区块链中的终结性
On Finality in Blockchains
摘要：
存在许多形式的区块链终结条件，从确定性终止到概率性终止。为了支持可用性，而不是面对分区的一致性，大多数区块链只提供概率性的最终结局：块在被添加到区块链后可能会被撤销，但随着它们在链中下沉得更深，概率会降低。其他区块链通过利用附加块的即时终结性（附加的块永远不会被撤销）来支持一致性，代价是额外的同步。在本文中，我们专注于实现具有确定性最终终结性的区块链的必要和充分条件，这确保了在不同进程中选择的主链共享一个共同的递增前缀。这是一种弱得多的终结性形式，它允许我们在异步系统中提供一个解决方案，该系统受到无限数量的拜占庭式故障的影响。我们研究了更强的形式的最终终局性，以及表明，不幸的是，它是不可能提供一个有界位移。通过有界位移，我们的意思是可以从当前区块链中撤销的（未知）块的数量是有界的。这个问题归结为共识或最终共识，取决于界限是否已知。我们还表明，经典的选择机制，如在比特币中，在最长的链上附加块是不符合最终的解决方案。
英文摘要：
There exist many forms of Blockchain finality conditions, from deterministic to probabilistic terminations. To favor availability against consistency in the face of partitions, most blockchains only offer probabilistic eventual finality: blocks may be revoked after being appended to the blockchain, yet with decreasing probability as they sink deeper into the chain. Other blockchains favor consistency by leveraging the immediate finality of Consensus-a block appended is never revoked-at the cost of additional synchronization. In this paper, we focus on necessary and sufficient conditions to implement a blockchain with deterministic eventual finality, which ensures that selected main chains at different processes share a common increasing prefix. This is a much weaker form of finality that allows us to provide a solution in an asynchronous system subject to unlimited number of byzantine failures. We study stronger forms of eventual finality as well and show that it is unfortunately impossible to provide a bounded displacement. By bounded displacement we mean that the (unknown) number of blocks that can be revoked from the current blockchain is bounded. This problem reduces to consensus or eventual consensus depending on whether the bound is known or not. We also show that the classical selection mechanism, such as in Bitcoin, that appends blocks at the longest chain is not compliant with a solution to eventual finality.
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