All articles tagged #containerd

If you're dealing with containers regularly, you've probably published ports many, many times already. A typical need for publishing arises like this: you're developing a web app, locally but in a container, and you want to test it using your laptop's browser. The next thing you do is docker run -p 8080:80 app and then open localhost:8080 in the browser. Easy-peasy!

But have you ever wondered what actually happens when you ask Docker to publish a port?

In this article, I'll try to connect the dots between port publishing, the term apparently coined by Docker, and a more traditional networking technique called port forwarding. I'll also take a look under the hood of different "single-host" container runtimes (Docker Engine, Docker Desktop, containerd, nerdclt, and Lima) to compare the port publishing implementations and capabilities.

As always, the ultimate goal is to gain a deeper understanding of the technology and get closer to becoming a power user of containers. Let the diving begin!

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A container image is a combination of layers where every layer represents some intermediary state of the final filesystem. Such a layered composition makes the building, storage, and distribution of images more efficient. But from a mere developer's standpoint, images are just root filesystems of our future containers. And we often want to explore their content accordingly - with familiar tools like cat, ls, or file. Let's try to see if we can achieve this goal using nothing but the means provided by Docker itself.

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The difference between docker (or podman, or containerd) attach and exec commands is a common source of confusion. And it's understandable - these two commands have similar arguments and, at first sight, similar behavior. However, attach and exec aren't interchangeable. They aim to cover different use cases, and the implementation of the commands also differs. But still, it might be hard to remember when to use which command.

Since I'm no fan of brute memorization, here is my recipe of how I managed to internalize the difference. Long story short, connecting the dots between the knowledge of what containers really are under the hood and these two commands helped to grasp the difference almost instantly. And like any true understanding, it freed me from relying only on my memory and gave me a chance to extrapolate the knowledge on a similar tech such as Kubernetes 😉

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For a long time, serverless has been just a synonym of AWS Lambda for me. Lambda offered a convenient way to attach an arbitrary piece of code to a platform event like a cloud instance changing its state, a DynamoDB record update, or a new SNS message. But also, from time to time, I would come up with a piece of logic that is not big enough to deserve its own service and at the same time doesn't fit well with the scope of any of the existing services. So, I'd often also put it into a function to invoke it later with a CLI command or an HTTP call.

I moved away from AWS a couple of years ago, and since then, I've been missing this ease of deploying serverless functions. So I was pleasantly surprised when I learned about the OpenFaaS project. It makes it simple to deploy functions to a Kubernetes cluster, or even to a VM, with nothing but containerd.

Intrigued? Then read on!

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Containers could have become a lightweight VM replacement. However, the most widely used form of containers, standardized by Docker/OCI, encourages you to have just one process service per container. Such an approach has a bunch of pros - increased isolation, simplified horizontal scaling, higher reusability, etc. However, there is a big con - in the wild, virtual (or physical) machines rarely run just one service.

While Docker tries to offer some workarounds to create multi-service containers, Kubernetes makes a bolder step and chooses a group of cohesive containers, called a Pod, as the smallest deployable unit.

When I stumbled upon Kubernetes a few years ago, my prior VM and bare-metal experience allowed me to get the idea of Pods pretty quickly. Or so thought I... 🙈

Starting working with Kubernetes, one of the first things you learn is that every pod gets a unique IP and hostname and that within a pod, containers can talk to each other via localhost. So, it's kinda obvious - a pod is like a tiny little server.

After a while, though, you realize that every container in a pod gets an isolated filesystem and that from inside one container, you don't see processes running in other containers of the same pod. Ok, fine! Maybe a pod is not a tiny little server but just a group of containers with a shared network stack.

But then you learn that containers in one pod can communicate via shared memory! So, probably the network namespace is not the only shared thing...

This last finding was the final straw for me. So, I decided to have a deep dive and see with my own eyes:

• How Pods are implemented under the hood
• What is the actual difference between a Pod and a Container
• How one can create Pods using Docker.

And on the way, I hope it'll help me to solidify my Linux, Docker, and Kubernetes skills.

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