# Frontend Technology Choices for ESP32-S3 Provider

**Authored by Claude Opus 4.6**
**Date:** 2026-03-02

---

## 1. Goal

Create a lightweight web frontend for the ESP32-S3 Provider that displays system information and provides a reboot control. The frontend must compile to static files small enough to live in ESP32 flash memory.

## 2. Chosen Stack

| Technology | Version | Role |
|---|---|---|
| **Svelte** | 5 | UI framework (compiles away at build time) |
| **Vite** | Latest | Build tool and dev server |
| **TailwindCSS** | 4 | Utility-first CSS (JIT, tree-shaken at build time) |
| **vite-plugin-singlefile** | Latest | Inlines all JS+CSS into one HTML file |
| **gzip** | — | Compresses final output for ESP32 flash |

## 3. Why Svelte Over Other Frameworks

Svelte is a **compile-time** framework. Unlike React or Vue, it does not ship a runtime to the browser. The compiler transforms `.svelte` components into highly optimized vanilla JavaScript at build time. This means:

- **No virtual DOM overhead** — direct DOM manipulation in the compiled output.
- **Tiny bundle size** — a simple Svelte app compiles to a few kilobytes of JS, compared to React's ~40kB runtime or Vue's ~30kB.
- **Ideal for constrained devices** — the ESP32-S3 has limited flash (typically 4-16MB shared with firmware) and serves files over a potentially slow connection.

Other frameworks considered:

| Framework | Runtime Size | Why Not |
|---|---|---|
| React | ~42kB min+gzip | Ships a runtime; too heavy for ESP32 |
| Vue | ~33kB min+gzip | Ships a runtime; unnecessary for a simple dashboard |
| Vanilla JS | 0kB | No tooling, harder to maintain, no component model |
| Svelte | **~2kB** min+gzip | ✅ Chosen — compiles away, smallest output |

Vanilla JS was considered but Svelte provides a component model, reactivity, and maintainability with effectively zero runtime cost.

## 4. Why TailwindCSS Over Vanilla CSS

TailwindCSS v4 uses a **Just-In-Time (JIT) compiler** that scans source files and generates only the CSS classes actually used. For a simple dashboard page, this produces **under 5kB of CSS** (before gzip).

Benefits over vanilla CSS:
- **Guaranteed small output** — unused styles are never generated (no manual purging needed).
- **Faster development** — utility classes directly in markup, no switching between files.
- **Consistent design system** — spacing, colors, typography are constrained to a scale.
- **Dark mode** — built-in `dark:` variant with zero extra effort.

TailwindCSS is a build-time tool only. It produces a plain `.css` file — no runtime, no JavaScript.

## 5. Why Single-File Build

The `vite-plugin-singlefile` plugin inlines all compiled JavaScript and CSS directly into `index.html`. The ESP32 serves **one file** instead of multiple.

Benefits for ESP32:
- **One HTTP request** — avoids multiple round-trips over a slow embedded server.
- **No filesystem overhead** — SPIFFS/LittleFS have per-file overhead; one file is optimal.
- **No filename length issues** — LittleFS has a 32-character filename limit; Vite normally generates hashed filenames like `assets/index-a1b2c3d4.js` that can exceed this.
- **Simpler backend** — the HTTP server only needs one route to serve the frontend.

## 6. Why Not ESP32-SvelteKit

[ESP32-SvelteKit](https://github.com/theelims/ESP32-sveltekit) is a full-featured framework that bundles SvelteKit + TailwindCSS + DaisyUI with an ESP32 backend. It was evaluated and rejected for the following reasons:

### Incompatible Build System

ESP32-SvelteKit requires **PlatformIO with the Arduino framework**. Our project uses **native ESP-IDF with CMake**. Adopting ESP32-SvelteKit would require rewriting all backend code (`connect.cpp`, `led_status.cpp`, `main.cpp`) to use Arduino APIs. This is not acceptable.

### Unnecessary Complexity

ESP32-SvelteKit includes features we do not need:
- MQTT client
- JWT-based user authentication
- WiFi provisioning UI (we already have our own wifi/ethernet logic in `connect.cpp`)
- DaisyUI theming system
- PsychicHttp web server
- ArduinoJson

These add weight to both the firmware and the frontend bundle without benefit for our use case (a simple system info dashboard).

### Licensing Concern

The ESP32-SvelteKit backend code is licensed under **LGPL v3**, which has copyleft implications for linked code. Our custom setup has no such constraints.

