Design (LLD) Paytm/Phone Wallet System - Machine Coding

Asked in Phonepe interview.

Design and implement a Paytm/PhonePe Wallet System that supports multiple concurrent users performing transactions at the same time. Your system should:

✅ Features Required

1. User & Wallet Management

   • Each user has a unique wallet with balance.

   • Support wallet creation and balance inquiry.

2. Concurrent Transactions

   • Add money to wallet (via UPI, Card, NetBanking).

   • Send money from one wallet to another.

   • Handle concurrent deposits and transfers without race conditions.

3. Transaction Lifecycle

   • Each transaction can be PENDING → SUCCESS / FAILED / REVERSED.

   • Ensure atomic debit/credit (no money disappears or duplicates even under concurrency).

4. Transaction Reliability

   • Idempotency (retries should not cause double debit).

   • Refund logic (if debit succeeds but credit fails, rollback debit).

5. Transaction History

   • Maintain mini-statement per user.

   • Show in descending order of time (latest first).

Solution approaches

Will be added by 09 Sept 2025