Active Merchant is a simple payment abstraction library used in and sponsored by Shopify. It is written by Tobias Luetke, Cody Fauser, and contributors. The aim of the project is to feel natural to Ruby users and to abstract as many parts as possible away from the user to offer a consistent interface across all supported gateways.
The File System State Monitor keeps track of the state of any number of paths and will fire events when said state changes (create/update/delete). FSSM supports using FSEvents on MacOS (with ruby 1.8), Inotify on GNU/Linux, and polling anywhere else.