Sys is yet another "dependency injection" / "system state management" library for Clojure.
More simply, it's a library to manage the start-up and shutdown of "stateful" parts of a web application (ex. reading the configuration, connecting to the database, starting a web server).
As functional programmers, Clojurians are obsessed with state (in an effort to avoid it), and there are many libraries related to this problem. Although, in my opinion, they are all over-engineered (or at least, they are trying to do too much for too many people).
For the past 10 years, I have happily avoided using any of these libraries.
In late 2025, a mentee at Clojure Camp asked for advice on which library to use, and I started to show him that you could "just do a reduce", and then "add this", "and this"... and, well, here we are.
(Jokes aside, it was a good excuse to pair code on something tractable, and also practise good testing, friendly error messages, writing documentation, etc.)
Sys works, it's released officially; but there a few more important quality-of-life improvements coming very soon.