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
February 11, 20260
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
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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Solve on CodemiaSample Answer
Requirements Clarification
Before diving into the architecture, clarify the scope with the interviewer. For Chat Service, key functional requirements include: what are the core ...
Capacity Estimation
Estimate the scale to drive design decisions. Assume 100M DAU with an average of 10 actions per user per day = 1B requests/day ~ 12K QPS average, ~36K...
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