NetWatch is an MIT-licensed JavaScript single-page application by
[@sneak](https://sneak.berlin) that provides real-time network latency
monitoring to common internet hosts, displayed with color-coded figures and
sparkline graphs, served from a static bucket or Docker container.

## Getting Started

```bash
# Install dependencies
yarn install

# Development server
yarn dev

# Production build
yarn build

# Preview production build
yarn preview

# Docker
docker build -t netwatch .
docker run -p 8080:8080 netwatch
```

## Rationale

When debugging network issues, it's useful to have a persistent at-a-glance view
of latency and reachability to multiple well-known internet endpoints. NetWatch
provides this as a zero-dependency SPA that can be deployed anywhere static
files are served, with no backend required.

## Design

The application is a single-page app built with Vite and Tailwind CSS v4. All
code lives in `src/main.js` with a class-based architecture:

- **`CONFIG`**: Frozen configuration object (update interval, timeouts, axis
  ticks, etc.)
- **`HostState`**: Per-host state management — history buffer, latency tracking,
  status transitions
- **`AppState`**: Top-level state container — WAN hosts, local hosts, pause
  state, aggregate stats
- **`SparklineRenderer`**: Canvas 2D sparkline drawing with fixed axes,
  color-coded line segments, error regions, and DPR-aware scaling
- **UI functions**: `buildUI()` constructs the DOM, `updateHostRow()` /
  `updateSummary()` / `updateHealthBox()` handle incremental updates
- **`tick()`**: Main loop — measures all hosts in parallel via `Promise.all`,
  pushes samples, redraws UI. When paused, pushes blank markers (no probes, no
  false outage)

### Monitoring targets

- **11 WAN hosts**: datavi.be, Anthropic API, OpenAI API, AWS, GCP, Azure,
  DigitalOcean, Cloudflare, Fastly, Akamai, GitHub
- **1 Local host**: Local Gateway (192.168.100.1), tracked separately from WAN
  stats

### Latency measurement

HEAD requests with `mode: 'no-cors'` and `cache: 'no-store'`, timed with
`performance.now()`. 1-second timeout; anything over 1000ms is clamped to
unreachable. IPv4 only.

### Color coding

| Latency     | Color  |
| ----------- | ------ |
| < 50ms      | Green  |
| < 100ms     | Lime   |
| < 200ms     | Yellow |
| < 500ms     | Orange |
| >= 500ms    | Red    |
| Unreachable | Gray   |

### Output structure

```
dist/
├── index.html
└── assets/
    ├── index-*.css
    └── index-*.js
```

## Features

- Real-time monitoring with 2s update interval and 300s history sparklines
- Health indicator: green (HEALTHY) or red (DEGRADED) based on WAN reachability
- Summary stats: reachable count, min/max/avg latency across WAN hosts only
- Fixed chart axes: Y-axis 0–1000ms, X-axis 0–300s
- Color-coded latency figures and sparkline line segments
- Play/pause: pause stops probes but history keeps scrolling (blank gaps, no
  false outage)
- Clickable service URLs
- Canvas-based sparkline rendering with devicePixelRatio scaling
- Zero runtime dependencies: all resources bundled into build artifacts

## Deployment

After running `yarn build`, deploy the contents of the `dist/` directory to any
static file host (S3, GCS, Cloudflare Pages, Vercel, Netlify, GitHub Pages) or
use the Docker image behind a reverse proxy.

The Docker image:

- Listens on port 8080 by default (override with `PORT` env var)
- Trusts `X-Forwarded-For` from RFC1918 reverse proxies (10/8, 172.16/12,
  192.168/16)
- Sends access logs to stdout
- Caches static assets with immutable headers

## Browser Compatibility

Requires a modern browser with ES modules, Fetch API, Canvas API, and CSS custom
properties.

## Limitations

- **CORS**: Some hosts may block cross-origin HEAD requests. The app uses
  `no-cors` mode which allows the request but provides opaque responses. Latency
  is still measurable based on request timing.
- **Local gateway**: The 192.168.100.1 endpoint requires the host to be
  accessible from your network.
- **Network conditions**: Measurements reflect browser-to-endpoint latency,
  which includes your local network, ISP, and internet routing.

## TODO

- Add unit tests
- Add eslint for JS linting (currently lint target runs prettier only)
- Add configurable host list (environment variable or config file)
- Add latency history export (CSV/JSON)
- Add notification/alert when status changes to DEGRADED

## License

MIT. See [LICENSE](LICENSE).

## Author

[@sneak](https://sneak.berlin)