Paytm Interview Questions & Answers 2026

Components: Merchant onboarding, payment initiation API, multiple payment methods (UPI, cards, wallet, netbanking), bank routing, PG integrations (Visa/Mastercard, NPCI for UPI), settlement engine, refund flow, webhooks. Handle: idempotency, double-charges prevention, reconciliation, PCI-DSS compliance.

User initiates payment via UPI app -> app calls UPI service -> NPCI's Unified Payments Interface -> payer's bank for debit -> NPCI for switching -> payee's bank for credit -> response back. VPA (Virtual Payment Address) maps to bank account. PSP banks provide UPI infrastructure. PIN entry via IMPS-grade security.

Multi-layered: device fingerprinting, behavioral biometrics, velocity checks (transactions/hour), geo-location anomalies, ML models on transaction patterns, blacklist databases (devices, accounts), 3D Secure for cards, OTP for high-value, rule engines for real-time decisions. Balance fraud prevention with false positive rate.

Operations: createWallet, getBalance, transfer (between wallets), refund, transactionHistory with pagination. Use database transactions for atomicity. Idempotency keys to prevent duplicates. Tables: wallets, transactions (with status states: PENDING, SUCCESS, FAILED). Handle concurrent transfers via row-level locks.

Use idempotency keys (UUIDs from client). Store in Redis with TTL of 24 hours. On request: check Redis - if exists, return cached response; else process and cache. Backup: detect via unique constraints in DB on (user_id, amount, timestamp_window). Critical for financial systems.

Iterative approach: count nodes, if >= K, reverse next K nodes using standard linked list reversal, recursively/iteratively process remaining. Edge case: don't reverse last group if size < K (or do it - based on requirement). Time O(n), Space O(1) iterative.

Paytm is India's leading mobile payments and financial services platform. Listed on NSE/BSE in 2021. Mention their wide product portfolio (payments, wallet, loans, insurance, equity broking, ticketing), large user base (300M+), focus on small merchant ecosystem (Paytm Soundbox, Soundbox Mini), and continuous innovation.

Use STAR method. Describe incident (e.g., payment failures, app crashes), severity, your role in war room, debugging approach, communication with stakeholders, fix deployment, post-incident actions. Paytm's payment scale demands strong incident management muscle.