Distributed Key-Value Store — Simple

Problem statement (interviewer prompt)

Design a distributed key-value store (Dynamo / Cassandra style). It must be always-writable, horizontally scalable, tunably consistent (R+W>N), and survive node + DC failures. Explain partitioning, replication, gossip, anti-entropy, and read repair.

flowchart LR
  C([Client])
  R[Coordinator / Router]
  N1[(Node 1<br/>range A-H)]
  N2[(Node 2<br/>range I-P)]
  N3[(Node 3<br/>range Q-Z)]
  C --> R
  R --> N1
  R --> N2
  R --> N3
  N1 <-. replicate .-> N2
  N2 <-. replicate .-> N3
  N3 <-. replicate .-> N1

    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 R service;
    class N1,N2,N3 datastore;