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Understanding Byzantine Fault Tolerance

Byzantine Fault Tolerance (BFT) is a characteristic of a decentralized system that indicates that it can tolerate Byzantine failures. A crash failure is when nodes just stopping to do anything (no messages at all) and Byzantine failure is when nodes just don't do anything or exhibit arbitrary behavior. Basically, Byzantine failures include crash failures.

In any decentralized computing environment where a blockchain data structure is used, there is a risk that one or more rogue or unreliable actors could be a reason for the environment to disband. A server cluster will not work well if a few servers within it lose out on passing data to other servers in a consistent manner. In order to be reliable, the decentralized computing environment has to be designed in a way that it has solutions to these kinds of Byzantine failures.

On blockchain-based decentralised applications, there is, by definition, no central authority, so a special kind of protocol called the consensus protocol is used to achieve BFT.

In simple terms, you must be wondering how to ensure that everyone has the same copy of the blockchain, and how to know which blockchain is correct when two nodes publish different blockchains? Also, how do you decide who creates the blocks, as there is nothing such as a master node in decentralized architecture? Well, consensus protocols provide an answer to these questions. A few examples of consensus protocols are Proof-of-Work (PoW), Proof-of-Stake (PoS), Proof-of-Authority (PoA), PBFT, and so on.

A consensus protocol is designed specially for permissioned or public blockchains. A consensus protocol made for a public blockchain is likely to create security and performance issues when implemented in a permissioned blockchain. Every consensus protocol has different performance and scalability vectors. You have to be alert while selecting a consensus protocol for your blockchain-based DApp.

Consensus protocols such as Raft and Paxos are not BFT; rather, they make the system only crash-tolerant. So, you should also consider this when choosing a consensus protocol.

You might have come across the term PoA. PoA is a categorisation of consensus protocols in which there is a set of authorities—nodes that are explicitly allowed to create new blocks and secure the blockchain. Ripple's iterative process, PBFT, Clique, Aura, and so on, are examples of PoA-based consensus protocols.