CRDTs — Simple

Problem statement (interviewer prompt)

Design a data structure that two replicas can update concurrently — without coordination — and still converge to the same state after they sync. Cover counters, sets, and text editing. Explain garbage collection, op-based vs state-based variants, and where you'd use them.

flowchart LR
  A[Replica A<br/>local op]
  B[Replica B<br/>local op]
  A -. async sync .-> B
  B -. async sync .-> A
  A --> CV[Converged state]
  B --> CV

    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,CV service;

Conflict-free Replicated Data Types: any order of merges yields the same state. No coordination needed for writes.