Besu Certified Professional: Consensus Protocols (PoW, PoA, IBFT 2.0, QBFT)

This domain tests protocol selection and the arithmetic of fault tolerance. Besu supports Ethash proof of work, Clique proof of authority, and the BFT protocols IBFT 2.0 and QBFT.

Core rules

• Ethash mining is unsuitable for small consortium chains because hash rate centralises control.
• Clique rotates block signing among authorised signers. It offers no immediate finality, can fork, and tolerates fewer than half of signers being malicious or offline.
• IBFT 2.0 and QBFT are Byzantine fault tolerant and give instant finality once a block is committed. With N validators, the protocol tolerates f = (N-1)/3 faulty validators and needs a quorum of 2f+1.
• Four validators therefore tolerate one fault and need three votes; three validators tolerate none.
• Validator changes happen through proposals and votes, and take effect at an epoch boundary. QBFT is the successor to IBFT 2.0 and can use a validator smart contract.
• blockperiodseconds sets the target block time; requesttimeoutseconds controls round changes when the proposer stalls.

Worked example

A network of seven validators: f = (7-1)/3 = 2, quorum = 2(2)+1 = 5. Two validators can be offline and blocks still commit; a third outage drops the live set to four and the chain stalls until connectivity returns. If you had chosen three validators for resilience, the arithmetic shows f = 0, so any single failure halts the chain.

Traps

• Expecting fault tolerance from three validators or from Clique.
• Running two validators with the same key, which double-signs and can stall or fork the chain.
• A validator key whose address is absent from genesis extraData.
• Editing genesis to change validators instead of voting.
• Assuming Clique provides finality.