Design a Digital Wallet

Advanced · 24 min read · Low-Level Design, Case Studies: Business Systems

Double-entry ledger, idempotent transfers, ordered locking, limits and refunds.

This is an Advanced lesson, part of Premium. What it covers:

• What the interviewer expects
• 1. Requirements
• 2. The core idea: double-entry ledger
• 3. Class design
• 4. Transfers: lock two accounts without deadlocking
• 5. Idempotency: a retried tap must not pay twice
• 6. Limits, freezing and refunds
• 7. Reconciliation
• 8. Patterns and principles used
• 9. Extensions
• Interview questions
• Further reading

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