The explosive advent of the blockchain technology has led to hundreds of blockchain systems in the industry, thousands of academic papers published over the last few years, and an even larger number of new initiatives and projects. Despite the emerging consolidation efforts, the area remains highly turbulent without systematization, educational materials, or cross-system comparative analysis. In this paper, we provide a systematic and comprehensive study of four popular yet widely different blockchain systems: Bitcoin, Ethereum, Hyperledger Fabric, and IOTA. The study is presented as a cross-system comparison, which is organized by clearly identified aspects: definitions, roles of the participants, entities, and the characteristics and design of each of the commonly used layers in the cross-system blockchain architecture. Our exploration goes deeper compared to what is currently available in academic surveys and tutorials. For example, we provide the first extensive coverage of the storage layer in Ethereum and the most comprehensive explanation of the consensus protocol in IOTA. The exposition is due to the consolidation of fragmented information gathered from white and yellow papers, academic publications, blogs, developer documentation, communication with the developers, as well as additional analysis gleaned from the source code. We hope that this survey will help the readers gain in-depth understanding of the design principles behind blockchain systems and contribute towards systematization of the area.

翻译：区块链技术的爆发式发展催生了数百个行业区块链系统、数千篇近年的学术论文，以及数量更为庞大的新倡议与项目。尽管学界已开始尝试整合相关成果，该领域仍处于高度动荡状态，缺乏系统化体系、教材资源或跨系统比较分析。本文对四种流行但架构迥异的区块链系统——比特币、以太坊、Hyperledger Fabric与IOTA——进行了系统全面的研究。研究以跨系统比较的形式呈现，通过明确界定的维度（定义、参与者角色、实体特征，以及跨系统区块链架构中各通用层的设计与特性）加以组织。相较于现有学术综述与教程，本研究的探索更为深入：例如，我们首次对以太坊的存储层进行了广泛覆盖，并对IOTA的共识协议给出了最全面的阐释。这些论述基于对分散信息的整合——这些信息源自白皮书与黄皮书、学术出版物、博客、开发者文档、与开发者的交流，以及从源代码中提取的补充分析。我们希望本综述能帮助读者深入理解区块链系统的设计原则，并推动该领域的系统化进程。