Distributed Systems & Consensus (Raft / Paxos) Interview Questions Hindi mein

जवाब: Parts of the system can fail independently while others keep running

Partial failure is the core difficulty: some nodes or links fail while others continue, and a node cannot reliably tell a slow peer from a dead one.

जवाब: A node that sends deliberately conflicting messages to different peers

That is a Byzantine failure. Raft and Paxos assume nodes may fail and messages may be lost, but nodes never lie.

जवाब: Because in a partially synchronous network a live node can be arbitrarily slow for a while

Without known delay bounds, a slow or paused node and a crashed node produce the same silence. The timeout picks a guess.

जवाब: Clock skew can make a later write carry a smaller timestamp, so it is discarded

If skew exceeds the real gap between two writes, the order by timestamp differs from the real order and the newer value is lost silently.

जवाब: b did not happen before a, but a and b may be concurrent

Lamport clocks guarantee a -> b implies L(a) < L(b). The reverse inference is invalid, so L(a) < L(b) only rules out b -> a.

जवाब: Concurrent

The first is larger in A, the second is larger in B. Neither dominates, so neither event saw the other: they are concurrent.

जवाब: W = 3, R = 3

Overlap needs W + R > N. Only 3 + 3 = 6 exceeds 5; the other combinations total 4 or 5.

जवाब: The value written by A, or a later one

Linearizability respects real time: an operation that starts after another finished must observe its effects.

जवाब: Latency vs consistency

The 'ELC' part: else (no partition), choose lower latency or stronger consistency.

जवाब: Validity

Validity requires the decided value to be one that some node proposed; a constant answer satisfies agreement and termination but not validity.

जवाब: Guaranteed termination under arbitrary timing

Raft is always safe; it only makes progress once timing is well-behaved enough to elect a stable leader.

जवाब: It does not know whether every other participant voted yes

Another participant may have voted no (so the result is abort) or all may have voted yes and the coordinator decided commit. The prepared participant cannot tell which.

जवाब: 'B', the value with the highest accepted ballot

The rule is to adopt the value with the highest accepted ballot among the promises. This preserves any value that might already be chosen.

जवाब: Any two majorities of acceptors share at least one acceptor

Quorum intersection means the new proposer's majority includes some acceptor that accepted the chosen value, and that acceptor reports it in its promise.

जवाब: Phase 1 (prepare/promise)

Phase 1 is done once for a range of slots, so steady-state commits need only phase 2: one round trip to a majority.

जवाब: Update its term to 8 and become a follower

Seeing a higher term means its leadership is stale. It adopts the higher term and reverts to follower.

जवाब: No, the voter's last entry has a higher term, so it is more up-to-date

Up-to-date is decided by last term first. Term 5 beats term 4, so the voter's log is more up-to-date and it refuses.

जवाब: Rejects the request so the leader retries from an earlier index

A mismatch means the logs diverge at or before that index. The follower rejects, and the leader backs nextIndex up until it finds the last matching entry.

जवाब: To commit an entry in its own term, which also commits all earlier entries

Earlier-term entries cannot be committed by counting replicas. Committing a current-term no-op commits everything before it through Log Matching.

जवाब: Voters reject candidates whose logs are less up-to-date than their own

The election restriction combined with majority overlap guarantees the winner's log contains all committed entries.

जवाब: Any majority of the old configuration overlaps any majority of the new one, so two leaders cannot be elected

With a single-server change, old and new majorities always share a server, which prevents disjoint majorities from electing separate leaders.

जवाब: When the follower needs entries the leader has already discarded during compaction

If nextIndex for a follower falls before the leader's snapshot point, the required entries no longer exist, so the leader ships the snapshot instead.

जवाब: Clock drift between servers is bounded

Lease reads skip the heartbeat round by trusting that time passes at roughly the same rate on all servers. Large drift or long pauses can break that.

जवाब: Leases

Keys attached to a lease are deleted when the lease expires, which happens when the client stops sending keep-alives.

जवाब: It is deleted automatically

Ephemeral znodes live only as long as their session, which is what makes them useful for liveness-based recipes like locks and leader election.

जवाब: In the storage or resource that receives the write

Only the resource sees writes from both clients in the order they arrive, so only it can reject the one with the stale token.

जवाब: 3 ms

The leader plus two followers form a majority of 5. The two fastest followers answer in 2 and 3 ms, so the commit waits about 3 ms.

जवाब: A rejoining partitioned node with an inflated term forcing an unnecessary election

Pre-vote lets a node check whether it could win before incrementing its term, so an isolated node cannot disrupt a healthy leader when it reconnects.

जवाब: Any failure can be reproduced exactly by replaying the same seed

Because every random choice comes from the seed, a failing run replays identically, which turns rare heisenbugs into debuggable ones.

जवाब: 7

Byzantine tolerance needs 3f + 1 replicas: 3 x 2 + 1 = 7. Five would be enough only for crash faults.

जवाब: Vector clocks: detect concurrent events; Lamport clocks: total order consistent with causality

Lamport clocks respect causality and can be made a total order with tie-breaking, but cannot prove concurrency; vector clocks can.

जवाब: It tolerates the same single failure as 3 nodes while needing 3 acknowledgements per commit

A majority of 4 is 3, so only one failure is tolerated, exactly like 3 nodes, while every write waits for more acknowledgements.

जवाब: The 3-node side elects a new leader and commits; the old leader cannot commit anything

Only the side with a majority can elect a leader and commit. The isolated old leader cannot reach a quorum, so its writes never commit, and it steps down when it sees the higher term.