I use mostly Ubuntu and Debian and defining systemd units just means you have to spit the right text into a .service file placed at the right spot.
How does NixOS make that easier?
How does NixOS make that easier?
Why I need NixModules in the first place? ELI5, please.
E.g. you could write a NixOS module to manage your web app. You declaratively configure that you want to run Nginx, Postgres, etc. open some ports, connect to your VPN and basically everything else you might want. There are a lot of existing modules, so in most cases you have to write very little code yourself. If you want to scale your system to run on multiple systems or containers, you can do that with relative ease.
You also have a singular source of truth for all of your information, down to your application binaries. Your configuration says you are using Postgres version X.Y.Z with compilation options A, B and C, so that's exactly what is running on your systems.
For example, systemd by default permanently gives up restarting services after a few number of tries (e.g. 5), even if you have set `Restart=always`. This is suboptimal for web servers that should recover by themselves after arbitrarily long failures (e.g. network downtimes outside of your control).
On NixOS, you can, from your machine config, set:
systemd.services.nginx.unitConfig.StartLimitIntervalSec = 0;
We use NixOS for our infra (having used Ansible before), and this ability to override anything cleanly and keeping defaults otherwise made for much easier to maintain infra code and less ugly/surprising compromises.
Why does this "not compose well" ?
You don't have to override the whole unit as /etc/systemd/system/nginx.service , which would have problems if two things wanted to override different parts of the original unit. Just drop an override file in /etc/systemd/system/nginx.service.d/90-restart-always.conf with that one specific config you want to override.
Because you cannot easily write libraries/components that do this.
In NixOS, other modules can override the options of other modules. For example, a a web app can set the nginx options that it needs, instead of requiring you (the admin) to "drop a file" in /etc.
This is one of the reasons why on Ansible Galaxy (community repository of Ansible roles) there are 527 nginx roles [1], and in NixOS there is 1 nginx module that everybody code-reuses.
[1]: https://galaxy.ansible.com/search?deprecated=false&tags=web&...
systemd.services.nginx.unitConfig.StartLimitIntervalSec = 0;
If the point is that it's not manipulating a system-level nginx service but a user-level one, then writing the systemd override file in the way I described doesn't require root either.
What I mean is that services can set other services' options, without you (the admin) having to to write such overrides manually.