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

User & Wallet Management
- Each user has a unique wallet with balance.
- Support wallet creation and balance inquiry.
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.
Transaction Lifecycle
- Each transaction can be PENDING → SUCCESS / FAILED / REVERSED.
- Ensure atomic debit/credit (no money disappears or duplicates even under concurrency).
Transaction Reliability
- Idempotency (retries should not cause double debit).
- Refund logic (if debit succeeds but credit fails, rollback debit).
Transaction History
- Maintain mini-statement per user.
- Show in descending order of time (latest first).

Solution approaches

Will be added by 09 Sept 2025

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

Table of contents