Antonopolous and Wood explained that the cryptocurrency maintains its dataset in DAG is to enable high data access frequency without slowing the performance by retrieving its entire large dataset (2019). In other words, it is not the smart contract nor its transaction that Ethereum is employing DAG to provide non-blockchain consensus. Yeow et al. has explicitly explained the differences between Blockchain and DAG and also classifying that Blockchain is used by Ethereuem as the data structure and also disclaimed that this does not apply to its “coins”. As described earlier by Saad et al., DAG does not duplicate its transactions over the entire network. However, Hedera has also employed the DAG structure, and its whitepaper explained that blockchain and Hashgraph ledgers are commonly using the same principle where created transactions are stored into a container or Block and then synchronised through the distributed network (2019). Hedera Hashgraph defined transactions as a signed transaction created by any member and subsequently distributed a copy of the transaction to all its member, which is the nodes (Baird,2019). This contradictory definition explained the principle of DAG data structure is of differing view of nodes, and the numbers of transaction storing in each node are of different size and numbers. This ambiguous definition is critical to this Systematic Literature finding that a block of transactions size is profoundly impacting its performance and also its use case. IOTA is employing DAG and storing its transaction on each node, and its Block is a single ledger of all nodes. This data structure is reasonably for IOTA as a use case for the IoT, which has limited resources to duplicates transaction in all nodes. On the other hand, without duplicating the ledger across the nodes, it has raised a question on how IOTA can warrant for an immutable transaction without replicating its transaction across the network.
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