A Survey on Consensus Algorithms of Blockchain Based on DAG

With the increasing research on blockchain, the study of consensus algorithms is also deepening. The blockchain trilemma, which includes decentralization, scalability, and security, is difficult to satisfy simultaneously. Fortunately, distributed ledgers based on directed acyclic graphs (DAGs) have the potential to solve these problems. DAG-based ledgers have no limits on the out-degree and in-degree of each node, allowing a node to be referenced by multiple nodes simultaneously, which greatly increases the transaction concurrency of the ledger. Due to its structural characteristics, the scalability of a DAG-based ledger can be greatly improved, making it a promising solution for the trilemma. This paper systematically categorizes classical and newer distributed ledgers based on DAGs into four categories based on the consensus process: DAG ledgers based on the main chain, DAG ledgers based on parallel chains, DAG ledgers based on the natural topology, and DAG ledgers based on layers. The paper provides a detailed description and analysis of the consensus process of these DAG ledgers and discusses their applications. Finally, the paper identifies some challenges that must be addressed by analyzing the existing problems of DAG ledgers, which is also an important direction for future research.