Payments
A shop ledger page, 1899. Every line adds up.
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
LLD Series: Digital Wallet (Low-Level Design)
Design a digital wallet (Paytm / PhonePe / PayPal style). Users can add money, transfer to other users, and view history. Make transfers safe under concurrent requests, idempotent on retries, and...
21 min read
LLD Series: Splitwise / Expense Sharing (Low-Level Design)
Design Splitwise. Users add expenses and split them equally, by exact amounts, or by percentage. Show who owes whom. Add a 'simplify debts' feature that minimises the number of transactions.
18 min read
LLD Series: ATM (Low-Level Design)
Design an ATM. A user inserts a card, enters a PIN, and can withdraw cash, deposit or check balance. The machine must dispense the right notes. Model the machine's behaviour so that invalid...
19 min read
HLD Fundamentals for Beginners: Estimation, Building Blocks and a Payments Lens
A beginner-friendly system design primer: the 6-step framework, back-of-envelope QPS math, L4 vs L7 load balancing, caching, databases, queues, reliability patterns and how a payment flows end to end.
7 min read
How to Add Payments and Subscriptions to a SaaS App
Implement subscription billing around provider-hosted checkout, verified webhooks, entitlement checks, and a clear billing state model.
2 min read