Consensus: Raft / Paxos — Simple

Problem statement (interviewer prompt)

Design a replicated state machine across 5 nodes such that all replicas agree on the order of operations even with crashes and message loss. Tolerate up to 2 node failures. Explain leader election, log replication, safety properties, and how reads are served.

A leader-based protocol where a quorum (majority) of replicas agree on each entry before commit. Tolerates (N-1)/2 failures.

flowchart LR
  C([Client])
  L[Leader]
  F1[Follower 1]
  F2[Follower 2]
  F3[Follower 3]
  F4[Follower 4]
  C --> L
  L --> F1
  L --> F2
  L --> F3
  L --> F4
  F1 -.ack.-> L
  F2 -.ack.-> L
  F3 -.ack.-> L
  F4 -.ack.-> L

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class C client;
    class L,F1,F2,F3,F4 service;