System Design: Basic Online Banking with Safe Money Movements

Context

Design a minimal but production-minded online banking service. It must support core money operations while ensuring correctness under retries and concurrency, and provide strong auditability and security.

Assume a single region deployment with a relational database (e.g., PostgreSQL). Currency is fiat (e.g., USD) with integer minor units (cents) to avoid floating point error.

Functional Requirements

1. Create accounts for users.
2. Deposit funds into an account.
3. Withdraw funds from an account.
4. Transfer funds between accounts.

Non-Functional Requirements

• Atomicity: Money movements are all-or-nothing.
• Idempotency: Safe client retries without duplicate movements.
• Concurrency safety: Prevent double-spend and race conditions.
• Immutable ledger/audit trail: Append-only financial records with reversals, not edits.
• Security: Authentication, authorization, input validation.
• Consistency and error handling: State guarantees and clear error semantics.

Deliverables

• Data model (schemas at the level of tables and key fields).
• API design (endpoints, key request/response fields, idempotency).
• Transaction flow for deposits, withdrawals, and transfers (including concurrency control).
• Approach to an immutable ledger and auditability.
• Security considerations.
• Consistency guarantees and error handling strategy.