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

SeniorPrefer not to say
Apr 2026

Summary

Meta system design round for a software engineer role. The whole question was basically 'build a competitive programming judge from scratch' and they wanted you to go deep on pretty much every layer of it. A lot of ground to cover in one session.

Questions Asked (1)

Q1

Design a competitive programming contest platform similar to LeetCode contests. Cover the full contest lifecycle, sandboxed code judging across multiple languages, real-time leaderboard at scale, anti-cheating measures, and surge handling when a contest starts.

System DesignTechnical Trade-offsData Modeling
Author's notes

This is a beast of a question.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and scale (e.g., number of concurrent users, contest frequency, languages supported). Then walk through the high-level architecture covering the contest lifecycle, judging system, leaderboard, anti-cheating, and surge handling. Finally, dive deep into 1-2 areas like sandboxing or real-time leaderboard, discussing trade-offs and optimizations.

Pro tip: Emphasize idempotency and exactly-once processing in judging to avoid duplicate submissions and ensure fairness. Also, mention using a CDN and edge caching for static assets to handle surge at contest start.

1. Clarify Requirements and Scale

Ask about expected number of concurrent participants, contest frequency, supported languages, and latency requirements for leaderboard updates. This sets the stage for design decisions.

2. High-Level Architecture

Outline the main components: API gateway, contest service, submission queue, judging workers, leaderboard service, and data stores. Explain how they interact during a contest.

3. Contest Lifecycle and Data Model

Describe how contests are created, scheduled, started, and ended. Detail the data model for contests, problems, submissions, and users, considering read/write patterns.

4. Sandboxed Judging and Anti-Cheating

Explain the sandboxing approach (e.g., containers, seccomp, cgroups) for running untrusted code. Discuss anti-cheating measures like plagiarism detection, monitoring, and randomized test cases.

5. Real-Time Leaderboard and Surge Handling

Design the leaderboard using in-memory data stores (e.g., Redis sorted sets) with pub/sub for updates. Address surge at contest start with autoscaling, queueing, and rate limiting.

Key Points to Mention

  • Use of message queues (e.g., Kafka) to decouple submission ingestion from judging, ensuring scalability and fault tolerance.
  • Sandboxing techniques: Docker containers with resource limits, seccomp for syscall filtering, and network isolation.
  • Leaderboard implementation with Redis sorted sets and WebSocket/SSE for real-time updates to clients.
  • Anti-cheating: code similarity detection (e.g., MOSS), monitoring for suspicious behavior, and randomized test cases per user.
  • Surge handling: autoscaling of judging workers, pre-warming instances, and rate limiting submissions per user.
  • Data consistency: eventual consistency for leaderboard vs. strong consistency for submission status, and idempotent submission processing.

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