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In the blockchain world, consensus is the process of developing an agreement among a group of commonly mistrusting shareholders. These are the full nodes on the network. The full nodes are validating transactions that are entered into the network to be recorded as part of the ledger.

Figure 1-2 shows the concept of how blockchains come to agreement.

ssss1 How blockchains work.

Each blockchain has its own algorithms for creating agreement within its network on the entries being added. There are many different models for creating consensus because each blockchain is creating different kinds of entries. Some blockchains are trading value, others are storing data, and others are securing systems and contracts.

Bitcoin, for example, is trading the value of its token between members on its network. The tokens have a market value, so the requirements related to performance, scalability, consistency, threat model, and failure model will be higher. Bitcoin operates under the assumption that a malicious attacker may want to corrupt the history of trades in order to steal tokens. Bitcoin prevents this from happening by using a consensus model called “proof of work” that solves the Byzantine general’s problem (which we discuss further in Book 6, ssss1): “How do you know that the information you are looking at has not been changed internally or externally?” Because changing or manipulating data is almost always possible, the reliability of data is a big problem for computer science.