Immutability is a core design goal of permissionless public blockchain systems. However, rewrites are more common than is normally understood, and the risk of rewrite, cyberattack, exploit, or black swan event is also high. Taking the position that strict immutability is neither possible on these networks nor the observed reality, this paper uses thematic analysis of node operator interviews to examine the limits of immutability in light of rewrite events. The end result is a qualitative definition of the conditional immutability found on these networks, which we call Practical Immutability. This is immutability contingent on the legitimate governance demands of the network, where network stakeholders place their trust in the governance topology of a network to lend it legitimacy, and thus manage ledger state.

翻译：不可变性是无许可公有区块链系统的核心设计目标。然而，账本重写现象比通常认知更为普遍，且重写风险、网络攻击、漏洞利用或黑天鹅事件的发生概率也较高。本文基于"严格不可变性在这些网络中既不可能实现，也非观察到的现实"这一立场，通过对节点运营者的访谈进行主题分析，结合重写事件审视不可变性的实际边界。最终提出对这些网络中存在的条件性不可变性的定性定义，我们称之为"实践不可变性"。这种不可变性取决于网络的合法治理需求，网络利益相关者通过信任网络的治理拓扑结构赋予其合法性，从而实现对账本状态的管理。