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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. One question, but it was a monster. They basically wanted you to design an entire banking platform from scratch, and the scope kept expanding the more you talked.

Questions Asked (1)

Q1

Design an online banking system that supports secure login, account opening, ATM transactions, intra-bank and inter-bank transfers, and querying transaction statistics over a time window. Walk through the architecture, data model, APIs, concurrency controls, external integrations, analytics approach, scalability, security, and monitoring.

System DesignAPI & IntegrationsData Modeling
Author's notes

I started with the usual stuff, auth, accounts, a ledger service, and thought I was doing fine.

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AI HintsAI Generated

Suggested Approach

Start by clarifying functional and non-functional requirements, then sketch a high-level architecture with microservices for authentication, accounts, transactions, and analytics. Dive into data modeling, concurrency controls, and security, and finally discuss scalability, monitoring, and trade-offs.

Pro tip: Emphasize idempotency and exactly-once processing for transfers, and discuss how you would handle reconciliation with external banks to ensure data consistency.

1. Clarify Requirements and Scope

Ask questions to understand expected scale, latency, consistency, compliance (e.g., PCI-DSS, GDPR), and whether the system is for a single bank or multiple banks.

2. High-Level Architecture

Propose a microservices-based architecture with separate services for authentication, account management, transaction processing, and analytics, and describe how they interact via APIs and events.

3. Data Model and Storage

Design a relational schema for core banking data (accounts, transactions, users) and consider using a distributed database or sharding for scalability; discuss ACID vs. BASE trade-offs.

4. APIs and Integrations

Define RESTful APIs for login, account opening, transfers, and statistics; detail integration with external systems like ATM networks, inter-bank networks (e.g., SWIFT, ACH), and fraud detection services.

5. Concurrency, Security, and Scalability

Explain concurrency controls (e.g., optimistic locking, distributed transactions), security measures (encryption, OAuth2, MFA), and scaling strategies (caching, read replicas, partitioning).

Key Points to Mention

  • Use idempotent APIs and exactly-once semantics for transfers to prevent duplicate transactions.
  • Implement strong authentication and authorization with OAuth2, JWT, and multi-factor authentication.
  • Employ event sourcing and CQRS for transaction history and analytics to ensure auditability and scalability.
  • Handle inter-bank transfers with a saga pattern or two-phase commit to maintain consistency across services.
  • Leverage a time-series database or data warehouse for efficient querying of transaction statistics over time windows.
  • Set up comprehensive monitoring with metrics, logging, and tracing (e.g., Prometheus, ELK, Jaeger) and alerting for anomalies.

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