A new data structure of Blockchain,named the Block Directed Acyclic Graph (DAG) structure,mainly designing for such practical applications where block sizes are large or blocks are created often,has been proposed by Yoad Lewenbery,Yonatan Sompolinsky and Aviv Zohar. In their work,a DAG structure was employed to log transactions from all blocks,including both the blocks on a main chain and on side chains. The precondition of the Block DAG is to ensure that all incorporated side-chain blocks will not conflict with prevIoUsly included content. Totally,the Block DAG strategy guarantees a high transaction throughput of Blockchian but it also makes Blockchain more vulnerable to attackers.
Q1：Block DAG是什么鬼？
顾名思义，有向无环图——在这里指一种采用有向&无环路的区块连接方式。
Q2：为什么要采用Block DAG这种连接方式？
传统的区块链中，主链之外的分叉是被丢弃的，换句话说，分叉部分的矿工的计算量被否认——这造成分叉部分计算资源的浪费以及该部分交易数据的确认延迟。为了解决这个问题，分叉链逐渐地被研究和使用，比如：“Inclusive Blockchain Protocols”一文采用了Block DAG协助使用分叉链资源，该方式如图（右边）所示：
相比于传统的Blockchain的分叉（如图左所示），Block DAG目的在充分地利用分叉部分的资源，直接体现为更多的分叉上面的交易数据被选择性地接受了，进而提高了整个区块链系统的交易流量。注意：这里的核心是“选择性”，选择策略如何保证安全性和高性能之间的平衡问题是有具有挑战性的！
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