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Calendar / Reminder Service — Notes#

Functional#

  • Create events with start, end, attendees.
  • Recurrence (RRULE).
  • Reminders via push/email/SMS.
  • Free/busy queries.
  • Sync via CalDAV / Google API.

Non-functional#

  • Sub-second reminder fire accuracy at minute granularity.
  • 99.95% uptime.

Trade-offs#

  • Expand recurrence on read vs materialize occurrences: hybrid — materialize next N occurrences for fast fan-out.
  • Time-bucket queue vs timing wheel: wheel scales to billions of pending timers in memory.

Refs#

  • RFC 5545 (iCalendar), RFC 4791 (CaldDAV).
  • Google Calendar engineering posts.
  • "Hashed and Hierarchical Timing Wheels" Varghese & Lauck.