Design a Messaging for Citadel
Last updated: October 20, 2025
Quick Overview
Design a event-driven messaging system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Citadel
October 20, 2025124
10
102 solved
Design a event-driven messaging system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
This is a common system design question asked during Onsite at Citadel. The interviewer expects you to demonstrate your ability to design large-scale distributed systems, make well-reasoned trade-offs, and communicate your thought process clearly. Citadel values engineers who can think about scalability from day one.
What the Interviewer Expects
- Drive the design discussion proactively with minimal interviewer guidance
- Perform detailed capacity estimation and use it to inform design decisions
- Design for global scale with multi-region deployment and data consistency
- Deep dive into 2-3 critical components with implementation-level detail
- Address complex trade-offs: CAP theorem, eventual consistency, conflict resolution
- Discuss operational excellence: deployment strategy, chaos engineering, SLOs/SLIs
- Propose a phased rollout plan from MVP to full-scale system
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 a 10x increase in traffic overnight?
- How would you handle schema migrations with zero downtime?
- What happens if one of your database nodes goes down?
Practice a Similar Problem on Codemia
Solve a related problem with our interactive workspace, get AI feedback, and view detailed solutions.
Solve on CodemiaSample Answer
Requirements
Functional Requirements
- Event Publishing: Users can publish messages to specific topics.
- Event Subscription: Users can subscribe to topics to receive messages in real-time.
- **Mess...
Capacity Estimation
Assuming Citadel has around 10,000 active users sending an average of 100 messages per day:
- Total Messages Per Day = 10,000 users * 100 messages = 1,000,000 messages/day.
- Total Messages Per Second...
Submit Your Answer
