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

IntermediatePrefer not to say

Jun 2026

Summary

Capital One system design round for a software engineering role. The whole session was focused on one question about building a transaction system for bank accounts, which sounds straightforward until you actually try to talk through all the edge cases.

Questions Asked (1)

Design a Transaction class for a bank account system. Walk through the class API (deposit, withdraw, getBalance), how transactions get recorded, handling concurrency and atomicity for simultaneous operations on the same account, error cases like insufficient funds, and extending the design to support transfers between accounts.

System DesignTechnical Trade-offsAPI & Integrations

Author's notes

I started with the easy stuff, deposit and withdraw returning updated balances, getBalance, all fine.

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Suggested Approach

Start by clarifying requirements and defining a clean API for the Transaction class, then walk through the core operations (deposit, withdraw, getBalance) with a focus on thread safety and atomicity. Use a lock or synchronized methods to handle concurrency, discuss error cases like insufficient funds, and finally extend the design to transfers by ensuring atomicity across two accounts.

Pro tip: Demonstrate awareness of real-world banking constraints: mention that in production, you'd likely use database transactions with ACID properties rather than in-memory locks, and discuss how to avoid deadlocks in transfers by ordering locks consistently.

1. Clarify requirements and define the API

Ask about expected concurrency level, persistence needs, and whether transactions are logged. Define the public methods: deposit(amount), withdraw(amount), getBalance(), and possibly transfer(toAccount, amount).

2. Design the class with thread safety in mind

Use a private lock object or synchronized methods to ensure atomicity. For getBalance, consider returning a snapshot. Explain how you prevent race conditions during simultaneous deposits/withdrawals.

3. Handle error cases and validation

Check for insufficient funds, negative amounts, and invalid account states. Throw appropriate exceptions (e.g., IllegalArgumentException, InsufficientFundsException) and ensure the account state remains consistent.

4. Record transactions for audit and history

Maintain a list of transaction records (e.g., TransactionRecord objects) with timestamp, type, amount, and resulting balance. Discuss whether to store in-memory or persist to a database.

5. Extend to transfers between accounts

Implement transfer by locking both accounts in a consistent order to avoid deadlocks, then atomically withdraw from one and deposit to the other. Discuss failure scenarios and rollback.

Key Points to Mention

Thread safety mechanisms: synchronized blocks, ReentrantLock, or atomic variables
Atomicity of compound operations like transfer: all-or-nothing execution
Deadlock avoidance: lock ordering or tryLock with timeout
Error handling: insufficient funds, invalid amounts, and exception design
Transaction logging for auditability and debugging
Trade-offs between in-memory locking and database transactions (ACID)

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