Decentralization has been touted as the principal security advantage which propelled blockchain systems at the forefront of developments in the financial technology space. Its exact semantics nevertheless remain highly contested and ambiguous, with proponents and critics disagreeing widely on the level of decentralization offered by existing systems. To address this, we put forth a systematization of the current landscape with respect to decentralization and we derive a methodology that can help direct future research towards defining and measuring decentralization. Our approach dissects blockchain systems into multiple layers, or strata, each possibly encapsulating multiple categories, and it enables a unified method for measuring decentralization in each one. Our layers are (1) hardware, (2) software, (3) network, (4) consensus, (5) economics ("tokenomics"), (6) client API, (7) governance, and (8) geography. Armed with this stratification, we examine for each layer which pertinent properties of distributed ledgers (safety, liveness, privacy, stability) can be at risk due to centralization and in what way. We also introduce a practical test, the "Minimum Decentralization Test" which can provide quick insights about the decentralization state of a blockchain system. To demonstrate how our stratified methodology can be used in practice, we apply it fully (layer by layer) to Bitcoin, and we provide examples of systems which comprise one or more "problematic" layers that cause them to fail the MDT. Our work highlights the challenges in measuring and achieving decentralization, and suggests various potential directions where future research is needed.

翻译：去中心化被标榜为区块链系统在金融科技领域占据前沿地位的核心安全优势。然而，其确切语义仍备受争议且模糊不清，支持者与批评者对现有系统的去中心化程度存在显著分歧。为此，我们系统梳理了当前去中心化领域的研究现状，并提出了一套方法论，旨在引导未来研究对去中心化进行定义与量化。我们的方法将区块链系统分解为多个层（或称"层级"），每个层级可能包含多个类别，并为每个层级提供了统一的去中心化度量方法。这些层级包括：(1)硬件层、(2)软件层、(3)网络层、(4)共识层、(5)经济层（代币经济学）、(6)客户端API层、(7)治理层以及(8)地理分布层。借助这一分层框架，我们逐一分析了每个层级中分布式账本的相关属性（安全性、活性、隐私性、稳定性）如何因中心化而面临风险及其具体影响机制。我们还提出了一项实用性测试——"最低去中心化测试"，可快速洞察区块链系统的去中心化状态。为展示分层方法论的实际应用，我们逐层将其完整应用于比特币系统，并列举了因一个或多个层级存在"问题"而导致MDT测试失败的案例。本工作揭示了量化与实现去中心化面临的挑战，并提出了若干亟需未来深入研究的潜在方向。