Introducing consensus

The distributed consensus problem has been studied extensively in distributed systems research since the late 1970s. Distributed systems are classified into two main categories, namely, message passing and shared memory. In the context of blockchain, we are concerned with the message-passing type of distributed systems, where participants on the network communicate with each other via passing messages to each other. Consensus is the process that allows all processes in a network to agree on some specific value in the presence of faults.

As we saw in Chapter 1, Blockchain 101, there are different types of blockchain networks. In particular, two types, permissioned and public (permissionless), were discussed. The consensus problem can also be classified based on these two paradigms. For example, Bitcoin is a public blockchain. It runs PoW, also called Nakamoto consensus. In contrast, many permissioned blockchains tend to run variants of traditional or classical...