Capital One·Software Engineer·Onsite - System Design / Architecture·Intermediate

Intermediate

Jun 2026

Summary

Capital One SWE interview focused on system design for an online banking app. Pretty open-ended prompt, which I wasn't fully prepared for.

Questions Asked (1)

Design an online banking application that supports checking, savings, and credit card account types.

System DesignData ModelingTechnical Trade-offs

Author's notes

I jumped straight into the data model and kind of forgot to clarify requirements first.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying functional and non-functional requirements, then propose a high-level architecture with microservices for account management, transactions, and user profiles. Dive into data modeling for the three account types, emphasizing how to handle commonalities and differences, and discuss trade-offs around consistency, scalability, and security.

Pro tip: Demonstrate awareness of financial regulations (e.g., PCI DSS, GDPR) and how they influence design decisions, such as encryption at rest and audit logging. Also, proactively discuss idempotency and exactly-once processing for transactions to prevent duplicate charges.

1. Clarify Requirements

Ask questions to understand scope: expected user base, read/write ratio, consistency needs, and regulatory constraints. Confirm core features like account creation, balance inquiry, transfers, and transaction history.

2. High-Level Architecture

Sketch a microservices-based architecture with separate services for accounts, transactions, users, and notifications. Include API gateway, load balancers, and caching layers for scalability.

3. Data Modeling

Design a flexible schema to represent checking, savings, and credit card accounts. Consider a base account entity with type-specific attributes, or separate tables with a common interface. Discuss normalization vs. denormalization for performance.

4. Transaction Handling

Explain how to process transactions atomically, ensuring ACID properties for financial operations. Discuss using distributed transactions or sagas for cross-service operations, and idempotency keys to avoid duplicates.

5. Trade-offs and Scalability

Analyze trade-offs: SQL vs. NoSQL for account data, strong vs. eventual consistency, and monolithic vs. microservices. Discuss scaling strategies like sharding by user ID and read replicas.

Key Points to Mention

Data model design: single table with type discriminator vs. separate tables per account type, and how to handle type-specific fields like credit limit or interest rate.
Transaction consistency: using ACID transactions for transfers, and idempotency to prevent duplicate submissions.
Security: encryption at rest and in transit, tokenization of sensitive data, and role-based access control.
Scalability: horizontal scaling of services, database sharding, and caching frequently accessed data like account balances.
Regulatory compliance: PCI DSS for card data, GDPR for user data, and audit trails for all financial transactions.
Trade-offs: SQL for strong consistency vs. NoSQL for scalability; microservices for modularity vs. monolith for simplicity.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.