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Openai·Software Engineer·Onsite - System Design / Architecture·Senior

SeniorPrefer not to say
Apr 2026

Summary

System design round at OpenAI where they had me build out a calendar app from scratch. The follow-up about multi-device sync is where things got interesting and honestly a bit uncomfortable.

Questions Asked (2)

Q1

Design a calendar application similar to Apple Calendar, supporting day, month, and year views where users can create and manage events.

System DesignData ModelingProduct Sense & Ideation
Author's notes

I jumped straight into the UI layer because the prompt mentioned views explicitly, which I think was a mistake.

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AI HintsAI Generated

Suggested Approach

Start by clarifying functional and non-functional requirements, then design a data model that efficiently supports day, month, and year views with event creation and management. Focus on scalability, performance, and user experience, and discuss trade-offs in your design choices.

Pro tip: Emphasize how you would handle recurring events and time zones, as these are common pitfalls in calendar systems and demonstrate depth of thought. Also, consider offline support and sync conflicts, which are critical for a production-grade calendar app.

1. Clarify Requirements

Ask questions to understand the scope: expected number of users, event types (recurring, all-day), sharing/collaboration features, and non-functional requirements like latency and availability.

2. Data Model Design

Design a schema for events, calendars, and users. Consider how to store recurring events efficiently (e.g., using RRULE) and how to index for fast range queries across different views.

3. API and View Rendering

Define APIs for CRUD operations on events and for fetching events for a given time range. Discuss how to render day, month, and year views efficiently, possibly using pre-aggregation or caching.

4. Scalability and Performance

Address how to scale the system for many users and events, including database sharding, caching strategies, and handling high read/write loads. Discuss trade-offs between consistency and availability.

5. Advanced Features and Edge Cases

Cover recurring events, time zones, reminders, sharing, and offline support. Discuss how to handle conflicts and sync across devices.

Key Points to Mention

  • Efficient storage and querying of recurring events using RRULE or similar standards
  • Time zone handling and conversion for events and views
  • Indexing strategies for fast range queries (e.g., by start time and calendar ID)
  • Caching and pre-aggregation for rendering month/year views quickly
  • Handling concurrent edits and sync conflicts (e.g., using versioning or CRDTs)
  • Scalability considerations: sharding, read replicas, and load balancing

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.

Q2

How would you extend the calendar to support multi-device sync, so an event created on one device automatically appears on a user's other devices?

System DesignTechnical Trade-offsAPI & Integrations
Author's notes

This is where I blanked for a second.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements like consistency, latency, and conflict resolution, then propose a sync architecture with a central server and real-time push notifications. Discuss trade-offs between different sync strategies and how to handle conflicts and offline scenarios.

Pro tip: Show awareness of incremental sync and conflict resolution using vector clocks or CRDTs, and mention how to handle offline changes and sync when connectivity resumes.

1. Clarify Requirements

Ask about consistency needs, expected latency, conflict frequency, and offline support to scope the solution.

2. Design Sync Architecture

Propose a central server with a database, and clients that sync via APIs and receive updates through push notifications (e.g., WebSocket, APNs, FCM).

3. Define Sync Protocol

Outline how clients fetch changes (e.g., delta sync with timestamps or version vectors) and push local changes, ensuring idempotency.

4. Handle Conflicts and Offline

Discuss conflict resolution strategies (last-write-wins, CRDTs) and how to queue changes when offline and sync upon reconnection.

5. Address Scalability and Reliability

Mention scaling the server horizontally, using message queues, and ensuring data consistency and fault tolerance.

Key Points to Mention

  • Real-time push notifications (WebSocket, APNs, FCM) for instant updates
  • Incremental/delta sync to minimize data transfer
  • Conflict resolution using vector clocks or CRDTs
  • Offline support with local change queue and sync on reconnect
  • Idempotent APIs to handle duplicate requests
  • Scalability considerations: sharding, load balancing, and message queues

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.