There may be several ways to implement such logic. If each subscriber has his own smart contact, you can simply save a list of his subscriptions in the data of this contract. But note that in this case, as the number of subscriptions becomes bigger, all operations with them will become quite expensive.

There may be several ways to implement such logic. If each subscriber has his own smart contact, you can simply save a list of his subscriptions in the data of this contract. But note that as the number of subscriptions becomes bigger, all operations with them will become quite expensive.