I want to track subscriptions of multiple applications and need to choose unique key.
Can transactionId from app1 match somehow transactionId from app2?
If yes, then Is it possible situation: app1 and app2 have products with same productId (e.g. "com.subscription") and have transactions with same transactionId?
So should I use key (appAppleId, productId, transactionId)?
(It would be more convinient to use transactionId, but I am not sure about uniqueness)
yes, it is unique that customer and that application. https://developer.apple.com/documentation/appstorereceipts/transaction_id