Design a Chat Service
Last updated: February 11, 2026
Quick Overview
Design a high-throughput chat system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Visa
System Design
Product Manager
Visa
February 11, 2026Product Manager
Onsite
System Design
Medium
0
4
2,892 solved
Design a high-throughput chat system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Visa asks this during the Onsite to assess your architectural thinking. They want to see how you decompose a complex problem, choose appropriate technologies, and reason about failure modes. Strong candidates proactively discuss monitoring, alerting, and operational concerns.
What the Interviewer Expects
- Systematically gather requirements and estimate capacity (QPS, storage, bandwidth)
- Design a scalable architecture with clear component responsibilities
- Make well-reasoned database and caching decisions with trade-off analysis
- Address consistency vs availability trade-offs specific to the use case
- Discuss partitioning strategy, replication, and data modeling
- Cover failure handling, monitoring, and alerting strategies
Key Topics to Cover
Database selection and data modeling
Monitoring, logging, and alerting
Consistency models and replication
Message queues and async processing
Load balancing and horizontal scaling
How to Approach This
- Start by clarifying functional and non-functional requirements with the interviewer.
- Estimate the scale: QPS, storage, bandwidth. This drives your design decisions.
- Draw a high-level architecture first, then deep dive into 1-2 critical components.
- Discuss trade-offs explicitly (e.g., consistency vs availability, SQL vs NoSQL).
- Address failure scenarios, monitoring, and how the system handles 10x traffic spikes.
Possible Follow-up Questions
- How would you handle schema migrations with zero downtime?
- How do you ensure data consistency across multiple services?
- What happens if one of your database nodes goes down?
- How would you handle a 10x increase in traffic overnight?
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Requirements
- Functional Requirements:
- Users can send and receive messages in real-time.
- Support for group chats with up to 1000 participants.
- Message persistence for at least 30 days.
...
Capacity Estimation
- User Base: Assume 10 million active users.
- Messages per User: Average of 100 messages per day.
- Total Messages per Day: 10 million users * 100 messages = 1 billion messages.
- *...
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