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Architecture of The CRI Plugin

This document describes the architecture of the cri plugin for containerd.

This plugin is an implementation of Kubernetes container runtime interface (CRI). Containerd operates on the same node as the Kubelet. The cri plugin inside containerd handles all CRI service requests from the Kubelet and uses containerd internals to manage containers and container images.

The cri plugin uses containerd to manage the full container lifecycle and all container images. As also shown below, cri manages pod networking via CNI (another CNCF project).

architecture

Let's use an example to demonstrate how the cri plugin works for the case when Kubelet creates a single-container pod:

  • Kubelet calls the cri plugin, via the CRI runtime service API, to create a pod;
  • cri uses containerd internal to create and start a special pause container (the sandbox container) and put that container inside the pod’s cgroups and namespace (steps omitted for brevity);
  • cri configures the pod’s network namespace using CNI;
  • Kubelet subsequently calls the cri plugin, via the CRI image service API, to pull the application container image;
  • cri further uses containerd to pull the image if the image is not present on the node;
  • Kubelet then calls cri, via the CRI runtime service API, to create and start the application container inside the pod using the pulled container image;
  • cri finally uses containerd internal to create the application container, put it inside the pod’s cgroups and namespace, then to start the pod’s new application container. After these steps, a pod and its corresponding application container is created and running.