Design a multi-device calendar application

System design prompt

Design a calendar application similar to Apple Calendar.

Core features

• Users can create, update, and delete events .
• The UI must support Day / Month / Year views (i.e., the backend must efficiently fetch events for a given time range).
• Multi-device support : if a user creates/edits an event on one device, it should sync to the user’s other devices .

Clarifications you should cover

• Assumptions about data scale (users, events per user, read/write QPS).
• Consistency expectations for cross-device sync (e.g., eventual vs “near real-time”).
• Handling tricky calendar cases (time zones, all-day events, recurring events, edits while offline, conflicts).

Output expected from candidate

• High-level architecture (clients, APIs, storage, background jobs, push/sync).
• Data model and key indexes.
• Sync protocol and conflict-resolution strategy.
• How to serve Day/Month/Year queries efficiently.
• Key tradeoffs and failure/edge cases.