Dockershim / Container Runtime Interface in the Context of Kubernetes.
What is Shim?
As per Wikipedia: In computer programming, a shim is a library that transparently intercepts API calls and changes the arguments passed, handles the operation itself, or redirects the operation elsewhere.[1][2] Shims can be used to support an old API in a newer environment or a new API in an older environment. Shims can also be used for running programs on different software platforms than they were developed for.
Shims for older APIs typically come about when the behavior of an API changes, thereby causing compatibility issues for older applications that still rely on the older functionality; in such cases, the older API can still be supported by a thin compatibility layer on top of the newer code. Shims for newer APIs are defined as: “a library that brings a new API to an older environment, using only the means of that environment.”[3]
Why Kubernetes used Dockershim?
Kubernetes uses Container runtime interface(CRI) to talk with the container, as docker does not have any in-built CRI, Kubernetes used dockershim/shim as CRI to talk with docker container. The CRI of docker (a.k.a. dockershim) is part of kubelet code, runs as part of kubelet, and is tightly coupled with kubelet’s lifecycle.
The architecture is as below:
- The dockershim is an alternate for Docker’s UX developed by Kubernetes to act as a bridge between Kubelet and Docker
- Docker engine is used as container runtime on the Kubernetes worker nodes .
- The Kubelet communicates the container creation request to dockershim through gRPC as they run on the same host.
- Dockershim forwards the request to Docker daemon, which again rerouted to containerd to invoke the OCI binary(runC) with an OCI bundle to create a container.
Why the deprecation of Dockershim now?
Kubernetes planned deprecation and later removal of Dockershim in Oct 2020. The reasons are:
- To promote open container initiative(OCI) and not to treat docker as a special one.
- Make releases of kubelet and other Kubernetes services independent of docker releases.
- The Kubernetes community can focus and move faster on the new container runtime-related enhancements after dropping dockershim.
Kubernetes is going to remove Dockershim completely after the 1.22 release.
What else?
Please refer to the Kubernetes FAQ(https://kubernetes.io/blog/2020/12/02/dockershim-faq/) for more details.
