Design a bank account ledger
Overview
This question evaluates understanding of distributed systems, transactional databases, double-entry ledger accounting, idempotency, concurrency control, reconciliation with external payment rails, and operational concerns for financial systems.
Coinbase
Sep 6, 2025, 12:00 AM
Software Engineer
Take-home Project
System Design
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Design a Bank Account Ledger Service
Context
You are building the core ledger service for a fintech wallet/banking platform. The service must support customer account balances with full auditability and correctness under concurrency and failures.
Functional Requirements
- Deposits, withdrawals, and intra-ledger transfers
- Balance queries (posted vs available)
- Transaction history with pagination and filters
Non-Functional and Architectural Requirements
- ACID semantics for single-account updates
- Atomicity for multi-account transfers
- Double-entry bookkeeping for auditability and reconciliation
- Idempotency and exactly-once effect for API requests and async processing
- Concurrency control under high throughput
- Integration and reconciliation with external payment rails (e.g., ACH, cards)
- Fraud checks, spending limits, and dispute/chargeback handling
- Scalability: sharding/partitioning, indexing strategy, read replicas, caching
- Recovery strategies: backups, point-in-time recovery, invariants validation
Deliverables
Describe:
- Public APIs (endpoints, request/response shape, idempotency usage)
- Data model (entities, relationships, constraints) using a double-entry ledger
- How to ensure ACID semantics and atomic multi-account transfers
- Idempotency and exactly-once processing design
- Concurrency control approach
- Reconciliation with external rails, fraud checks, limits, and dispute flows
- Sharding, indexing, read replicas, and recovery strategies for scale and consistency
Solution
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