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docs update for cri-containerd to cri move

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Signed-off-by: Mike Brown <brownwm@us.ibm.com>
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Mike Brown
2018-03-13 17:38:04 -05:00
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docs/architecture.md
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# Architecture of CRI-Containerd
This document describes the architecture of `cri-containerd`.
# Architecture of The CRI Plugin
This document describes the architecture of the `cri` plugin for `containerd`.
Cri-containerd is a containerd based implementation of Kubernetes [container runtime interface (CRI)](https://github.com/kubernetes/kubernetes/blob/master/pkg/kubelet/apis/cri/runtime/v1alpha2/api.proto). It operates on the same node as the [Kubelet](https://kubernetes.io/docs/reference/generated/kubelet/) and [containerd](https://github.com/containerd/containerd). Layered between Kubernetes and containerd, cri-containerd handles all CRI service requests from the Kubelet and uses containerd to manage containers and container images.
This plugin is an implementation of Kubernetes [container runtime interface (CRI)](https://github.com/kubernetes/kubernetes/blob/master/pkg/kubelet/apis/cri/runtime/v1alpha2/api.proto). Containerd operates on the same node as the [Kubelet](https://kubernetes.io/docs/reference/generated/kubelet/). The `cri` plugin inside containerd handles all CRI service requests from the Kubelet and uses containerd internals to manage containers and container images.
Cri-containerd uses containerd to manage the full container lifecycle and all container images. As also shown below, cri-containerd manages pod networking via [CNI](https://github.com/containernetworking/cni) (another CNCF project).
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](https://github.com/containernetworking/cni) (another CNCF project).
![architecture](./architecture.png)
Let's use an example to demonstrate how cri-containerd works for the case when Kubelet creates a single-container pod:
* Kubelet calls cri-containerd, via the CRI runtime service API, to create a pod;
* cri-containerd uses containerd to create and start a special [pause container](https://www.ianlewis.org/en/almighty-pause-container) (the sandbox container) and put that container inside the pod’s cgroups and namespace (steps omitted for brevity);
* cri-containerd configures the pod’s network namespace using CNI;
* Kubelet subsequently calls cri-containerd, via the CRI image service API, to pull the application container image;
* cri-containerd further uses containerd to pull the image if the image is not present on the node;
* Kubelet then calls cri-containerd, via the CRI runtime service API, to create and start the application container inside the pod using the pulled container image;
* cri-containerd finally calls containerd to create the application container, put it inside the pod’s cgroups and namespace, then to start the pod’s new application container.
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](https://www.ianlewis.org/en/almighty-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.