design_payment_system

Payment System

• Build a Payment System for E-commerce site
• Data should be consistent
• Do not allow users to double submit payments
• Actual handling of payments can be done by an external service
  • Example: Stripe, RazorPay
  • Batch Payments to vendors: Tipalti??
• NFR:
  • Latency is not important, it can take up to few seconds
  • Generate Analytics for:
    • Orders per user
    • Revenue per seller

Database

• payments table
  • (idempotency key, amount, status)
  • (1234, ₹104, pending)
• The Database should be strongly consistent
  • Google Spanner
  • CockroachDB

Payment Gateway

• https://www.youtube.com/watch?v=m6DtqSb1BDM
• https://www.youtube.com/watch?v=rT4sS4l51PY
• The moment payment confirmation page is loaded, payment service generates a unique identifier called idempotency key
  • https://docs.stripe.com/api/idempotent_requests
• All the subsequent communication with Payment Gateway will have this identifier as header:
  • Idempotency-Key: <your-key>
• If the request is sent multiple times to Gateway, gateway can ignore duplicate operations and make it idempotent
• Payment Gateway does not complete the processing instantly, instead it provides a webhook when the processing is complete
  • Payment Service exposes an API which is used as webhook

Failures

• Payment Server dies after the table is populated with pending status
• Payment Gateway dies and webhook is never called, hence payment stuck on pending
• Ideas:
  • Do polling after some time, if we don’t hear back from gateway
  • If gateway does not recognize idempotency key, then delete from payments table
  • if completed/failed, update the table accordingly
  • If in-progress, do nothing!

Analytics

• Payments DB ⇒ CDC ⇒ Time-Series DB
  • Orders DB sharding based on seller ID
  • Revenue DB sharding based on buyer ID