Quadlet is an opinionated tool for easily running podman system containers under systemd in an optimal way.

Containers are often used in a cloud context, and they are then used in combination with an orchestrator like Kubernetes. They are also commonly used during development and testing to manually manage containers on an ad-hoc basis.

However, there are also use cases where you want some kind of automatic container management, but on a smaller, single-node scale, and often more tightly integrated with the rest of the system. Typical examples of this can be embedded or automotive use, where there is no system administrator, or disconnected or EDGE servers.

The recommended way to do this is to use systemd to orchestrate the containers, since this is an already running process manager, and since podman containers are just child processes. There are many documents that describe how to use podman with systemd directly, but the end result are generally large, hard to maintain systemd config files. And often the container setup isn't optimal.

With quadlet, you describe how to run a container in a format that is very similar to regular systemd config files. From these actual systemd configurations are automatically generated (using systemd generators).

The container descriptions focus on the relevant container details, with no technical details about how the podman integration works. This means they are easy to write, easy to maintain and integration can automatically improve over time as new podman features become available.

Here is a minimal container config:

[Unit]
Description=A minimal container

[Container]
Image=centos
Exec=sleep 60

[Service]
Restart=always

This is very similar to a regular systemd service file, except for the [Container] section. It will run sleep 60 in a centos container, and then exit only for systemd to restart it again.

If you put this in /etc/containers/systemd/minimal.container and then run systemctl daemon-reload and podman pull centos you can immediately start the container using systemctl start minimal.service and watch the status:

# systemctl status minimal.service
● minimal.service - A minimal container
     Loaded: loaded (/etc/containers/systemd/minimal.container; generated)
     Active: active (running) since Thu 2021-09-23 13:05:33 CEST; 1s ago
    Process: 839846 ExecStartPre=rm -f /run/minimal.cid (code=exited, status=0/SUCCESS)
   Main PID: 839894 (conmon)
      Tasks: 4 (limit: 38375)
     Memory: 1.4M
        CPU: 193ms
     CGroup: /system.slice/minimal.service
             ├─container
             │ ├─839898 /dev/init -- sleep 60
             │ └─839943 /usr/bin/coreutils --coreutils-prog-shebang=sleep /usr/bin/sleep 60
             └─supervisor
               └─839894 /usr/bin/conmon ...

The generated service file is in /run/systemd/generator/minimal.service for people interested in all the technical details.

Quadlet builds using meson. You can build and install it with these steps:

$ meson builddir  --prefix /usr
$ cd builddir
$ meson compile
$ meson install

This will install quadlet-generator in /usr/lib/systemd/system-generators, which will read configuration files from /etc/containers/systemd.

Here are some further documentations:

• File formats
• Container setup

Quadlet also ships with some example containers.