From Classical to Blockchain Consensus: What Are the Exact Algorithms?

Abstract

This tutorial describes well-known algorithms for distributed consensus problems, from classical consensus to blockchain consensus, and discusses exact algorithms that are high-level as in pseudocode and directly executable at the same time. The tutorial consists of five parts: (1) A introduction to different distributed consensus problems, from classical consensus and Byzantine consensus to blockchain consensus. (2) An overview of well-known algorithms, from Paxos for classical consensus to the Bitcoin algorithm for blockchain consensus, including important variants such as Viewstamped Replication and Virtual Synchrony, as well as Proof-of-Stake vs. Proof-of-Work. (3) An overview of a method and language, DistAlgo, for expressing distributed algorithms precisely at a high-level as pseudocode and having them be directly executable at the same time. (4) A study of exact algorithms expressed at a high level for the most extensively studied algorithm variants, including Lamport's Paxos for classical consensus and Nakamoto's Bitcoin algorithm for blockchain consensus. (5) A demo of the direct execution of these exact algorithms by distributed processes.