Consensus Mechanism Interview Questions

[luhuimao]

(1) What’s consensus mechanism?
A. The purpose of a consensus mechanism is to verify that information being added to the ledger is valid.
B. Is a method of action for network or project creators to decide on changes to the decentralized system.
C. A consensus makes sure that an agreement is reached which could benefit the entire group as a whole, a method by which consensus decision-making is achieved is called “Consensus Algorithm”.
D. All of them.

(2) What are the most common consensus algorithms used by blockchain?
A. POW
B. POS
C. DPOS
D. PBFT

(3) What is POW?
A. POW is the first and most well-known consensus mechanism.
B. In Pow a miner who finds the hash first will be allowed to add a new block of the transaction to the blockchain.
C. The process of mining is extremely computation-intensive, so having a high hashrate is the key for miners to calculate the hash, thus getting the rewards.
D. All of them.

(4) What are the disadvantages of POW?
A. Huge expenditures.
B. Useless of computations.
C. 51 percent attack.
D. ALL of them.

(5) How do miners make sure they generate a hash below the target in Bitcoin network?
A. They alter the input by adding an integer, called a nonce(“number used once”).
B. Once a valid hash is found, it is broadcast to the network, and the block is added to the blockchain.
C. They ask other peers to get the valid hash.
D. None.

(6) How the miners to increate their chances of mining blocks?
A. Pool together.
B. Increate memory capacity.
C. Increate computing power.
D. Increate bandwidth.

(7) Why is say that Proof of work makes it extremely difficult to alter any aspect of the blockchain?
A. An alteration would require re-mining all subsequent blocks.
B. It is difficult for a user or pool of users to monopolize the network’s computing power, since the machinery and power required to complete the hash functions are expensive.
C. A and B.
D. None.

(8) What hash functions was used in Bitcoin?
A. BLAKE-256.
B. LBAKE2b.
C. SHA-256.
D. MD5.

(9) How Bitcoin network sets a certain level of difficulty of mining?
A. Increate the block size.
B. Setting the target hash starts with a long string of zeros: the lower the target, the smaller the set of valid hashed, and the harder it is to generate one.
C. A and B.
D. None.

(10) How POW work?
A. Take the current block of transactions and add it onto the hash of the previous block.
B. Pick a nonce and add it to the block of text.
C. Perform a hash of the block, the resulting hash needs to be a string that has a certain number of zeros in front of it.
D. If the hash lower than the target then broadcast to the network.

(11) What is Proof of stake?
A. Is a type of algorithm by which a cryptocurrency blockchain network aims to achieve distributed consensus.
B. Proof of stake requiring users to stake an amount of their tokens so as to have a chance of being selected to validate blocks of transactions.
C. In chain-based proof of stake, the algorithm pseudo-randomly selects a validator during each time slot (e.g. every period of 10 seconds might be a time slot), and assigns that validator the right to create a single block, and this block must point to some previous block.
D. In PoS-based public blockchains (e.g. Ethereum's upcoming Casper implementation), a set of validators take turns proposing and voting on the next block, and the weight of each validator's vote depends on the size of its deposit (i.e. stake).

(12) What are the benefits of proof of stake as opposed to proof of work?
A. No need to consume large quantities of electricity.
B. Because of the lack of high electricity consumption, there is not as much need to issue as many new coins in order to motivate participants to keep participating in the network.
C. Proof of stake opens the door to a wider array of techniques that use game-theoretic mechanism design in order to better discourage centralized cartels from forming.
D. Reduced centralization risks.

(13) How can be a block added to the chain in BFT-style proof of stake?
A. Validators are randomly assigned the right to propose blocks, but agreeing on which block is canonical is done through a multi-round process.
B. Every validator in this multi-round process sends a "vote" for some specific block during each round, and at the end of the process all (honest and online) validators permanently agree on whether or not any given block is part of the chain.
C. A and B.
D. None.

