Step 1: System Definition

A reminder is a promise about a future moment. A user says "tell me at 9:00 AM to take my medication," and the system must keep that promise every morning, in the user's own time zone, through every daylight saving change, for as long as the reminder exists. This case study designs the service that keeps such promises for millions of people at once.

A reminder alert system stores scheduled reminders and, when each one comes due, delivers an alert through the channels the user chose: email, SMS, mobile push, or an alert inside the app. Slack's `/remind