cron_runner
Use the Job trait to define Jobs on your structs, pass the jobs to the Runner and start it with run(). The Runner will spawn a main "monitoring" thread to schedule your jobs, and additional worker threads to run your jobs.
The Runner can run in 2 modes.
Pooled
Spawn it with n > 0 threads and those threads will live in a thread pool waiting for new jobs from a mpsc channel ( in this context used like a FIFO queue). While the worker threads are waiting, they are blocked on recv(). If all workers are busy new jobs will not be run untill a worker is free, the first job to be scheduled is the first to be picked up and run by a worker.
Non Pooled
If you start the runner with 0 worker threads, any time a job should run a new thread will be spawned for the job that will execute it and return. If for some reason a worker could not be spawned, the Runner will try to run the job in its main thread, blocking scheduling for the rest of the jobs untill it is done.