The Science of the Blockchain.pdf

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The Science of the Blockchain Roger Wattenhofer

Copyright c 2016 by Roger Wattenhofer First Edition 2016 Inverted Forest Publishing ISBN-13 978-1522751830 ISBN-10 1522751831

Preface When hanging out with colleagues from ﬁnancial technology (Fin- Tech), you cannot help but notice that the so-called blockchain is all the rage, almost as important as the Internet. Some FinTech colleagues seem to understand the blockchain as a magic piece of code that allows the participants of a distributed system to agree on a common view of the system, to track changes in the system. In the distributed systems community, agreement techniques have been known long before cryptocurrencies such as Bitcoin (where the term blockchain is borrowed) emerged. Various concepts and protocols exist, each with its own advantages and disadvantages. The idea of this book is to give a scientiﬁcally precise overview of the most interesting approaches that have emerged in the past few decades. If you are a developer (in FinTech or not), this book will help you to get a better understanding what is right and what is wrong for your distributed system, what is possible and what is not. This Book This book introduces the basic techniques when building fault- tolerant distributed systems. We will present diﬀerent protocols and algorithms that allow for fault-tolerant operation, and we will discuss practical systems that implement these techniques. The book presents the main ideas independently from each other, each topic has its own chapter. Each chapter starts with an introductory story that motivates the content of the chapter. Al- gorithms, protocols and deﬁnitions are presented formally, so that you know how to implement the ideas. Some insights are proved in theorems, so that you learn why the concepts or algorithms are correct, and what they can guarantee. Most of the other text is iii

iv presented as so-called remarks. These remarks discuss various in- formal thoughts, and often set the stage for the next idea. However, one will get the essence of each chapter even without reading any of these remarks. Each chapter also discusses the history of the ideas presented in the chapter, so that you can ﬁnd the original research. In this book, we will see diﬀerent models (and combinations of models) that may make sense in diﬀerent scenarios. The focus of the book is on protocols and systems that matter in practice. In other words, we do not discuss concepts because they are fun, but because they are practically relevant. Nevertheless, have fun!

Contents 1 Introduction 1.1 What are Distributed Systems? . . . . . . . . . . . . 1.2 Book Overview . . . . . . . . . . . . . . . . . . . . . 1 1 2 2 Fault-Tolerance & Paxos 2.1 Client/Server . . . . . . . . . . . . . . . . . . . . . . 2.2 Paxos 5 5 . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3 Consensus 17 3.1 Two Friends . . . . . . . . . . . . . . . . . . . . . . . 17 3.2 Consensus . . . . . . . . . . . . . . . . . . . . . . . . 18 Impossibility of Consensus . . . . . . . . . . . . . . . 18 3.3 3.4 Randomized Consensus . . . . . . . . . . . . . . . . 25 3.5 Shared Coin . . . . . . . . . . . . . . . . . . . . . . . 29 4 Byzantine Agreement 33 4.1 Validity . . . . . . . . . . . . . . . . . . . . . . . . . 34 4.2 How Many Byzantine Nodes? . . . . . . . . . . . . . 35 4.3 The King Algorithm . . . . . . . . . . . . . . . . . . 38 . . . . . . . . . 40 4.4 Lower Bound on Number of Rounds 4.5 Asynchronous Byzantine Agreement . . . . . . . . . 40 5 Authenticated Agreement 45 5.1 Agreement with Authentication . . . . . . . . . . . . 45 5.2 Zyzzyva . . . . . . . . . . . . . . . . . . . . . . . . . 47 6 Quorum Systems 61 6.1 Load and Work . . . . . . . . . . . . . . . . . . . . . 62 6.2 Grid Quorum Systems . . . . . . . . . . . . . . . . . 64 6.3 Fault Tolerance . . . . . . . . . . . . . . . . . . . . . 66 6.4 Byzantine Quorum Systems . . . . . . . . . . . . . . 70 v

vi CONTENTS 7 Eventual Consistency & Bitcoin 77 7.1 Consistency, Availability and Partitions . . . . . . . 78 7.2 Bitcoin . . . . . . . . . . . . . . . . . . . . . . . . . . 79 7.3 Smart Contracts . . . . . . . . . . . . . . . . . . . . 87 7.4 Weak Consistency . . . . . . . . . . . . . . . . . . . 91 8 Distributed Storage 95 8.1 Consistent Hashing . . . . . . . . . . . . . . . . . . . 95 8.2 Hypercubic Networks . . . . . . . . . . . . . . . . . . 97 8.3 DHT & Churn . . . . . . . . . . . . . . . . . . . . . 105

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