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Digital Wallet (PayPal / Paytm) — Simple#

Problem statement (interviewer prompt)

Design a digital wallet (PayPal / Paytm): users top up, send money to friends, pay merchants, and withdraw to a bank. Multi-currency, multi-rail (UPI / ACH / SEPA / cards). Strict ledger correctness — no funds appear or disappear — even with concurrent transfers.

flowchart LR
  U([User])
  WAL[Wallet Service]
  LED[(Ledger)]
  BANK[Bank rail]
  CARD[Card network]
  MERCH[Merchant API]
  U --> WAL
  WAL --> LED
  WAL --> BANK
  WAL --> CARD
  WAL --> MERCH

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class U client;
    class WAL,BANK,CARD,MERCH service;
    class LED datastore;