allora

https://github.com/flippa/allora

Allora (Italian for "at that time") provides a replacement for the classic UNIX cron, using nothing but ruby. It is very small, easy to follow and relatively feature-light. It does support a locking mechanism, backed by Redis, or any other custom implementation, which makes it possible to run the scheduler on more than one server, without worrying about jobs executing more than once per scheduled time.