How to Build Scalable Payment Integrations with APIs, Webhooks and Fault Tolerance

Scalable payment integrations require strict controls at every layer, from outbound API calls to inbound webhook handling. Developers are advised to set explicit timeouts, implement exponential backoff retries, and use idempotency keys to prevent duplicate transactions caused by network uncertainty. Inbound webhooks should be acknowledged immediately with an HTTP 200 response, while all business logic is offloaded to background workers to avoid retry storms from payment gateways like Razorpay, which expect responses within five seconds. Deduplication using unique event IDs with database-level constraints ensures each webhook is processed only once, even when gateways redeliver events multiple times. Circuit breakers, token bucket rate limiting, and explicit state machines further protect application stability under high transaction loads.