Mastering Ethereum

Chapter 14. Consensus

Throughout this book we have talked about “consensus rules”—the rules that everyone must agree to for the system to operate in a decentralized, yet deterministic, manner. In computer science, the term consensus predates blockchains and is related to the broader problem of synchronizing state in distributed systems, such that different participants in a distributed system all (eventually) agree on a single system-wide state. This is called “reaching consensus.”

When it comes to the core function of decentralized record keeping and verification, it can become problematic to rely on trust alone to ensure that information derived from state updates is correct. This rather general challenge is particularly pronounced in decentralized networks because there is no central entity to decide what is true. The lack of a central decision-making entity is one of the main attractions of blockchain platforms, because of the resulting capacity to resist censorship and the lack of dependence on authority for permission to access information. However, these benefits come at a cost: without a trusted arbitrator, any disagreements, deceptions, or differences need to be reconciled using other means. Consensus algorithms are the mechanism used to reconcile security and decentralization.

In blockchains, consensus is a critical property of the system. Simply put, there is money at stake! So, in the context of blockchains, consensus is about being able to arrive at a common state, ...