(14) What is the “nothing at stake” problem?
A. There are only rewards for producing blocks, and no penalties in many early (all chain-based) proof of stake algorithms, in the case that there are multiple competing chains, it is in a validator's incentive to try to make blocks on top of every chain at once.
B. May caused a blockchain never reach consensus even there are no attackers if all actors are narrowly economically rational.
C. A and B.
D. None.

(15) How to fix the “nothing at stake” problem?
A. involves penalizing validators if they simultaneously create blocks on multiple chains, by means of including proof of misbehavior (i.e. two conflicting signed block headers) into the blockchain as a later point in time at which point the malfeasant validator's deposit is deducted appropriately.
B. punish validators for creating blocks on the wrong chain.
C. A and B.
D. None.

(16) What is Raft?
A. It’s equivalent to Paxos in fault-tolerance and performance.
B. it's decomposed into relatively independent subproblems, and it cleanly addresses all major pieces needed for practical systems.
C. Raft will make consensus available to a wider audience, and that this wider audience will be able to develop a variety of higher quality consensus-based systems than are available today.
D. It solves the problem of getting multiple servers to agree on a shared state even in the face of failures.

(17) What is the possible results of leader election?
A. The candidate receives votes from the majority of the servers and becomes the leader. It then sends a heartbeat message to others in the cluster to establish authority.
B. If other candidates receive AppendEntries RPC, they check for the term number. If the term number is greater than their own, they accept the server as the leader and return to follower state. If the term number is smaller, they reject the RPC and still remain a candidate.
C. The candidate neither loses nor wins. If more than one server becomes a candidate at the same time, the vote can be split with no clear majority. In this case a new election begins after one of the candidates times out.
D. All of them.

(18) When a leader gets a client request, it adds it to its own log as a new entry, What is the contents of the log?
A. Contains the client specified command
B. Has an index to identify the position of entry in the log (the index starts from 1)
C. Has a term number to logically identify when the entry was written.
D. All of them.

(19) Each server exists in one of the three states, what are they?
A. Follower.
B. Candidate.
C. Leader.
D. All of them.

(20) What is Paxos?
A. Paxos is one of the oldest, simplest, and most versatile algorithms in the field of distributed consensus.
B. Paxos is a family of protocols for solving consensus in a network of unreliable processors.
C. The Paxos family of protocols includes a spectrum of trade-offs between the number of processors, number of message delays before learning the agreed value, the activity level of individual participants, number of messages sent, and types of failures.
D. All of them.

(21) Paxos describes the actions of the processors by their roles in the protocol:
A. Client.
B. Acceptor.
C. Proposer.
D. Learner and Leader.

(22) A successful round has two phases, what are they?
A. Prepare.
B. Promise.
C. Accept Request.
D. Accepted.

(23) What is OCC?
A. OCC is a concurrency control method applied to transactional systems such as relational database management systems and software transactional memory.
B. OCC assumes that multiple transactions can frequently complete without interfering with each other.
C. While running, transactions use data resources without acquiring locks on those resources.
D. Before committing, each transaction verifies that no other transaction has modified the data it has read. If the check reveals conflicting modifications, the committing transaction rolls back and can be restarted.

(24) What phases are involve in OCC transactions?
A. Begin: Record a timestamp marking the transaction’s beginning.
B. Modify: Read database values, and tentatively write changes.
C. Validate: Check whether other transactions have modified data that this transaction has used( read or written).
D. Commit/Rollback: If there is no conflict, make all changes take effect. If there is a conflict, resolve it, typically by aborting the transaction, although other resolution schemes are possible.

(25) When transaction wants to commit:
A. Send read/write info to server for “validation”.
B. Validation decides if OK to commit -- if serializable.
C. If yes then send invalidates to clients with cached copies of written records.
D. If no, abort, discard writes.

(26) What should validation do ?
A. It looks at what the executing transactions read and wrote.
B. Decides if there’s a serial execution order that would have gotten.
C. A and B.
D. None.