API Versioning & Evolution — Simple

flowchart LR
  V1[v1 contract]
  V2[v2 contract]
  V3[v3 contract]
  V1 -- coexists --> V2
  V2 -- coexists --> V3
  V1 -. deprecated, sunsets after window .-> X([retired])

  classDef p fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
  classDef s fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
  class V1,V2,V3 s;
  class X p;

    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 V1,V2,V3,X service;

Public APIs live forever. Version them so you can evolve without breaking clients: ideally additive changes only; if breaking, run multiple versions side-by-side and sunset older ones predictably.