Resolving Forks Using the Longest-Chain-Criterion

Why fork

We showed in this section that each node is working on its own candidate block. When a node finds a solution for the Proof-of-Work, it broadcasts the candidate block immediately to the network, and all nodes accept the new block immediately if it’s valid.

Now, it can happen that two competing nodes find a solution almost simultaneously, leading to the situation that not every node receives the same block first. As a result, the nodes of the network don’t have identical information, and the individual copies of the blockchain diverge, and the system ends up with a fork, as shown in this figure. The nodes have now a different view of the world state, and the network will no longer be able to maintain a unique authoritative chronology of the blocks.

Figure 1