Consensus

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A blockchain is an immutable ledger maintained by a decentralised, distributed, peer-to-peer network of nodes. Each mutually distrusting node performs in accordance with a consensus algorithm. Consensus can be thought of as ‘voting’ to build agreement among the nodes on a common, unambiguous ordering of transactions and blocks.

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The consequences of failure

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The consensus algorithm plays a crucial part in determining whether a blockchain can succeed for its intended purpose. The consensus mechanism should guarantee the blockchain’s integrity and consistency across all nodes, to safeguard the key blockchain properties of immutability and auditability. A poor choice of consensus mechanism can render the blockchain platform useless by compromising the data recorded on it.

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An inferior consensus mechanism can result in a Blockchain fork which occurs when different nodes converge on different blocks as the ‘true’ blockchain.

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Consensus failure

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Occurs when the nodes fail to reach consensus (e.g. from failure to reach required supermajority, failure due to node or network failure, non-compliant nodes, or inability to resolve consensus due to inconsistent messages).

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Dominance

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Occurs when a group of nodes can manipulate consensus to confirm and process transactions according to their own rules (e.g. Sybil1 or ‘51%’2 attack).

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Cheating

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Occurs when validating nodes can maintain parallel and different forks in the blockchain, allowing them to ‘double spend’ fraudulently.

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Inferior Performance

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Occurs when the consensus is unacceptably slow to converge under certain conditions.

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