Step 4: High-Level Design

The system has two jobs: decide when each alert should go out, and deliver it on the right channel. We follow one reminder through both. Priya, who lives in Los Angeles, creates a daily reminder, "Take medication", for 9:00 AM, delivered by push notification. We watch it being created, then watch it fire the next morning.

One rule shapes the whole design: the scheduler never scans a table to ask what is due now. With 100 million reminders, a scan is too slow to run every few seconds