Logical Clocks — Simple

Problem statement (interviewer prompt)

Across distributed nodes there is no global wall clock. Design a way to order events causally for use in a multi-region key-value store. Compare Lamport timestamps, vector clocks, hybrid logical clocks (HLC), and TrueTime; pick one and explain why.

flowchart LR
  A[Node A] -- send msg ts=3 --> B[Node B]
  A -- local event ++ --> A
  B -- on recv: ts = max local, msg + 1 --> B

    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 A,B service;

Lamport / vector clocks order events causally without synchronized wall clocks. HLC mixes wall-clock + logical to get both.