Kubernetes Container Runtime Concepts

External

• https://kubernetes.io/blog/2016/12/container-runtime-interface-cri-in-kubernetes/
• https://kubernetes.io/docs/setup/production-environment/container-runtimes/
• https://kubernetes.io/blog/2020/12/02/dont-panic-kubernetes-and-docker/

Internal

• Kubernetes Concepts
• Docker Concepts
• containerd
• CRI-O
• OCI

Container Runtime Interface (CRI)

The Container Runtime Interface (CRI) is an abstraction layer that standardizes the way third-party container runtimes interface with Kubernetes. Irrespective of the container runtime in use, the regular Kubernetes commands and patterns apply. CRI exposes a clean documented interface for third-party container runtimes to plug in to.

Among other functions, CRI exposes container resource metrics to the kubelet. TODO: https://github.com/kubernetes/community/blob/master/contributors/devel/sig-node/cri-container-stats.md

Container Runtime

Each Kubernetes node runs a container runtime, usually Docker. However, support for other container runtimes is available, via Container Runtime Interface (CRI). A container runtime performs container-related tasks such as pulling images and starting and stopping containers.

Available Container Runtimes

Docker

Kubernetes and Docker are complementary technologies. Docker is the technology that starts and stops containers under Kubernetes' supervision. Kubernetes is the higher-level technology that decides which nodes to run containers on, when to scale an application up or down or when to apply updates.

Docker

containerd

A container runtime that consists in a stripped-down version of Docker. More details:

containerd

cri-containerd

A community-based Open Source port of containerd.

CRI-O

CRI-O

Runtime Classes

Runtime classes is a feature that was introduced in Kubernetes 1.2. It allows for different classes of runtimes (gVisor, Kata Containers, etc).