Blockchain Developer Consensus Algorithms and Protocols — Questions and Answers

Question 1: In Practical Byzantine Fault Tolerance (PBFT), how many total nodes are required to tolerate f faulty nodes?

2f + 1
3f + 1 (Correct answer)
f + 1
4f

Correct answer: 3f + 1

PBFT requires 3f + 1 total nodes to safely tolerate f Byzantine faults.

Question 2: What problem does the 'nothing-at-stake' issue describe in Proof of Stake systems?

Validators can cheaply vote on multiple competing chains (Correct answer)
Miners waste electricity
Nodes cannot reach finality
Transactions have zero fees

Correct answer: Validators can cheaply vote on multiple competing chains

Nothing-at-stake means validators face no cost to validate multiple forks, undermining consensus.

Question 3: In Proof of Work, what does the 'difficulty' parameter primarily control?

The block reward amount
The number of transactions per block
The expected time to find a valid block (Correct answer)
The maximum chain length

Correct answer: The expected time to find a valid block

Difficulty adjusts how hard the hash puzzle is to keep block times roughly constant.

Question 4: Which consensus mechanism does Ethereum use after 'The Merge'?

Proof of Work
Proof of Stake (Correct answer)
Proof of Authority
Delegated Proof of Stake

Correct answer: Proof of Stake

Ethereum transitioned from Proof of Work to Proof of Stake during The Merge in 2022.

Question 5: What is a key characteristic of Delegated Proof of Stake (DPoS)?

Every node mines blocks
Token holders vote for a limited set of block producers (Correct answer)
It requires solving hash puzzles
It has no validators

Correct answer: Token holders vote for a limited set of block producers

In DPoS, stakeholders elect a small group of delegates who produce blocks on their behalf.

Question 6: What does 'finality' mean in a blockchain consensus context?

The point at which a transaction cannot be reversed (Correct answer)
The last block ever produced
The total supply cap
The end of a mining epoch

Correct answer: The point at which a transaction cannot be reversed

Finality is the guarantee that a committed transaction is permanent and irreversible.

Question 7: Which attack involves an entity controlling more than half of a network's mining or staking power?

Sybil attack
51% attack (Correct answer)
Eclipse attack
Replay attack

Correct answer: 51% attack

A 51% attack lets an attacker control the majority of hashpower or stake to rewrite history.

Question 8: In the Raft consensus algorithm, what role is responsible for handling all client requests?

Follower
Candidate
Leader (Correct answer)
Observer

Correct answer: Leader

In Raft, the elected leader handles client requests and replicates the log to followers.

Question 9: What is the primary purpose of a 'slashing' mechanism in Proof of Stake?

To reward validators
To penalize validators for misbehavior (Correct answer)
To increase token supply
To speed up block times

Correct answer: To penalize validators for misbehavior

Slashing destroys part of a validator's stake to punish malicious or faulty behavior.

Question 10: Which consensus property guarantees that all honest nodes eventually agree on the same value?

Liveness
Agreement (safety) (Correct answer)
Availability
Partition tolerance

Correct answer: Agreement (safety)

Agreement, a safety property, ensures all honest nodes decide on the same value.

Question 11: In Proof of Stake, what generally determines a validator's chance of being selected to propose a block?

CPU power
Amount of stake committed (Correct answer)
Internet speed
Account age only

Correct answer: Amount of stake committed

Selection probability typically scales with the amount of tokens a validator stakes.

Question 12: What does the FLP impossibility result state about distributed consensus?

Consensus is impossible with any faults
No deterministic protocol guarantees consensus in an async system with one faulty process (Correct answer)
Consensus requires a leader
Consensus needs unlimited nodes

Correct answer: No deterministic protocol guarantees consensus in an async system with one faulty process

FLP proves no deterministic asynchronous protocol can guarantee consensus if even one process may fail.

Question 13: Which consensus mechanism is commonly used by permissioned blockchains like Hyperledger Fabric?

Proof of Work
Raft / BFT-style ordering (Correct answer)
Proof of Stake
Proof of Burn

Correct answer: Raft / BFT-style ordering

Hyperledger Fabric uses ordering services based on Raft or BFT rather than mining.

Question 14: What is the main goal of a Sybil attack on a consensus network?

To create many fake identities to gain disproportionate influence (Correct answer)
To double-spend a single coin
To delay block propagation
To corrupt the genesis block

Correct answer: To create many fake identities to gain disproportionate influence

A Sybil attack floods the network with fake nodes to gain outsized control.

Question 15: In Ethereum's PoS, what is the role of an 'attestation'?

Proposing a new block
A validator's vote on the validity of a block (Correct answer)
Burning gas fees
Generating a private key

Correct answer: A validator's vote on the validity of a block

An attestation is a validator's signed vote confirming a block and the current chain head.

Question 16: What is 'chain reorganization' (reorg) in a blockchain?

Compressing the chain
Replacing blocks with a different competing branch (Correct answer)
Reordering transactions in a block
Archiving old blocks

Correct answer: Replacing blocks with a different competing branch

A reorg occurs when nodes switch to a longer or heavier competing chain, discarding some blocks.

Question 17: Which rule does Bitcoin use to choose the canonical chain among forks?

Longest/heaviest cumulative work chain (Correct answer)
First block seen
Lowest hash value
Most transactions

Correct answer: Longest/heaviest cumulative work chain

Bitcoin follows the chain with the greatest cumulative proof-of-work.

Question 18: What is the purpose of the GHOST / fork-choice rule (e.g., LMD-GHOST) in Ethereum?

To set gas prices
To select the head of the chain using accumulated votes (Correct answer)
To mint new tokens
To compress state

Correct answer: To select the head of the chain using accumulated votes

LMD-GHOST chooses the canonical head based on the latest attestations (votes) by validators.

Question 19: In Proof of Stake, what is a 'validator deposit' or bond?

A fee paid per transaction
Capital locked as collateral to participate in consensus (Correct answer)
A block reward
A gas refund

Correct answer: Capital locked as collateral to participate in consensus

Validators lock up a bonded deposit as collateral that can be slashed for misbehavior.

Question 20: What does 'liveness' guarantee in a consensus protocol?

Nodes never crash
The system continues to make progress and process new transactions (Correct answer)
All blocks are identical
Finality is instant

Correct answer: The system continues to make progress and process new transactions

Liveness ensures the protocol keeps producing decisions and does not stall.

Question 21: Which consensus approach uses verifiable random functions (VRFs) to select block proposers privately?

Bitcoin PoW
Algorand's pure PoS (Correct answer)
Raft
PBFT

Correct answer: Algorand's pure PoS

Algorand uses VRFs for cryptographic sortition to secretly and randomly select proposers.

Blockchain Developer Consensus Algorithms and Protocols

In Practical Byzantine Fault Tolerance (PBFT), how many total nodes are required to tolerate f faulty nodes?

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