Docker components and how they interact.

Docker components and how they interact.
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When we talk about containers a big name that comes into picture is docker. Docker is a leading container platform and provides all the functionalities required in the life cycle of a container. In this article, we will talk about docker components and how they work together.

Docker components and how they interact.

Docker consists of the following components.

Docker Daemon:

Docker Daemon runs as dockerd and is a continuous running process. This daemon helps you in connecting docker-cli to containers itself. Docker talks to containerd using gRPC protocol.

You can see by typing below command in your Linux machine.

ps aux | grep docker
/usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock

You can see the output contains dockerd and also tells that it is using containerd.

Containerd:

Containerd daemon runs and expose gRPC API to interact with it. It handles all the low-level container management tasks, storage, image distribution, network attachment, etc. If you wanna see this you can type below command.

ps aux| grep container
0:01 /usr/bin/containerd

This shows you that containerd daemon is running and no containerd-shim is running.

Containerd-ctr:

A lightweight CLI to directly communicate with containerd. You can control containerd using this cli tool.

runc:

It is used for actually running containers. This binary deal with the low-level things like cgroups and namespaces that are required to create a container.

Whenever you run any container this process will come into the picture. To see this in action. Run the below commands in two different terminals.

watch 'ps aux| grep runc'

And then this

docker run image_name

You will see a process will appear, create the container and then exit.

containerd-shim:

Once runc launches containers it exits. This means there is no long-running process for this container. The flow is containerd asked containerd-shim to launch container which calls runc to create container and then exits leaving a few of the things like file descriptors so that container can pass essential signals to containerd-shim.

If your container is running you can see containerd-shim is running like below

root     16097  1.0  0.0  10612  4912 ?        Sl   22:03   0:00 containerd-shim -namespace moby -workdir /var/lib/containerd/io.containerd.runtime.v1.linux/moby/61fd4688eae0b6b938aa07d609f55150cf0231af2d6043cd01d29cee5ee63d14 -address /run/containerd/containerd.sock -containerd-binary /usr/bin/containerd -runtime-root /var/run/docker/runtime-runc

So here is a glance of how docker components talk to each other.

Docker instruct containerd to launch a container, containerd tell containerd-shim to launch a container, containerd uses runc to launch the container and then keep the signals line open with it while runc exits.

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This was how docker components interact with each other. To know more about Docker and Kubernetes keep following my blog. A lot of such articles will be coming soon.

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Gaurav Yadav

Gaurav is cloud infrastructure engineer and a full stack web developer and blogger. Sportsperson by heart and loves football. Scale is something he loves to work for and always keen to learn new tech. Experienced with CI/CD, distributed cloud infrastructure, build systems and lot of SRE Stuff.

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