Grokking the System Design Interview, Volume II
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Step 5: Data Model and Storage

Two questions drive every table in this step. The scheduler asks "what is due now?" many times a second. The app asks "what reminders does this user have?" whenever a user opens it. No single key answers both quickly, so the design stores each active reminder twice: its definition keyed by user, and its next fire time keyed by time. Storing data more than once so that each read is a direct lookup is called denormalization.

Reminders

The Reminder Service owns this table. Each row holds one reminder's definition:

  • user_id and reminder_id
  • title, the message text
  • `channels

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