Update middleware docs

src/routes/solid-start/advanced/middleware.mdx

@@ -2,28 +2,277 @@
 title: "Middleware"
 ---
 
-Middlewares may be included by passing file you specify in your start config.
+Middleware intercepts HTTP requests and responses to perform tasks like authentication, redirection, logging, and more.
+It also enables sharing request-scoped data across the application using the `event.locals` object.
 
-```js
+## Common use cases
+
+Here are some common use cases for middleware:
+
+- **Request and response header management:** Middleware allows modifying headers to control caching (e.g., `Cache-Control`), improve security (e.g., `Content-Security-Policy`), or implement custom behaviour based on request characteristics.
+- **Global data sharing:** The `event.locals` object allows storing and sharing request-scoped data between middleware and any server-side context (e.g., API routes, server-only queries/actions). This is useful for passing information like user authentication status, feature flags, or other request-related data.
+- **Server-side redirects:** Middleware can redirect users based on various request properties, such as locale, authentication state, or custom query parameters.
+- **Request preprocessing:** Middleware can perform lightweight preprocessing tasks, such as validating tokens or normalizing paths.
+
+## Limitations
+
+While middleware is powerful, certain tasks are better handled in other parts of your application for performance, maintainability, or security reasons:
+
+- **Authorization:** Middleware does _not_ run on every request, especially during client-side navigations.
+  Relying on it for authorization would create a significant security vulnerability.
+  As a result, authorization checks should be performed as close to the data source as possible.
+  This means it within API routes, server-only queries/actions, or other server-side utilities.
+- **Heavy computation or long-running processes:** Middleware should be lightweight and execute quickly to avoid impacting performance.
+  CPU-intensive tasks, long-running processes, or blocking operations (e.g., complex calculations, external API calls) are best handled by dedicated route handlers, server-side utilities, or background jobs.
+- **Database operations:** Performing direct database queries within middleware can lead to performance bottlenecks and make your application harder to maintain.
+  Database interactions should be handled by server-side utilities or route handlers, which will create better management of database connections and handling of potential errors.
+
+## Basic usage
+
+Middleware is configured by exporting a configuration object from a dedicated file (e.g., `src/middleware/index.ts`).
+This object, created using the [`createMiddleware`](/solid-start/reference/server/create-middleware) function, defines when middleware functions execute throughout the request lifecycle.
+
+```ts title="src/middleware/index.ts"
+import { createMiddleware } from "@solidjs/start/middleware";
+
+export default createMiddleware({
+	onRequest: (event) => {
+		console.log("Request received:", event.request.url);
+
+		event.locals.startTime = Date.now();
+	},
+	onBeforeResponse: (event) => {
+		const endTime = Date.now();
+		const duration = endTime - event.locals.startTime;
+		console.log(`Request took ${duration}ms`);
+	},
+});
+```
+
+For SolidStart to recognize the configuration object, the file path is declared in `app.config.ts`:
+
+```ts title="app.config.ts"
 import { defineConfig } from "@solidjs/start/config";
 
