How nodes reach agreement

Imagine two payments sent at nearly the same moment, both spending the same funds. In a bank, one central system looks at the clock and rejects the second. A blockchain has no such system, so every node must be able to reach the same verdict on its own. That is the job of a consensus mechanism: a published set of rules for who may propose the next block, how others check it, and which chain to follow when candidates compete. Each node applies the rules independently, and because the rules are identical, honest nodes land on the same answer. The double-spend problem is exactly the case consensus was designed to settle.

Two designs dominate public networks. In proof of work, participants compete to find a number that makes a block's hash meet a difficulty target, which uses real computing effort, and the network follows the chain with the most accumulated work. In proof of stake, participants lock up units of the network's asset as a deposit and are selected to propose blocks, while others vote to attest to them. Cheating can cost the deposit. Confirmation also differs. Under proof of work, certainty grows as more blocks pile on top. Under proof of stake, validator votes can mark a block as finalized, after which reversing it would be extremely costly.