Fintech Logic

Preventing Duplicate Charges in Subscription Apps

In e-commerce, double-charging is a reputation killer. I built a robust billing state machine to handle the delicate 'AppSubscription' flow.

Idempotency Keys: I generate a hash of the plan_id and billing_cycle. If a request is retried, the system recognizes the hash and blocks a second API call.
Atomic Locks: I use Redis locks to ensure that even if a merchant clicks "Subscribe" twice rapidly, only one process can interact with the Shopify API.
Webbook Verification: We only activate the subscription in our DB after receiving the APP_SUBSCRIPTIONS_UPDATE webhook.