A consensus algorithm is a computer program or set of rules that provides the basis for how the blockchain network agrees on which transactions should be added to its ledger. Without this agreement, attacks known as double spending would be possible. The consensus algorithm helps ensure that all nodes in the network agree on which transactions are valid, and thus it's an essential component of blockchain technology.
Let's take an example of how a consensus algorithm works. Let's say you want to send money to someone. You submit your transaction request, and it is then sent to nodes in the network. Some of these nodes want to generate blocks with incoming (legitimate) transactions. Let's call these nodes Generators, and let's take the Proof of Justice consensus algorithm as a baseline for our example.
Once your transaction has been established as legitimate and other criteria are met, such as by having an external Enforcer node validate it, a Generator adds it to the blockchain ledger in the form of a block. A blockchain ledger can be thought of as a public, impossible-to-modify database where all validated transactions are stored. The consensus algorithm ensures that all nodes agree on which transactions should be added to this ledger so there isn't any fraud or disputes.
Now let's look at why consensus algorithms are so important in blockchain technology. Every node in the network is responsible for maintaining a copy of the ledger, and they must all agree on which transactions should be added. Without consensus algorithms, it would be difficult to keep track of who has spent what money and to detect any fraudulent activity. Consensus algorithms help ensure that all nodes are in agreement about what data should be stored in the ledger.
Lastly, consensus algorithms are also important for creating trust between participants in a blockchain network. Since Generators can only add validated transactions to the ledger, it provides an extra layer of security against double-spending and other malicious attacks.
