Design a Merchant Ledger Service
Company: Stripe
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Onsite
Design a ledger service for a payments platform.
The system must provide:
- An API to record a transaction for a merchant.
- An API to query the current balance of a merchant.
Your design should explain:
- How to model money and price values safely.
- How to represent debits, credits, and currencies.
- How to prevent double posting and support idempotent retries.
- How to maintain an auditable transaction history.
- How to keep balances correct under concurrency, retries, and partial failures.
- What storage model, APIs, and consistency guarantees you would choose.
- How the system would scale as transaction volume grows.
Quick Answer: This question evaluates system design skills related to modeling monetary values, transactional integrity, idempotency, auditability, concurrency control, storage choices, and scalability for payment-ledger systems, testing knowledge of distributed systems and data consistency trade-offs.