Design a Blockchain Monitoring System

Design a blockchain transaction monitoring system. Each blockchain has many blocks, and each block has transactions with incrementally assigned IDs. Three APIs are provided: (1) given a block_id, return the latest transaction ID on that chain; (2) given a subscription_id and block_id, return the transaction ID at the time the subscription was created; (3) given a block_id and transaction_id, return the real transaction hash. Design a system where a user can create a subscription that, when queried, returns all transactions from the subscription's creation point up to the most recent transaction. For example, if blockchain bc_1 has transactions 1→2→3→4→5→6→7 and subscription_1 was created when transaction 4 was the head, the system should return transactions 4, 5, 6, and 7.