Thousands of independent nodes receive transactions at slightly different times and must arrive at one shared history without trusting each other or any coordinator. The consensus mechanism is the procedure that produces that agreement, and it has to work even when some participants are faulty or actively hostile.
The approaches differ in what they make expensive. Proof of work requires computation, so rewriting history costs electricity. Proof of stake requires posted collateral, so attacking the chain costs the attacker's own capital through slashing. Both make honest participation cheaper than dishonest participation, which is the property that matters; they simply buy it with different resources.
Why does a blockchain need a consensus mechanism?
Because there is no central authority to declare which version of history is correct. Consensus is how independent nodes that do not trust each other converge on the same ledger.
Last updated October 17, 2026