Blockchain consensus unraveled

Since the introduction of Bitcoin---the first wide-spread application driven by blockchains---the interest of the public and private sector in blockchains has skyrocketed. At the core of this interest are the ways in which blockchains can be used to improve data management, e.g., by enabling federated data management via decentralization, resilience against failure and malicious actors via replication and consensus, and strong data provenance via a secured immutable ledger. In practice, high-performance blockchains for data management are usually built in permissioned environments in which the participants are vetted and can be identified. In this setting, blockchains are typically powered by Byzantine fault-tolerant consensus protocols. These consensus protocols are used to provide full replication among all honest blockchain participants by enforcing an unique order of processing incoming requests among the participants. In this tutorial, we take an in-depth look at Byzantine fault-tolerant consensus. First, we take a look at the theory behind replicated computing and consensus. Then, we delve into how common consensus protocols operate. Finally, we take a look at current developments and briefly look at our vision moving forward.