Observer pattern
A shop ledger page, 1899. Every line adds up.
LLD Series: Notification Service (Low-Level Design)
Asked at: PayU, Amex, Visa and Mastercard (OTP / transaction alerts / fraud alerts), NPCI-ecosystem apps (UPI collect and debit alerts). Interviewers watch for: Strategy per channel, Observer for...
13 min read
LLD Series: Payment Gateway with Routing, Retries and Webhooks (Low-Level Design)
Design a payment gateway (Razorpay / PayU / Stripe style). Merchants submit payments by card, UPI, net-banking or wallet. The gateway routes to a bank/acquirer, falls back if one is down, retries...
29 min read