摘要:自2008年比特币出现以来，研究****相继提出了多种分布式账本技术，其中，区块链是当前分布式账本最主要的实现形式之一.但当前区块链中存在一个核心问题：可扩展性瓶颈.具体而言，区块链的吞吐量严重不足，且其交易确认也较为缓慢，这些因素极大地限制了它的实际应用.在此背景下，基于DAG（有向无环图）的分布式账本因其具有高并发特性，有望突破传统区块链中的性能瓶颈，从而受到了学术界和产业界越来越多的关注和研究.在基于DAG的分布式账本中，最为核心和关键的技术是其共识机制，为此，对该关键技术进行了系统深入的研究.首次从共识形态出发将现有基于DAG的分布式账本分为以下3类：基于主干链的DAG账本；基于平行链的DAG账本；基于朴素DAG的账本.在此基础上，对不同类型的共识机制本质原理及特性进行了深入阐述，并从不同层面对它们进行了详细的对比分析.最后，指出基于DAG的共识机制研究中存在的问题与挑战，并给出进一步的研究方向.



Abstract:Since the emergence of Bitcoin in 2008, various decentralized consensus schemes have been brought about to realize a decentralized ledger. Most existing schemes adopt a blockchain, which is the fundamental building block of the consensus of Bitcoin, to store and extend the ledger. However, classical blockchain is heavily bounded in its scalability. Specifically, its throughput of transactions is far from satisfactory and transactions are confirmed at a slow rate, which greatly limit the practical application of blockchain. To face this issue, novel consensus schemes based on direct acyclic graphs (DAGs) are introduced in an attempt of achieving better performance. Due to its high concurrency feature, the research of DAG-based distributed ledger is getting more and more attention. With a systematic survey, it is proposed that DAG ledgers can be classified into three categories by the feature of its underground consensus mechanisms, i.e., DAG with a main chain, DAG of parallel chains, and naive DAG. To begin with, the pivotal features and characteristics of current consensus system associated with each category are introduced. After that, a comprehensive evaluation regarding different aspects of current systems is conducted. Finally, several open challenges on DAG-Based consensus schemes are identified to consider in future research endeavors.



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