Duolingo's Implementation of Optimistic Locking to Solve Database Contention in Notification System

In order to schedule and send hundreds of millions of notifications each day, we use a system known as comeback.

When a user does things like finishing a lesson, we will calculate when we should send them reminders for the following days and store that schedule of reminders as a row in our database.

Here's how we used *optimism* to fix our notifications service and save computing costs.

The Duolingo app is famous for encouraging users to do their lessons. While Duo the owl's methods vary, the most effective method for reminding users about their learning goals is via notifications.