Reactive programming is an approach to writing software that embraces asynchronous I/O. Asynchronous I/O is a small idea that portends big changes for software. The idea is simple: alleviate inefficient resource utilization by reclaiming resources that would otherwise be idle, as they waited for I/O activity. Asynchronous I/O inverts the normal design I/O processing: the clients are notified of new data instead of asking for it, which frees the client to do other work while waiting for these notifications.