It’s troublesome matching iOS in-app purchase renewals with their original buyer. You could use the original_transaction_id if it was reliable, but it is not. It does not change with new subscriptions, which might be made by another user using the same Apple ID.
For this reason, the documentation suggests to “populate the applicationUsername property” as an opaque identifier uniquely identifying the buyer. It is definitely provided with the initial receipt.
If an auto renewing subscriptions renews, the TransactionObserver receives a new receipt and a transactionIdentifier for the renewal, right?
Does a renewal also comprise the applicationUsername?