Payment integration is rarely just about creating a checkout button. Once real money is involved, the difficult part is making sure your application remains correct when networks fail, users retry requests, providers resend events, and multiple systems disagree about the current payment state.
The problem with trusting a single payment request
A browser request is not a reliable source of truth for payment completion. A customer can close the browser, lose their connection, or return to the application after the payment provider has already processed the transaction. Webhooks provide a server-to-server mechanism for receiving payment events, but webhook delivery itself is not guaranteed to happen exactly once.
Why idempotency matters
An idempotent webhook handler can safely receive the same event more than once without creating duplicate business effects. The application first identifies the event, checks whether it has already been processed, and only then performs the required state transition.
Separating payment state from business state
A useful architecture treats the payment provider as an external system and your database as the source of truth for your application's business state. Payment records, orders, invoices, and fulfillment should have explicit relationships instead of being inferred from a single callback.
What I learned building payment workflows
The most important lesson is that payment code should be designed around failure scenarios rather than the happy path. Retries, duplicate events, partial failures, delayed webhooks, and reconciliation should be considered part of the normal operating environment.
