Blockchain CAP Theorem Allows User-Dependent Adaptivity and Finality
Blockchain CAP Theorem Allows User-Dependent Adaptivity and Finality
复制标题
区块链 CAP 定理允许用户依赖的适应性和最终性
DOI：
10.1007/978-3-662-64331-0_5
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发表时间：
2021
期刊：
影响因子：
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通讯作者：
Viswanath, P
中科院分区：
文献类型：
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作者：
Sankagiri, S;Wang, X;Kannan, S;Viswanath, P
最长链区块链协议，如比特币，即使在积极参与用户数量可变的情况下，也能保证活跃度，即它们是自适应的。然而，它们在网络分区下是不安全的，也就是说，它们不能保证最终性。另一方面，经典区块链协议，如PBFT，实现了最终性，但没有自适应性。事实上，区块链背景下的CAP定理断言，没有任何协议可以同时提供自适应性和最终性。我们提出了一个新的区块链协议，称为检查点最长链，它为个人用户提供了最终性和适应性之间的选择，而不是在系统范围内强加它。该协议的显著特点是它支持两种不同的确认规则：一种保证自适应，另一种保证最终性。更乐观的自适应规则总是确认被更保守的规则标记为最终确定的块，并且可能在不同的参与水平下确认更多的块。客户端（用户）根据个人喜好在两种确认规则之间进行本地选择，而矿工则遵循与两种确认规则一致的固定块提议规则。提议的协议具有内在有效性的额外好处：最终确定的区块始终位于单个区块链上，因此矿工可以在提议区块时证明交易的有效性。我们的协议建立在终末性小工具的概念之上，终末性小工具是一种为最长链协议添加终末性的流行技术。
Longest-chain blockchain protocols, such as Bitcoin, guarantee liveness even when the number of actively participating users is variable, i.e., they are adaptive. However, they are not safe under network partitions, i.e., they do not guarantee finality. On the other hand, classical blockchain protocols, like PBFT, achieve finality but not adaptivity. Indeed, the CAP theorem in the context of blockchains asserts that no protocol can simultaneously offer both adaptivity and finality. We propose a new blockchain protocol, called the checkpointed longest chain, that offers individual users the choice between finality and adaptivity instead of imposing it at a system-wide level. This protocol’s salient feature is that it supports two distinct confirmation rules: one that guarantees adaptivity and the other finality. The more optimistic adaptive rule always confirms blocks that are marked as finalized by the more conservative rule, and may possibly confirm more blocks during variable participation levels. Clients (users) make a local choice between the confirmation rules as per their personal preference, while miners follow a fixed block proposal rule that is consistent with both confirmation rules. The proposed protocol has the additional benefit of intrinsic validity: the finalized blocks always lie on a single blockchain, and therefore miners can attest to the validity of transactions while proposing blocks. Our protocol builds on the notion of a finality gadget, a popular technique for adding finality to longest-chain protocols.