ADR-001 — React SPA and Python exit (Phase 1)

• Status: Accepted (2026-07-31); Phase 2 cutover completed 2026-08-01 (React sole product UI — see PHASE_2_QUALIFICATION.md)
• Program: tools/open-fdd-modernization/
• Tracking: docs/migration/react-rust/
• Phase 3: outlook only (PHASE_3_EDGE_STREAMING_OUTLOOK.md); not started.

Context

The operator UI is a React SPA (frontend/web → openfdd-web). Production FDD runs in DataFusion SQL on central. Several checked-in instructions historically forbade recreating a React UI during Milestone A convergence; that lock is superseded by this ADR.

Decision

1. Product UI: React + TypeScript SPA (frontend/web).
2. Browser backend: existing central Rust service (services/central/). No FastAPI (or other Python) compatibility sidecar.
3. Deterministic analytics / FDD: DataFusion SQL (sql_rules/, crates/fdd_*). Never move fault math into React or TypeScript.
4. Delivery: openfdd-web nginx container with same-origin /api proxy to central.
5. Auth / browser security direction: JWT (or equivalent) already used by central; SPA must carry request IDs; document CSRF/CORS/CSP and safe artifact Content-Disposition in the contract milestone (P1-M2). Prefer same-origin /api to simplify cookies/CORS.
6. Python during Phase 1: frozen for product features. Allowed: oracle / characterization / fixtures. Disallowed: new production Python services, silent pandas FDD fallback, new persistence formats React must later consume.
7. BACnet / fieldbus / MQTTS: ownership and write-safety rules are unchanged. React never owns UDP 47808 or protocol writes.

Consequences

• Instruction files that said “do not recreate React” are superseded by this ADR for Phase 1+ work.
• New production packages under a React app must not depend on pandas for runtime FDD.
• Policy CI rejects a production React client aimed at a Python service URL.
• Pandas cookbooks and open_fdd.rules remain the oracle until an explicit product decision relocates or archives them (not this ADR).

Non-goals (this ADR)

• Changing live BACnet write policy.
• Choosing a large UI component library (deferred to P1-M2/M3 evaluation).
• Byte-identical HTML/report formats (semantic parity is the default).

Related

• DataFusion-first — computation boundary (kept, tightened).
• Job workspaces — durable Jobs remain central SoT.
• Phase docs: tools/open-fdd-modernization/PHASE_1_PREP_AND_REACT_PARITY.md.