Hot reload

GoodOne and any Arqen app can get Rust hot reload while keeping Expo HMR and the native single-process Thingd topology.

Recommended: arqen up with cargo watch

Define the Rust service in arqen.toml as a cargo watch command and let arqen up supervise it alongside the frontend:

[[dev.services]]
name = "backend"
command = "cargo"
args = ["watch", "-q", "-x", "run --quiet"]
cwd = "backend"
ready_url = "http://127.0.0.1:8888/ready"
ready_timeout_seconds = 90

[[dev.services]]
name = "frontend"
command = "pnpm"
args = ["dev"]
cwd = "."

Run everything through Arqen:

cargo install cargo-watch   # once
pnpm dev:up                 # or: arqen up
# or: arqen up backend frontend

• cargo watch restarts the backend on Rust source, arqen.toml, or env-file changes; arqen up prefixes logs as [backend]│ and waits for ready_url.
• Expo HMR (pnpm dev / expo start) is untouched - frontend hot reload is independent.
• Native single-process storage is preserved: scripts/dev-up.sh strips worker when ARQEN_STORAGE_MODE=native|memory, so the Thingd file is never opened by two processes.
• Do not use arqen dev --watch for this stack - arqen dev is a single-process runner; the arqen up service table is the correct place for watch.

Alternative: external cargo watch without Arqen supervision

For ad-hoc backend-only work:

cargo watch -q -x "run --quiet"  # in backend/

This bypasses arqen up and loses the unified log prefix/ready-url handling.

Production

The watcher must not be used in production. arqen start runs one process with graceful shutdown; production uses JSON logs (ARQEN_LOG_FORMAT=json) and the standard cargo run path.