 export default defineConfig({
-  middleware: "./src/middleware.ts"
+	middleware: "src/middleware/index.ts",
+});
+```
+
+## Lifecycle events
+
+A middleware function executes at specific points in the request lifecycle, using two key events: `onRequest` and `onBeforeResponse`.
+
+### `onRequest`
+
+The `onRequest` event is triggered at the beginning of the request lifecycle, before the request is handled by the route handler.
+This is the ideal place to:
+
+- Store request-scoped data in `event.locals` for use in later middleware functions or route handlers.
+- Set or modify request headers.
+- Perform early redirects.
+
+### `onBeforeResponse`
+
+The `onBeforeResponse` event is triggered after a request has been processed by the route handler but before the response is sent to the client.
+This is the ideal place to:
+
+- Set or modify response headers.
+- Log response metrics or perform other post-processing tasks.
+- Modify the response body.
+
+## Locals
+
+In web applications, there's often a need to share request-specific data across different parts of the server-side code.
+This data might include user authentication status, trace IDs for debugging, or client metadata (e.g., user agent, geolocation).
+
+The `event.locals` is a plain JavaScript object that can hold any JavaScript value.
+This object provides a temporary, request-scoped storage layer to address this need.
+Any data stored within it is only available during the processing of a single HTTP request and is automatically cleared afterward.
+
+```ts
+import { createMiddleware } from "@solidjs/start/middleware";
+
+export default createMiddleware({
+	onRequest: (event) => {
+		event.locals.user = {
+			name: "John Wick",
+		};
+		event.locals.sayHello = () => {
+			return "Hello, " + event.locals.user.name;
+		};
+	},
+});
+```
+
+Within middleware, `event.locals` can be accessed and modified directly.
+Other server-side contexts must use the [`getRequestEvent`](/reference/server-utilities/get-request-event) function to access the `event.locals` object.
+
+```tsx title="src/routes/index.tsx"
+import { getRequestEvent } from "solid-js/web";
+import { query, createAsync } from "@solidjs/router";
+
+const getUser = query(async () => {
+	"use server";
+	const event = getRequestEvent();
+	return {
+		name: event?.locals?.user?.name,
+		greeting: event?.locals?.sayHello(),
+	};
+}, "user");
+
+export default function Page() {
+	const user = createAsync(() => getUser());
+
+	return (
+		<div>
+			<p>Name: {user()?.name}</p>
+			<button onClick={() => alert(user()?.greeting)}>Say Hello</button>
+		</div>
+	);
+}
+```
+
+## Headers
+
+Request and response headers can be accessed and modified using the `event.request.headers` and `event.response.headers` objects.
+These follow the [standard Web API `Headers`](https://developer.mozilla.org/en-US/docs/Web/API/Headers) interface, exposing built-in methods for reading/updating headers.
+
+```ts
+import { createMiddleware } from "@solidjs/start/middleware";
+
+export default createMiddleware({
+	onRequest: (event) => {
+		// Reading client metadata for later use
+		const userAgent = event.request.headers.get("user-agent");
+		// Adding custom headers to request/response
+		event.request.headers.set("x-custom-request-header", "hello");
+		event.response.headers.set("x-custom-response-header1", "hello");
+	},
+	onBeforeResponse: (event) => {
+		// Finalizing response headers before sending to client
+		event.response.headers.set("x-custom-response-header2", "hello");
+	},
+});
+```
+
+Headers set in `onRequest` are applied **before** the route handler processes the request, allowing downstream middleware or route handlers to override them.
+Headers set in `onBeforeResponse` are applied **after** the route handler and are finalized for the client.
+
+## Cookies
+
+HTTP cookies are accessible through the `Cookie` request header and `Set-Cookie` response header.
+While these headers can be manipulated directly, [Vinxi](https://vinxi.vercel.app), the underlying server toolkit powering SolidStart, provides helpers to simplify cookie management.
+See the [Vinxi Cookies documentation](https://vinxi.vercel.app/api/server/cookies.html) for more information.
+
+```ts
+import { createMiddleware } from "@solidjs/start/middleware";
+import { getCookie, setCookie } from "vinxi/http";
+
+export default createMiddleware({
+	onRequest: (event) => {
+		// Reading a cookie
+		const theme = getCookie(event.nativeEvent, "theme");
+
+		// Setting a secure session cookie with expiration
+		setCookie(event.nativeEvent, "session", "abc123", {
+			httpOnly: true,
+			secure: true,
+			maxAge: 60 * 60 * 24, // 1 day
+		});
+	},
+});
+```
+
+## Custom responses
+
+Returning a value from a middleware function immediately terminates the request processing pipeline and sends the returned value as the response to the client.
+This means no further middleware functions or route handlers will be executed.
+
+```ts
+import { createMiddleware } from "@solidjs/start/middleware";
+
+export default createMiddleware({
+	onRequest: () => {
+		return new Response("Unauthorized", { status: 401 });
+	},
+});
+```
+
+Only [`Response`](https://developer.mozilla.org/en-US/docs/Web/API/Response) objects can be returned from middleware functions.
+Returning any other value will result in an error.
+
+### Redirects
+
+[Solid Router](/solid-router) provides the [`redirect` helper function](/solid-router/reference/response-helpers/redirect) which simplifies creating redirect responses.
+
+```ts
+import { createMiddleware } from "@solidjs/start/middleware";
+import { redirect } from "@solidjs/router";
+
+const REDIRECT_MAP: Record<string, string> = {
+	"/signup": "/auth/signup",
+	"/login": "/auth/login",
+};
+
+export default createMiddleware({
+	onRequest: (event) => {
+		const { pathname } = new URL(event.request.url);
+
+		// Redirecting legacy routes permanently to new paths
+		if (pathname in REDIRECT_MAP) {
+			return redirect(REDIRECT_MAP[pathname], 301);
+		}
+	},
+});
+```
+
+This example checks the requested path and returns a redirect response if it matches a predefined path.
+The 301 status code indicates a permanent redirect.
+Other redirect status codes (e.g., 302, 307) are available as needed.
+
+### JSON responses
+
+Solid Router provides the [`json` helper function](/solid-router/reference/response-helpers/json) which simplifies sending custom JSON responses.
+
+```ts
+import { createMiddleware } from "@solidjs/start/middleware";
+import { json } from "@solidjs/router";
+
+export default createMiddleware({
+	onRequest: (event) => {
+		// Rejecting unauthorized API requests with a JSON error
+		const authHeader = event.request.headers.get("Authorization");
+		if (!authHeader) {
+			return json({ error: "Unauthorized" }, { status: 401 });
+		}
+	},
 });
 ```
 
-Inside the middleware file, you can export a `createMiddleware` function.
+## Chaining middleware functions
+
+`onRequest` and `onBeforeResponse` options in `createMiddleware` can accept either a single function or an array of middleware functions.
+When an array is provided, these functions execute sequentially within the same lifecycle event.
+This enables composing smaller, more-focused middleware functions, rather than handling all logic in a single, large middleware function.
 
-```tsx
+```ts
 import { createMiddleware } from "@solidjs/start/middleware";
+import { type FetchEvent } from "@solidjs/start/server";
+
+function middleware1(event: FetchEvent) {
+	event.request.headers.set("x-custom-header1", "hello-from-middleware1");
+}
+
+function middleware2(event: FetchEvent) {
+	event.request.headers.set("x-custom-header2", "hello-from-middleware2");
+}
 
 export default createMiddleware({
-  onRequest: [
-    event => {
-      console.log("GLOBAL", event.request.url);
-    }
-  ]
+	onRequest: [middleware1, middleware2],
 });
 ```
 
-Middleware supports 2 lifecycles: `onRequest` and `onBeforeResponse`. If you return a value from middleware it will respond with that, otherwise it will run the next one in the chain.
+The order of middleware functions in the array determines their execution order.
+Dependent middleware functions should be placed after the middleware functions they rely on.
+For example, authentication middleware should typically run before logging middleware.