Idempotency

Why Idempotency Matters: A Real-Life Lesson from My Webhook System.

In backend development, idempotency is one concept that sounds complex but is absolutely critical. In simple terms, idempotency means ensuring that running the same request multiple times does not produce unintended side effects. Whether crediting a wallet, processing a payment, or creating a transaction record, the action should only be executed once, even if it's triggered 10 times by mistake.
This sounds basic, right? But trust me, it can be the difference between a safe system and one that loses money overnight.

My Recent Experience: The Webhook That Refused to Stop
Recently, I had an interesting situation on my platform. For most users, everything was working perfectly. Each webhook call was processed once as expected.
But suddenly, one particular user triggered multiple webhook calls back-to-back, faster than I could refresh my logs.
If I hadn’t implemented idempotency handling, this user would have been credited multiple times for a single transaction. Imagine approving a ₦10,000 credit five times by accident just because the external system retried the webhook🤔🤔

Thank God my system was built with idempotency protection. Each webhook came with a unique reference, and before processing any request, my backend checked:
“Have I already processed this transaction before?”
If yes, I simply ignored duplicates and responded with success without re-crediting the user.
Which simply means that:
✅ No duplicate credits.
✅ No financial loss.
✅ Happy user, safe system.

Why Webhooks Repeat?
It’s Not Always a Bug. Most people assume that if their webhook fires twice, something is wrong. Not exactly!

Many payment providers intentionally resend webhooks if they don’t receive a successful response immediately. Network delays, timeouts, or processing lag can make them believe your server didn’t receive the notification, so they retry to be safe.
Meaning, you should never assume a webhook will come only once.

How to Implement Idempotency (Conceptually)
The idea is simple:
Every transaction or webhook must have a unique reference (ID).
Before processing, check if that reference has been handled before.
If no, process it and store it as completed.
If yes, ignore it and return success without doing anything extra.
You can store processed references in your database, Redis, or any persistent store.
If your system handles payments, wallet funding, withdrawals, or sensitive financial operations, idempotency is not optional; it’s mandatory. Never let external retries become internal losses.
Build smart. Plan for duplicates. Embrace idempotency.