### What We Borrow From It

Although we don't use the framework, we adopt the same **proven patterns**:
- Svelte + TailwindCSS + Vite as the frontend toolchain
- Gzipped static files served from ESP32 flash
- The concept of embedding frontend into firmware binary for OTA (future task)

## 7. Build Pipeline

```
Source Files                Build Steps                     Output
─────────────              ───────────                     ──────
src/App.svelte    ─┐
src/app.css        ├─→  Vite + Svelte Compiler  ─→  dist/index.html  (47kB)
src/lib/api.js    ─┘     + TailwindCSS JIT               │
                          + vite-plugin-singlefile         ▼
                                                    gzip compression (level 9)
                                                          │
                                                          ▼
                                                    dist/index.html.gz  (16kB)
                                                          │
                                                          ▼
                                                    ESP32 Flash (SPIFFS/LittleFS)
                                                    or embedded in firmware binary
```

## 8. Environment-Based API URL

The frontend calls the ESP32's REST API. The base URL depends on the environment:

| Environment | `VITE_API_BASE` | Reason |
|---|---|---|
| **Development** (PC) | `http://<ESP32_IP>` | Frontend on PC, API on ESP32 over network |
| **Production** (ESP32) | *(empty string)* | Frontend and API on same device, relative URLs |

This is handled via Vite's `.env.development` and `.env.production` files. The value is baked in at compile time — zero runtime overhead.

## 9. OTA & Versioning Implementation

Instead of embedding the UI directly into the firmware binary as originally considered, we implemented a **Standalone Partition OTA** for maximum flexibility:

1. **A/B Partitioning**: The frontend is staged to an inactive LittleFS slot (`www_0` or `www_1`).
2. **Semantic Versioning**: `version.json` tracks `major.minor.revision`.
3. **Auto-Increment**: A custom `node scripts/package.js` script automatically increments the revision and generates a versioned binary (e.g., `www_v0.1.6.bin`).
4. **Resilient UX**: The Svelte app implements "Resilient Recovery Polling" — it enters a dedicated `isRecovering` state during reboot that ignores connection errors until the device is confirmed back online.

This decoupled approach allows for rapid frontend iteration without touching the 1M+ firmware binary.

## 10. Summary

We chose **Svelte + TailwindCSS + Vite** because they are build-time tools that produce the smallest possible static output, ideal for ESP32 constraints. We build to a **single gzipped HTML file** of ~16kB. We did not adopt ESP32-SvelteKit because it requires PlatformIO/Arduino, which is incompatible with our native ESP-IDF project, and includes unnecessary complexity for our simple dashboard.

---

## 11. Implementation Results

*Added 2026-03-02 after implementation was completed.*

### Final Build Output

| File | Size | Notes |
|---|---|---|
| `dist/index.html` | **47.0 kB** | Single self-contained file (all JS + CSS inlined) |
| `dist/index.html.gz` | **16.1 kB** | Gzipped at max level 9 (65% reduction) |

### Size Breakdown (before inlining)

| Asset | Uncompressed | Gzipped |
|---|---|---|
| HTML shell | 0.4 kB | 0.3 kB |
| CSS (TailwindCSS JIT) | 12.4 kB | 3.2 kB |
| JS (Svelte compiled) | 34.9 kB | 13.5 kB |
| **Total** | **47.7 kB** | **17.0 kB** |

The singlefile plugin reduces total size by ~0.8kB (removes multi-file boilerplate). Its main value is operational (one file, one HTTP request, no filesystem overhead).

### File Structure

```
Provider/frontend/
├── .env.development          # VITE_API_BASE=http://ESP32_IP_HERE
├── .env.production           # VITE_API_BASE= (empty, same-origin)
├── index.html                # Vite entry point
├── package.json              # npm scripts: dev, build, build:esp32
├── vite.config.js            # Svelte + TailwindCSS + singlefile plugins
├── scripts/
│   └── gzip.js               # Node.js gzip script (built-in zlib, no deps)
└── src/
    ├── main.js               # Svelte mount point
    ├── app.css               # TailwindCSS v4 imports + @theme tokens
    ├── App.svelte            # Dashboard UI (system info, reboot button)
    └── lib/
        └── api.js            # API helpers (getSystemInfo, reboot)
```

### Dashboard Features

- System info display: chip model, free heap, uptime, firmware version, connection type
- Reboot button with confirmation modal
- **Resilient Auto-Reload**: Targeted polling during reboot that handles intermediate connection failures.
- **OTA Dashboard**: Dedicated card showing version, active slot, and real-time partition statistics.
- Dark theme with custom color tokens
- Fully responsive layout

### npm Scripts

| Script | Command | Purpose |
|---|---|---|
| `npm run dev` | `vite` | Dev server with HMR (for PC development) |
| `npm run build` | `vite build` | Production build (single `index.html`) |
| `npm run build:esp32` | `vite build && node scripts/gzip.js` | Build + gzip for ESP32 deployment |

### Versions Used

| Package | Version |
|---|---|
| Svelte | 5.45.2 |
| Vite | 7.3.1 |
| TailwindCSS | 4.2.1 |
| @tailwindcss/vite | 4.2.1 |
| vite-plugin-singlefile | 2.3.0 |

### Known Issues

- **ESP-IDF Header Ordering**: Some C++ linting errors persist regarding unused headers (e.g., `esp_log.h`) that are actually required for macros; these are suppressed or ignored to maintain compatibility with the unity build pattern.
