# Concurrent Requests Support for ESP32-S3 Provider

**Authored by Antigravity**
**Date:** 2026-03-08

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## 1. Goal (What)

Enable the ESP32-S3 HTTP server to gracefully handle multiple concurrent web clients. Currently, if one browser connects, it consumes all available server sockets with "keep-alive" connections and blocks the `static_file_handler` via a single shared scratch buffer. The goal is to allow up to 5-10 clients (e.g., PCs, tablets, phones) on the local network to open the dashboard simultaneously without hanging, and to add a frontend safeguard (a loading spinner) to improve the user experience during slow network responses.

## 2. Rationale (Why)

The default ESP-IDF HTTP server (`esp_http_server`) is configured for minimal resource usage:
- `max_open_sockets = 7`: A single modern browser tries to open up to 6 connections simultaneously to fetch HTML, CSS, JS, and API data.
- `lru_purge_enable = false`: When a browser finishes loading, it keeps those 6 sockets open (Keep-Alive) for future requests. If a second device tries to connect, it is rejected because the server has no free sockets, even though the first device's sockets are idle.

Furthermore, the current `static_file_handler` relies on a single shared `rest_context->scratch` buffer allocated globally. If the server is modified to multiplex handlers concurrently, this shared buffer would be overwritten by competing requests, causing data corruption in the served files.

## 3. Chosen Approach (How)

### 3.1 Backend Configuration (ESP-IDF)
Instead of implementing complex multi-threading (spawning multiple FreeRTOS worker tasks), we will leverage the HTTP server's built-in event loop multiplexing by tuning its configuration:
1. **Increase LwIP Socket Limit**: `LWIP_MAX_SOCKETS` is set to `32` in `sdkconfig.defaults`.
2. **Increase HTTP Socket Limit**: Set `config.max_open_sockets = 24`. This deliberately reserves `8` sockets for LwIP internals and outwards connections, guaranteeing the network stack always has headroom to accept a TCP handshake from a new client.
3. **Enable Stale Socket Purging**: Set `config.lru_purge_enable = true`. This is the critical fix. When the 24 socket limit is reached and a new device attempts to connect, the server will intentionally drop the oldest idle keep-alive socket to make room, allowing the new device to load the page seamlessly.

### 3.2 Backend Scratch Buffer Pooling
To safely support multiplexed file serving without heavy `malloc`/`free` overhead on every request, we will replace the single shared scratch buffer with a **Static Shared Buffer Pool**:
- We allocated a global struct with a fixed array of `MAX_SCRATCH_BUFFERS = 10`.
- When `static_file_handler` begins, it will request an available chunk from the pool, allocating a 4KB chunk on the heap only the first time it is used.
- When the handler finishes, the chunk is marked as available yielding it for the next request.
- This provides isolation between up to 10 active transmission connections while minimizing heap fragmentation compared to per-request `mallocs`.

### 3.3 Frontend Safety (Loading Spinner)
Even with backend improvements, network latency or heavy load might cause delays. We will implement a global request tracker to improve perceived performance:
- A new Svelte writable store `pendingRequests` will track the count of active API calls.
- `api.js` will wrap the native `fetch` in a `trackedFetch` function that increments/decrements this store.
- A new `<Spinner />` component will be rendered at the root (`App.svelte`). It will overlay the screen when `$pendingRequests > 0`, optionally with a small delay (e.g., 300ms) to prevent flashing on fast requests.

## 4. Design Decisions & Trade-offs

| Approach | Pros | Cons | Decision |
|---|---|---|---|
| **True Multi-Threading (Multiple Worker Tasks)** | Can process files fully in parallel on both cores. | High memory overhead for stack space per task; over-engineered for simple static file serving. | **Rejected**. Relying on the event loop's multiplexing is sufficient for local network use cases. |
| **Per-Request `malloc` / `free`** | Simplest way to isolate scratch buffers. | High heap fragmentation risk; computationally expensive on every HTTP request. | **Rejected**. |
| **Fixed Pool (10 buffers)** | Low overhead; memory footprint only grows organically to the maximum concurrent need limit (10 * 4KB = 40KB) and stabilizes. | Strict limit on how many connections can be actively transmitting data at the exact same millisecond. | **Selected**. Best balance of performance and memory safety. |

## 5. Potential Future Improvements
- If the `realloc` pool grows too large during an unexpected spike, we could implement a cleanup routine that periodically shrinks the pool back to a baseline size when the server is idle.
- If true parallel processing is needed later, the HTTP server's `config.core_id` and `async_workers` could be utilized, but this requires ensuring all API handlers are perfectly thread-safe.
