IPNS

IPNs, or Instant Payment Notifications, are a messaging mechanism used by online payment services to inform merchants about events related to financial transactions. The most widely known implementation is PayPal IPN, though other processors offer similar capabilities. IPN enables asynchronous updates without the merchant polling the processor for status changes.

How IPN works: When a registered merchant experiences a transaction event, the payment processor sends an HTTP POST to a configured listener URL on the merchant’s site, containing details such as txn_id, payment_status, mc_gross, mc_currency, receiver_email, and custom fields. To verify authenticity, the merchant sends back a POST with cmd=_notify-validate to the processor’s IPN verification endpoint. If the processor responds with VERIFIED, the merchant can process the payment; if INVALID, the message is discarded. Common checks include ensuring payment_status is Completed, that the txn_id has not been processed before, that receiver_email matches the intended account, and that the amount and currency match expected values.

Reliability and best practices: IPN is asynchronous and may arrive out of order or be retried after failures, so processing should be idempotent. Use TLS/HTTPS, log all IPN traffic, and implement deduplication. The listener should respond quickly, typically with a 200 OK, to minimize retries, with any longer validation performed after a quick acknowledgment if needed. Merchants should also monitor for anomalies, such as unexpected currencies or amounts.

Variants and evolution: In modern ecosystems, many processors offer webhooks as a replacement or complement to IPN, providing signed payloads and easier verification. While IPN remains in use for legacy integrations, webhooks are increasingly common for new implementations.