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

SeniorPrefer not to say
May 2026

Summary

Capital One system design round for a software engineer role, and they went full depth on credit card infrastructure. One long question that basically covered the entire lifecycle of a credit card product, from auth flows to PCI compliance. Left feeling like I'd only scratched the surface on half the topics.

Questions Asked (1)

Q1

Design a bank credit card system end-to-end, covering the core domain entities, authorization flow, settlement and reconciliation, billing cycle, and how you'd handle scaling, consistency, and compliance.

System DesignData ModelingTechnical Trade-offs
Author's notes

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and scope, then walk through the core domain model and the end-to-end flow from authorization to settlement and billing. Emphasize how you'd handle scale, consistency, and compliance at each stage, making explicit trade-offs and referencing real-world patterns like idempotency, event sourcing, and PCI DSS.

Pro tip: Show that you understand the financial and regulatory constraints by proactively discussing idempotency, audit trails, and data retention—these are often overlooked but critical in fintech. Also, mention that authorization and settlement are separate systems with different consistency needs.

1. Clarify Requirements and Scope

Ask questions to understand scale (e.g., transactions per second, number of cards), compliance needs (PCI DSS, GDPR), and key features like rewards, fraud detection, and multi-currency support. Define functional and non-functional requirements.

2. Design Core Domain Model

Identify main entities: Card, Account, Customer, Transaction, Authorization, Settlement, Billing Cycle, Statement, Payment. Define relationships and key attributes, considering data partitioning and storage choices.

3. Outline Authorization Flow

Describe real-time authorization: merchant sends request via payment network, system checks credit limit, fraud rules, and holds funds. Ensure low latency, high availability, and idempotency to handle retries.

4. Explain Settlement, Reconciliation, and Billing

Cover batch settlement with networks, reconciliation to ensure accuracy, and billing cycle generation (statement, interest, fees). Discuss consistency models (e.g., eventual consistency for settlement, strong for balances) and error handling.

5. Address Scaling, Consistency, and Compliance

Propose scaling strategies (sharding, caching, async processing), consistency trade-offs (CAP theorem), and compliance measures (encryption, tokenization, audit logs, access controls). Mention monitoring and disaster recovery.

Key Points to Mention

  • Idempotency in authorization and settlement to handle duplicate requests and ensure exactly-once processing.
  • Separation of authorization (real-time, low latency) and settlement (batch, high throughput) with different consistency requirements.
  • Use of event sourcing or append-only ledger for auditability and reconciliation.
  • Sharding strategies for scalability, e.g., by customer ID or card number, and caching for hot data.
  • Compliance: PCI DSS for card data, tokenization, encryption at rest and in transit, and audit trails.
  • Handling failures and reconciliation: retries, dead-letter queues, and automated reconciliation jobs.

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