UPI / Real-time Payments — Simple

Problem statement (interviewer prompt)

Design India's UPI-style real-time payments network: payer scans a QR / enters a VPA, money moves from payer-bank to payee-bank via a central switch in 2–5 seconds, 24×7. Cover idempotency, reversal on partial failure, end-of-day reconciliation, and PSP-level integration.

flowchart LR
  P[Payer App / PSP]
  NPCI[NPCI Switch / UPI]
  PB[Payer Bank]
  RB[Payee Bank]
  R[Payee App / PSP]
  P --> NPCI
  NPCI --> PB
  NPCI --> RB
  R --> NPCI

    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 P,NPCI,PB,RB,R service;