Project: Live Lecture Board

One educator annotates PDFs/slides in handwriting; many remote viewers watch ink appear in near-real-time; sessions are recorded for faithful stroke-by-stroke replay. Single-writer INK broadcast — NOT symmetric multi-writer. Vector strokes, never video.

Stack (fixed — do not substitute)

• pnpm monorepo. Frontend: React + Vite + TypeScript. Backend: Node + Fastify + TypeScript.
• Realtime: raw ws (NOT Socket.IO). Drawing: Excalidraw (@excalidraw/excalidraw, MIT).
• Shared wire protocol lives in packages/types and is the SINGLE source of truth.
• Persistence (Postgres) and object storage (R2/S3) added ONLY when a subsystem needs them.

Architecture rules

• Modular monolith: one server process, clean module boundaries. No microservices, no k8s.
• Coordinates: store ink/cursor positions in NORMALIZED 0–1 document-page space, never pixels.
• Two traffic classes: committed ink/scene = reliable + ordered; live cursor/laser = lossy, latest-wins (drop stale).
• Capture-time timestamps: stamp events on the ADMIN machine, ms since session start. Never use server-arrival time for anything that gets recorded.

Guardrails (do NOT do these unless the active prompt says so)

• No Redis, no auth/JWT, no database until the subsystem that introduces it.
• No localStorage/sessionStorage anywhere.
• Build each subsystem against FAKE neighbors first; keep the app runnable at every step.
• TypeScript strict mode, ESM, no any. Small focused files. No premature abstraction.
• When unsure about scope, prefer the smallest thing that passes the stated Acceptance test.

Feature / protocol tracking

• Keep FEATURES.md updated when adding session features or changing wire messages (compat notes).