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

Senior
Jun 2026

Summary

Google SWE system design round, just one question about building a group chat app. Pretty open-ended and they clearly wanted to see how deep you'd go on scale and architecture.

Questions Asked (1)

Q1

Design a group chat application.

System DesignTechnical Trade-offsData Modeling
Author's notes

I started with the basics, WebSocket connections, message storage, delivery guarantees, and the interviewer kept pushing on scale.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and scale, then design the high-level architecture covering message flow, storage, and real-time delivery. Dive into data models and trade-offs for consistency, latency, and scalability, and discuss how to handle failures and scale.

Pro tip: Emphasize the difference between delivering messages to online users (via WebSockets) and offline users (via push notifications and persistent queues), and discuss how to ensure exactly-once delivery semantics.

1. Clarify Requirements and Scale

Ask about expected user base, message volume, features (group size, media support, read receipts), and consistency vs. availability trade-offs.

2. High-Level Architecture

Outline components: clients, WebSocket servers for real-time communication, message queue, database for persistence, and push notification service for offline users.

3. Data Modeling and Storage

Design schemas for users, groups, messages, and memberships. Choose appropriate databases (e.g., NoSQL for messages, SQL for user data) and discuss indexing for efficient retrieval.

4. Message Flow and Delivery Guarantees

Detail how a message is sent, stored, and delivered to online and offline users. Discuss ordering, exactly-once delivery, and handling failures.

5. Scalability and Trade-offs

Address scaling WebSocket servers, sharding databases, and trade-offs between consistency, latency, and cost. Mention monitoring and rate limiting.

Key Points to Mention

  • Use WebSockets for real-time bidirectional communication with online users.
  • Implement a message queue (e.g., Kafka) to decouple message ingestion from delivery and ensure reliability.
  • Store messages in a distributed database with appropriate partitioning (e.g., by group ID) for scalability.
  • Handle offline users via push notifications (APNs/FCM) and store undelivered messages for later retrieval.
  • Ensure message ordering and exactly-once delivery using sequence numbers and idempotent processing.
  • Consider trade-offs: consistency vs. availability (CAP theorem), latency vs. durability, and cost implications.

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