Design a Payment Service
Last updated: April 28, 2026
Quick Overview
Design a event-driven payment system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
Instacart
System Design
Software Engineer
Instacart
April 28, 2026Software Engineer
Technical Screen
System Design
Easy
12
6
3,991 solved
Design a event-driven payment system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.
This ML system design question from Instacart's Technical Screen tests your ability to think about ML systems at scale. The interviewer expects discussion of data quality, feature stores, model serving infrastructure, and A/B testing strategy.
What the Interviewer Expects
- Map the business problem to a concrete ML objective
- Propose reasonable features and a baseline model
- Discuss basic model evaluation metrics
- Outline a simple serving architecture
Key Topics to Cover
Data collection and labeling strategy
Model serving and latency optimization
ML objective formulation and metric selection
Model selection and architecture
Feedback loops and model retraining
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
- What is your model retraining strategy?
- How would you handle a 10x increase in prediction requests?
- How would you ensure fairness and reduce bias in the model?
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Requirements
Functional Requirements
- User Authentication: Enable secure user login and verification for payment processing.
- Payment Processing: Accept multiple payment methods including credit ca...
Capacity Estimation
To estimate capacity, we can make some assumptions:
- Active Users: Assume Instacart has 10 million active users.
- Transactions per User: Assume each user makes about 5 transactions per wee...
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