Distributed Lock Service — Simple

Problem statement (interviewer prompt)

Design a distributed lock service (Zookeeper / etcd-based) used by 1000s of microservices for leader election + critical sections. Cover lease semantics, fencing tokens to prevent stale clients holding locks, and how to handle clock skew and network partitions.

flowchart LR
  C([Client A])
  C2([Client B])
  L[Lock Service<br/>etcd / ZK / Redis]
  C -->|acquire key X| L
  L --> C
  C2 -->|acquire key X| L
  L -.fenced wait.-> C2

    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,C2 client;
    class L cache;