I created this playground to make it easier to understand systemd.
In my experience, the best way to learn about a topic is through experimentation. With systemd, this seems hard to do at first. When you are working on a Linux machine, you have access to a systemd setup that you could do experiments with, but the danger is that if anything goes wrong with your experiment, then your system might end up in a bad state. (And if your experiments never go wrong, your learning experience is likely limited).
In my first blog post on systemd, I describe a way to work around this problem by using a systemd setup in podman containers. This has worked well for me, but I have to admit that it is a bit cumbersome. It involves juggling with a few different terminal windows to prepare the container setup, build and start the container, and interact with it. And if you simply want to try out one of the examples mentioned in the blog post, you first have to get the example files from GitHub or re-create them yourself. This creates a pretty big barrier for people to actually try out the examples and do their own experiments, and I'm not aware of many people who did.
In December 2021, Julia Evans and Marie Claire LeBlanc Flanagan published Mess with DNS, a tool that allows you to do DNS experiments right from your browser! It removes any barrier that might keep you from playing with DNS; there is no danger of breaking your real life setup (sound familiar?), and there are zero steps for you to set it up.
When I saw this, I immediately knew that I wanted to have something similar for systemd. This playground is an attempt in that direction. It provides a way to play with systemd from the browser. When you click on "Start system", it creates a podman container on the backend and allows you to interact with it through a command line. The container output and the journal are automatically shown, so there is no juggling with terminal windows, all information is in one place. Zero setup is required, you can dive right in and start with examples from the blog posts. And if you are like me and read blog posts mostly from your phone, you should also be able to use this tool on there (although using it on a computer is probably a lot more comfortable).
Most importantly, you can also change the examples: see which parts are really important, and check out what happens when you change certain aspects. Or create new setups from scratch. The description is also editable, so when you are happy with your experiment, you can document it and then share it (or simply bookmark it for yourself).
So have fun experimenting with systemd! (And if you find interesting examples, let me know on Mastodon.)