The Payment Succeeded. The Response Never Arrived.

A customer clicks Pay £100.

The payment succeeds — but the response is lost.
The customer retries.
Without idempotency:
Retry → second charge ❌
With an idempotency key:
Same logical payment → same key → same result ✅

A robust design should:

• Generate the key once at the caller
• Reuse it for every retry
• Protect it with a DB unique constraint
• Store a request hash
• Persist the final response
• Pass the same key to the payment provider when supported

The key lesson:

A retry should repeat the request for the result — not repeat the business side effect.

Payments #SpringBoot #SoftwareArchitecture