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

Senior
Apr 2026

Summary

System design round at Roblox for a software engineering role, centered entirely on building a matchmaking system. Pretty deep dive, they wanted real architecture not just buzzwords.

Questions Asked (1)

Q1

Design a matchmaking system that groups multiple players into game sessions, covering skill-based matching, latency, queue time, party support, and regional pools.

System DesignData ModelingTechnical Trade-offs
Author's notes

This one sprawled fast.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and scale, then design a modular system that separates matchmaking logic from game session allocation. Focus on trade-offs between match quality and queue time, and explain how to handle parties and regional latency constraints.

Pro tip: Emphasize that matchmaking is a multi-objective optimization problem; propose a scoring function that balances skill, latency, and wait time, and discuss how to tune weights dynamically based on player feedback and queue health.

1. Clarify Requirements and Scale

Ask about player base size, game types, skill metrics, latency thresholds, and party sizes. Establish non-functional requirements like queue time targets and match quality expectations.

2. High-Level Architecture

Outline components: matchmaker service, player/party queue, skill rating service, regional pools, and game session allocator. Explain data flow from player join to match formation.

3. Matching Algorithm Design

Describe how to group players: use a scoring function combining skill difference, latency, and wait time. Discuss algorithms like greedy matching, bucket-based, or expandable search windows.

4. Party and Regional Handling

Explain how to treat parties as atomic units, compute party skill (e.g., average or weighted), and assign parties to regional pools based on member latencies. Discuss cross-region matching fallbacks.

5. Trade-offs and Scalability

Discuss trade-offs: strict skill matching increases queue time; strict latency limits pool size. Propose dynamic tuning, sharding by region/game mode, and monitoring for queue health.

Key Points to Mention

  • Skill rating systems (e.g., Elo, Glicko, TrueSkill) and how to update ratings after matches.
  • Latency measurement and regional pool assignment; use of ping or geographic data.
  • Queue time management: expanding search criteria over time, backfilling, and priority queues.
  • Party support: treating parties as single entities, handling skill disparity within parties, and preventing exploitation.
  • Scalability: sharding matchmakers by region/game mode, using distributed queues, and caching player data.
  • Trade-offs: match quality vs. queue time, fairness vs. latency, and how to measure success (e.g., player retention, match balance).

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