SDE2 Interview Questions at PhonePe

Design PhonePe's UPI transaction system to handle 1 billion transactions per day. Cover: request routing to the correct bank, NPCI integration, handling duplicate transactions, consistency vs availability trade-offs, and how you build a reliable audit trail for regulatory compliance.

Design PhonePe's switch - the internal routing layer that forwards UPI requests to the correct bank. It must be highly available (99.999%), handle 50k TPS, and complete each request within 30 seconds (NPCI timeout). Cover: circuit breakers, retries, and bank-specific timeouts.

Explain how PhonePe handles transaction deduplication. A user taps "Pay" twice due to network lag. The same debit must not happen twice. Walk through idempotency at the API level, database level, and bank integration level.

Design PhonePe's merchant settlement system. Thousands of merchants receive daily settlements of their collected payments minus fees. How do you compute net payables, batch into bank transfers, and provide a reconciliation report?

Tell me about a time you had to debug a critical production issue affecting financial transactions. What was your systematic approach, how did you communicate during the incident, and what was the resolution?

You use Redis to cache OTPs with a 5-minute TTL. How does Redis actually expire keys internally — does it scan the whole keyspace on a timer?