Container Engine

Introduction to Container Engines

Container engines, one of the most important components of Kubernetes, manage the lifecycle of images and containers. The kubelet interacts with a container runtime through the Container Runtime Interface (CRI).

CCE supports containerd and Docker. containerd is recommended for its shorter traces, fewer components, higher stability, and less consumption of node resources.

**Table 1** Comparison between container engines
Item	containerd	Docker
Tracing	kubelet --> CRI plugin (in the containerd process) --> containerd	Docker (Kubernetes 1.23 and earlier versions): kubelet --> dockershim (in the kubelet process) --> docker --> containerd Docker (community solution for Kubernetes 1.24 and later versions): kubelet --> cri-dockerd (kubelet uses CRI to connect to cri-dockerd) --> docker--> containerd
Command	crictl	docker
Kubernetes CRI	Native support	Support through dockershim or cri-dockerd
Pod startup delay	Low	High
kubelet CPU/memory usage	Low	High
Runtime's CPU/memory usage	Low	High

Common Commands of containerd and Docker

containerd does not support Docker APIs and Docker CLI, but you can run crictl commands to implement similar functions.

**Table 2** Image-related commands
No.	Docker Command	containerd Command	Remarks
1	docker images [Option] [Image name[:Tag]]	crictl images [Option] [Image name[:Tag]]	List local images.
2	docker pull [Option] Image name[:Tag\|@DIGEST]	crictl pull [Option] Image name[:Tag\|@DIGEST]	Pull images.
3	docker push	None	Pushing images.
4	docker rmi [Option] Image...	crictl rmi [Option] Image ID...	Delete a local image.
5	docker inspect Image ID	crictl inspecti Image ID	Check images.

**Table 3** Container-related commands
No.	Docker Command	containerd Command	Remarks
1	docker ps [Option]	crictl ps [Option]	List containers.
2	docker create [Option]	crictl create [Option]	Create a container.
3	docker start [Option] Container ID...	crictl start [Option] Container ID...	Start a container.
4	docker stop [Option] Container ID...	crictl stop [Option] Container ID...	Stop a container.
5	docker rm [Option] Container ID...	crictl rm [Option] Container ID...	Delete a container.
6	docker attach [Option] Container ID	crictl attach [Option] Container ID	Connect to a container.
7	docker exec [Option] Container ID Startup command [Parameter...]	crictl exec [Option] Container ID Startup command [Parameter...]	Access the container.
8	docker inspect [Option] Container name\|ID...	crictl inspect [Option] Container ID...	Query container details.
9	docker logs [Option] Container ID	crictl logs [Option] Container ID	View container logs.
10	docker stats [Option] Container ID...	crictl stats [Option] Container ID	Check the resource usage of the container.
11	docker update [Option] Container ID...	crictl update [Option] Container ID...	Update container resource limits.

**Table 4** Pod-related commands
No.	Docker Command	containerd Command	Remarks
1	None	crictl pods [Option]	List pods.
2	None	crictl inspectp [Option] Pod ID...	View pod details.
3	None	crictl start [Option] Pod ID...	Start a pod.
4	None	crictl runp [Option] Pod ID...	Run a pod.
5	None	crictl stopp [Option] Pod ID...	Stop a pod.
6	None	crictl rmp [Option] Pod ID...	Delete a pod.

Containers created and started by containerd are immediately deleted by kubelet. containerd does not support suspending, resuming, restarting, renaming, and waiting for containers, nor Docker image build, import, export, comparison, push, search, and labeling. containerd does not support file copy. You can log in to the image repository by modifying the configuration file of containerd.

Differences in Tracing

Docker (Kubernetes 1.23 and earlier versions):
kubelet --> docker shim (in the kubelet process) --> docker --> containerd
Docker (community solution for Kubernetes v1.24 or later):
kubelet --> cri-dockerd (kubelet uses CRI to connect to cri-dockerd) --> docker--> containerd
containerd:
kubelet --> cri plugin (in the containerd process) --> containerd

Although Docker has added functions such as swarm cluster, docker build, and Docker APIs, it also introduces bugs. Compared with containerd, Docker has one more layer of calling. Therefore, containerd is more resource-saving and secure.

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