##### Question Design and implement APIs to create an account, deposit money into an existing account, and transfer funds between two accounts while maintaining atomic balance updates and proper error handling.

This question evaluates skills in transactional system design, concurrency control, idempotency, API specification, data modeling, and robust error handling for money-movement and account operations.

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This is a medium difficulty System Design question, commonly asked during Take-home Project rounds at HubSpot.

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Implement account CRUD and transfer | HubSpot Interview Question

Design and Implement Money Movement APIs

Context

You are building a small money-movement service for a single currency. The system must support creating accounts, depositing money, and transferring funds between accounts. Operations must be atomic and resilient to concurrency issues, and should provide clear, consistent error handling.

Assume a service with a relational database. You may choose any language/framework; document your choices and trade-offs.

Requirements

APIs
- Create account
- Deposit into an existing account
- Transfer funds between two existing accounts
Correctness and Atomicity
- Balance updates must be atomic and consistent under concurrent requests.
- Prevent overdrafts; a transfer should fail if the source account lacks funds.
- Ensure no partial updates (transfer must either fully succeed or have no effect).
Concurrency Control
- Handle concurrent deposits/transfers safely (no lost updates, no race conditions).
- Avoid deadlocks.
Idempotency and Error Handling
- Make deposit and transfer endpoints idempotent (e.g., via an Idempotency-Key header).
- Define clear error codes/messages (validation, not found, insufficient funds, conflicts).
Data Modeling
- Use an integer-based amount (e.g., cents) to avoid floating-point errors.
- Persist a minimal audit trail (e.g., ledger entries) for deposits/transfers.
Documentation & Tests
- Document API contracts (requests/responses), validation rules, and error schema.
- Provide a basic test plan covering core and edge cases (including concurrency and idempotency).

Deliverables

Data model (DDL or schema definition).
API specification (routes, payloads, responses, error schema).
Implementation outline with transaction logic ensuring atomicity.
Notes on concurrency control, idempotency, and validation.
Test plan (and tests if you implement code).

Context

Assume a service with a relational database. You may choose any language/framework; document your choices and trade-offs.

Requirements

APIs

Create account
Deposit into an existing account
Transfer funds between two existing accounts

Correctness and Atomicity

Balance updates must be atomic and consistent under concurrent requests.
Prevent overdrafts; a transfer should fail if the source account lacks funds.
Ensure no partial updates (transfer must either fully succeed or have no effect).

Concurrency Control

Handle concurrent deposits/transfers safely (no lost updates, no race conditions).
Avoid deadlocks.

Idempotency and Error Handling

Make deposit and transfer endpoints idempotent (e.g., via an Idempotency-Key header).
Define clear error codes/messages (validation, not found, insufficient funds, conflicts).

Data Modeling

Use an integer-based amount (e.g., cents) to avoid floating-point errors.
Persist a minimal audit trail (e.g., ledger entries) for deposits/transfers.

Documentation & Tests

Document API contracts (requests/responses), validation rules, and error schema.
Provide a basic test plan covering core and edge cases (including concurrency and idempotency).

Implement account CRUD and transfer

Quick Overview

Design and Implement Money Movement APIs

Context

Requirements

Deliverables

Solution

Submit Your Answer to Earn 20XP

Implement account CRUD and transfer

Quick Overview

Design and Implement Money Movement APIs

Context

Requirements

Deliverables

Solution

Submit Your Answer to Earn 20XP