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This is a hard difficulty System Design question, commonly asked during Onsite rounds at Uber.

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This question is commonly asked for Software Engineer candidates at Uber during technical interviews.

Compare Paxos and Raft

Last updated: Mar 29, 2026

Quick Overview

This question evaluates understanding of distributed consensus protocols, replicated logs, linearizability, fault tolerance, and operational concerns such as leader election, quorum semantics, log replication, membership changes, and failure recovery.

Uber

Sep 6, 2025, 12:00 AM

Software Engineer

Onsite

System Design

Consensus over a Replicated Log: Paxos vs. Raft

Context

You are designing a fault-tolerant replicated log to back a stateful service (e.g., a key-value store). The system must be linearizable, tolerate crash-recovery failures and network partitions, and run on 2f+1 nodes to survive up to f failures.

Tasks

Explain how Paxos (specifically Multi-Paxos) and Raft achieve consensus over a replicated log.
Compare the two across:
- Leader election
- Quorum requirements
- Log replication mechanics
- Membership changes (cluster reconfiguration)
- Behavior under network partitions
Walk through failure scenarios and recovery:
- Leader crash
- Message delays/reordering
- Disk loss
Given goals of operational simplicity and debuggability, choose one for a new service and justify the trade-offs.

Solution

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