Projects:Kubernetes

According to Wikipedia, Kubernetes (commonly stylized as k8s) is an open-source container orchestration system for automating application deployment, scaling, and management. I've heard a lot about it, and it seems to solve some problems I'm encountering sometimes, so I'd like to get to know it better. While doing so, I wanted to make a write-up of what I found out.

This page is not intended as a tutorial, but it will link to various tutorials I found useful. In this way, I hope that it can be used as a reference for someone setting up Kubernetes to be able to learn about it more quickly.

Disclaimer: I'm a beginner in the Kubernetes area, so this tutorial will contain errors. Please correct errors where you find them, and if possible, write some explanation! The "I" in this article is Sjors, but parts are possibly written by other people.

Problem

We start with a Linux desktop, running some version of Arch Linux. Someone wants to run an application on it for which only binaries are available, but those binaries were compiled on Ubuntu. Now, system libraries are different between Arch Linux and Ubuntu, and while it's possible to create binaries that run independent of system libraries (called "static binaries"), in this particular situation let's assume the binaries aren't static, but you still want to run them on your Arch installation.

You can get a second machine and run Ubuntu on it, or similarly you could use a Virtual Machine. But, there's a simpler and more efficient solution: Docker allows you to install, within your Linux distribution (the "host"), another Linux distribution (let's call it "guest" for now). The host and the guest may be completely different – technically, only the kernel of the host is used, and of course the host and guest must have compatible processor architectures. You run the guest environment within Docker (a "Docker Container") and run that binary in it.

This is how easy that is:

sjors@somebox:~$ lsb_release -a
LSB Version: 1.4
Distributor ID: Arch
Description: Arch Linux
Release: rolling
Codename: n/a
sjors@somebox:~$ docker run -ti ubuntu:bionic bash
root@a5b210d251c2:/# apt-get update && apt-get -y install lsb-release
[....]
root@a5b210d251c2:/# lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 18.04.1 LTS
Release:	18.04
Codename:	bionic

The -it flag makes this an interactive container that can run bash; just as easily, you can run detached containers running webservers, and use the integrated port-forwarding features to make them accessible to the outside.

Now, Docker has some problems of its own:

• You start Docker containers by accessing the Docker daemon; the daemon runs containers as root and allows you to start a container with a bind-mount of "/". Basically, having access to the Docker daemon means you have root on the system.
• When your Docker machine goes down, all containers are gone. You'll have to either restart all containers manually, or have boot-scripts that set them up, but there's no automatic restart mechanism.
• When you want to run more Docker containers than fit on one machine, there's no horizontal scaling mechanism built-in.

Kubernetes mainly provides a solution for these three problems, while otherwise looking very much like Docker. In fact, when you're familiar with Docker some of the commands below will also be very familiar to you.

Concepts

In this section, I'll explain some of Kubernetes' concepts quickly, and add links if you want to know more.