation to see what configuration
options are available as you migrate things over.

For instructions on how to use containerd and CRI-O with Kubernetes, see the
Kubernetes documentation on Container Runtimes

What if I have more questions?

If you use a vendor-supported Kubernetes distribution, you can ask them about
upgrade plans for their products. For end-user questions, please post them
to our end user community forum: https://discuss.kubernetes.io/.

You can also check out the excellent blog post
Wait, Docker is deprecated in Kubernetes now? a more in-depth technical
discussion of the changes.

Can I have a hug?

Always and whenever you want! 🤗🤗

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