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Coinbase·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Coinbase software engineer interview with a coding round focused on building a small in-memory banking system from scratch. Pretty straightforward on the surface but they pushed into concurrency and edge cases pretty quickly.

Questions Asked (1)

Q1

Design and implement an in-memory bank system with addAccount, deposit, and transfer operations. How do you handle insufficient funds, missing accounts, and concurrent access on transfers?

System DesignAlgorithms & Data StructuresTechnical Trade-offs
Author's notes

The core implementation wasn't bad, hashmap from account ID to balance, pretty much the obvious choice.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then design a simple in-memory data model with accounts and balances. Discuss error handling for missing accounts and insufficient funds, and explain concurrency control mechanisms like locks or atomic operations to ensure thread safety. Finally, analyze trade-offs and potential optimizations.

Pro tip: Emphasize the importance of atomicity and consistency in financial operations, and mention that using fine-grained locks or lock-free data structures can improve performance while maintaining correctness.

1. Clarify Requirements

Ask about expected load, consistency requirements, and whether operations need to be thread-safe. Confirm the API and error handling expectations.

2. Design Data Model

Propose a simple structure: a map from account IDs to account objects containing balance. Discuss using concurrent data structures for thread safety.

3. Implement Operations

Describe addAccount, deposit, and transfer. For transfer, ensure atomicity by locking accounts in a consistent order to avoid deadlocks, or use a single lock for simplicity.

4. Handle Errors

Define exceptions or error codes for missing accounts and insufficient funds. Ensure operations fail gracefully without partial updates.

5. Discuss Concurrency and Trade-offs

Explain how you handle concurrent transfers: use locks, optimistic concurrency, or transactional memory. Discuss trade-offs between simplicity and performance.

Key Points to Mention

  • Thread safety: use of locks (e.g., ReentrantLock) or synchronized blocks, or concurrent collections like ConcurrentHashMap.
  • Atomicity of transfer: ensure both debit and credit happen or neither, to avoid inconsistent state.
  • Deadlock avoidance: lock accounts in a consistent order (e.g., by account ID) when transferring between two accounts.
  • Error handling: throw specific exceptions (e.g., AccountNotFoundException, InsufficientFundsException) and ensure no partial updates.
  • Performance considerations: fine-grained locking vs. coarse-grained, and potential use of optimistic concurrency with versioning.
  • Scalability: discuss limitations of in-memory approach and how it might extend to distributed systems.

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