Calendar / Reminder Service — Simple

Problem statement (interviewer prompt)

Design Google Calendar: events with attendees, RSVP, recurring rules (RFC 5545 RRULE), time-zone correctness, sharing, free/busy queries, and reminder delivery (push + email) at the right local time for each attendee.

flowchart LR
  U([User])
  CAL[Calendar Service]
  REM([Reminder Scheduler])
  Q[[(Delayed queue)]]
  NOTIF[Notification Service]
  U --> CAL --> REM --> Q --> NOTIF --> U

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class U client;
    class CAL,NOTIF service;
    class Q datastore;
    class REM compute;