At a high level, what do Paxos and Raft consensus algorithms do and what failure tolerance do they provide?

• A. Paxos and Raft are cryptographic protocols for securing data.
• B. They coordinate agreement on a value or log entry; tolerate crash failures; require a minority of nodes.
• C. They guarantee consensus without any communication overhead.
• D. They coordinate agreement on a value or log entry, tolerate crash failures, and require a majority of nodes.

Answer: (D)

Paxos and Raft are distributed consensus protocols that ensure multiple machines agree on a single value or the next entry in a replicated log, even if some of the machines fail. They’re designed to tolerate crash failures (fail-stop scenarios) rather than arbitrary or Byzantine faults. The key safety mechanism is the requirement that a majority of nodes must agree (a quorum) before a value is considered chosen. This majority requirement guarantees that two different values cannot both be chosen, because any two majorities have at least one node in common, preventing conflicting outcomes. As long as a majority of nodes remain operational, progress can be made, but if a majority is unavailable, the system may stall to preserve safety. This combination—agreement on a value or log entry, crash-fault tolerance, and majority-based progress—captures what Paxos and Raft do. They are not cryptographic protocols, and consensus requires inter-node communication; claims of zero overhead are inaccurate.