Limitations on high latency and low scalability of classical blockchain systems retard their adoptions and applications. Reconstructed blockchain systems have been proposed to avoid the consumption of competitive transactions caused by linear sequenced blocks. These systems, instead, structure transactions/blocks in the form of Directed Acyclic Graph (DAG) and consequently rebuild upper layer components. The promise of DAG-based blockchain systems is to enable fast confirmation (complete transactions within million seconds) and high scalability (attach transactions in parallel) without significantly compromising security. However, this field still lacks systematic work that summarises DAG techniques. To bridge the gap, this Systematization of Knowledge (SoK) provides a comprehensive analysis of ever-existing and ongoing DAG-based blockchain systems. We abstract a general model to capture the main features and identify six types of design patterns. Then, we evaluate these systems from the perspectives of structure, consensus, property, security, and performance. We further discuss the trade-off between different factors, open challenges, and the potentiality of DAG-based solutions, indicating their promising directions for future research.

经典区块链系统的高延迟和低可扩展性的限制阻碍了它们的采用和应用。重构的区块链系统已经被提出来避免由线性排序的块引起的竞争性交易的消耗。相反，这些系统以有向无环图 (DAG)的形式构建事务/块，并因此重建上层组件。基于 DAG 的区块链系统的承诺是在不显着影响安全性的情况下实现快速确认（在百万秒内完成交易）和高可扩展性（并行附加交易）。然而，该领域仍然缺乏总结 DAG 技术的系统性工作。为了弥合差距，知识系统化 (SoK)提供了对现有和正在进行的基于 DAG 的区块链系统的全面分析。我们抽象出一个通用模型来捕捉主要特征并确定六种类型的设计模式。然后，我们从结构、共识、属性、安全和性能的角度评估这些系统。我们进一步讨论了不同因素、开放挑战和基于 DAG 的解决方案的潜力之间的权衡，指出了它们未来研究的有希望的方向。