Architecture & LLD Preview
Digital Wallet
Add money, pay, and transfer between wallets with a consistent balance ledger and transaction history.
Balance is derived from an append-only ledger of CREDIT/DEBIT entries, never stored as a mutable field. Every mutation carries an idempotency key so retries are answered from cache, and transfer locks both wallets in a fixed order to avoid deadlock.
Core Design Challenge
"Design a digital wallet. How do you guarantee a retried transfer never double-applies?"
What You'll Learn:
- ✦Double-entry bookkeeping and ledger accuracy rules
- ✦ACID transaction safety, idempotency, and ledger balance consistency
- ✦Optimizing running balance computations for fast queries
- ✦Auditing, logging, and immutable historical records