What's used by the most - Cron? But a task or rather script executed by Cron won't access to the context of an application. Meaning, a task will have be an independent unit. Whereas I want is a library to use inside a project such that it'll have access to everything.
For my project I just run them. I'm using async so it is just spawning a task in the background, but you can do the same with threads. Don't underestimate the number of threads that you can run on a modern computer.
If you want some sort of throttling you can stuff tasks into a queue and just run N background threads pulling them off an processing them.
If you need durability then you start to have more trouble. This is where I would start looking at a library. IDK if there are any libraries that handle logging, retries and similar, but if not you can probably get the basics down pretty easily.
Works pretty well. Maybe add a bit of code to crash the whole process on panic or some other logging. Wastes a few KiB of memory per loop but probably not a major issue. Doing this with async will waste only the tiniest amount of memory.
I looked into this last week for recurring tasks in an actix-web project and ended up picking fang, but haven't really battle tested it.. But seems to do what I need. I'm using cron tasks with Postgres as the backend to run a recurring task every minute.