Architect a low-latency Order Processing Engine
Last updated: July 17, 2025
Quick Overview
Design a low-latency order processing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Expedia
System Design
Software Engineer
Expedia
July 17, 2025Software Engineer
Onsite
System Design
Hard
70
14
1,314 solved
Design a low-latency order processing system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
System design interviews at Expedia typically last 45-60 minutes. You are expected to drive the conversation, starting from requirements gathering through to a detailed architecture. The interviewer will evaluate your ability to handle ambiguity and make practical engineering decisions.
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
Consistency models and replication
Caching strategies (local, distributed, CDN)
API design and rate limiting
Monitoring, logging, and alerting
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 region-wide outage?
- How would you migrate from a monolithic to a microservices architecture?
- How would you handle schema migrations with zero downtime?
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Requirements
Functional Requirements
- Process millions of order requests per second with low latency (sub-100ms response time).
- Support various order types (flight bookings, hotel reservations, car rental...
Capacity Estimation
Back-of-Envelope Calculations
- User Traffic: Assume 10 million users hitting the platform during peak hours.
- Requests: If each user initiates 5 requests per minute, that results in 50...
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