docs: add an Undici 7 to 8 migration guide

docs/docs/best-practices/migrating-from-v7-to-v8.md

@@ -0,0 +1,231 @@
+# Migrating from Undici 7 to 8
+
+This guide covers the changes you are most likely to hit when upgrading an
+application or library from Undici v7 to v8.
+
+## Before you upgrade
+
+- Make sure your runtime is Node.js `>= 22.19.0`.
+- If you have custom dispatchers, interceptors, or handlers, review the
+  handler API changes before updating.
+- If you rely on HTTP/1.1-only behavior, plan to set `allowH2: false`
+  explicitly.
+
+## 1. Update your Node.js version
+
+Undici v8 requires Node.js `>= 22.19.0`.
+
+If you are still on Node.js 20 or an older Node.js 22 release, upgrade Node.js
+first:
+
+```bash
+node -p "process.version"
+```
+
+If that command prints a version lower than `v22.19.0`, upgrade Node.js before
+installing Undici v8.
+
+## 2. Migrate custom dispatcher handlers to the v2 API
+
+Undici v8 uses the newer dispatcher handler API consistently.
+
+If you implemented custom dispatchers, interceptors, or wrappers around
+`dispatch()`, update legacy callbacks such as `onConnect`, `onHeaders`, and
+`onComplete` to the newer callback names.
+
+### Old handler callbacks vs. v8 callbacks
+
+| Undici 7 style | Undici 8 style |
+|---|---|
+| `onConnect(abort, context)` | `onRequestStart(controller, context)` |
+| `onHeaders(statusCode, rawHeaders, resume, statusText)` | `onResponseStart(controller, statusCode, headers, statusText)` |
+| `onData(chunk)` | `onResponseData(controller, chunk)` |
+| `onComplete(trailers)` | `onResponseEnd(controller, trailers)` |
+| `onError(err)` | `onResponseError(controller, err)` |
+| `onUpgrade(statusCode, rawHeaders, socket)` | `onRequestUpgrade(controller, statusCode, headers, socket)` |
+
+### Example
+
+Before:
+
+```js
+client.dispatch(options, {
+  onConnect (abort) {
+    this.abort = abort
+  },
+  onHeaders (statusCode, headers, resume) {
+    this.resume = resume
+    return true
+  },
+  onData (chunk) {
+    chunks.push(chunk)
+    return true
+  },
+  onComplete (trailers) {
+    console.log(trailers)
+  },
+  onError (err) {
+    console.error(err)
+  }
+})
+```
+
+After:
+
+```js
+client.dispatch(options, {
+  onRequestStart (controller) {
+    this.controller = controller
+  },
+  onResponseStart (controller, statusCode, headers, statusText) {
+    console.log(statusCode, statusText, headers)
+  },
+  onResponseData (controller, chunk) {
+    chunks.push(chunk)
+  },
+  onResponseEnd (controller, trailers) {
+    console.log(trailers)
+  },
+  onResponseError (controller, err) {
+    console.error(err)
+  }
+})
+```
+
+### Pause, resume, and abort now go through the controller
+
+In Undici v7, legacy handlers could return `false` or keep references to
+`abort()` and `resume()` callbacks. In Undici v8, use the controller instead:
+
+```js
+onRequestStart (controller) {
+  this.controller = controller
+}
+
+onResponseData (controller, chunk) {
+  controller.pause()
+  setImmediate(() => controller.resume())
+}
+
+onResponseError (controller, err) {
+  controller.abort(err)
+}
+```
+
+### Raw headers and trailers moved to the controller
+
+If you need the raw header arrays, read them from the controller:
+
+- `controller.rawHeaders`
+- `controller.rawTrailers`
+
+## 3. Update `onBodySent()` handlers
+
+If you implemented `onBodySent()`, note that its signature changed.
+
+Before, handlers received counters:
+
+```js
+onBodySent (chunkSize, totalBytesSent) {}
+```
+
+In Undici v8, handlers receive the actual chunk:
+
+```js
+onBodySent (chunk) {}
+```
+
+If you need a notification that the whole body has been sent, use
+`onRequestSent()`:
+
+```js
+onRequestSent () {
+  console.log('request body fully sent')
+}
+```
+
+## 4. If you need HTTP/1.1 only, disable HTTP/2 explicitly
+
+Undici v8 enables HTTP/2 by default when a TLS server negotiates it via ALPN.
+
+If your application depends on HTTP/1.1-specific behavior, set `allowH2: false`
+explicitly.
+
+Before:
+
+```js
+const client = new Client('https://example.com')
+```
+
+After, to keep HTTP/1.1 only:
+
+```js
+const client = new Client('https://example.com', {
+  allowH2: false
+})
+```
+
+The same applies when you configure an `Agent`:
+
+```js
+const agent = new Agent({
+  allowH2: false
+})
+```
+
+## 5. Use real `Blob` and `File` instances
+
+Undici v8 no longer accepts fake Blob-like values that only imitate `Blob` or
+`File` via properties such as `Symbol.toStringTag`.
+
+If you were passing custom objects that looked like `Blob`s, replace them with
+actual `Blob` or `File` instances:
+
+```js
+const body = new Blob(['hello'])
+```
+
+## 6. Avoid depending on the internal global dispatcher symbol
+
+`setGlobalDispatcher()` and `getGlobalDispatcher()` remain the public APIs and
+should continue to be used.
+
+Internally, Undici v8 stores its dispatcher under
+`Symbol.for('undici.globalDispatcher.2')` and mirrors a v1-compatible wrapper
+for legacy consumers such as Node.js built-in `fetch`.
+
+If your code was reading or writing `Symbol.for('undici.globalDispatcher.1')`
+directly, migrate to the public APIs instead:
+
+```js
+import { setGlobalDispatcher, getGlobalDispatcher, Agent } from 'undici'
+
+setGlobalDispatcher(new Agent())
+const dispatcher = getGlobalDispatcher()
+```
+
+If you must expose a dispatcher to legacy v1 handler consumers, wrap it with
+`Dispatcher1Wrapper`:
+
+```js
+import { Agent, Dispatcher1Wrapper } from 'undici'
+
+const legacyCompatibleDispatcher = new Dispatcher1Wrapper(new Agent())
+```
+
+## 7. Verify the upgrade
+
+After moving to Undici v8, it is worth checking these paths in your test suite:
+
+- requests that use a custom `dispatcher`
+- `setGlobalDispatcher()` behavior
+- any custom interceptor or retry handler
+- uploads that use `Blob`, `File`, or `FormData`
+- integrations that depend on HTTP/1.1-only behavior
+
+## Related documentation
+
+- [Dispatcher](/docs/api/Dispatcher.md)
+- [Client](/docs/api/Client.md)
+- [Global Installation](/docs/api/GlobalInstallation.md)
+- [Undici Module vs. Node.js Built-in Fetch](/docs/best-practices/undici-vs-builtin-fetch.md)

docs/docsify/sidebar.md

@@ -41,6 +41,7 @@
   * [Undici Examples](/examples/ "Undici Examples")
 * Best Practices
   * [Undici vs. Built-in Fetch](/docs/best-practices/undici-vs-builtin-fetch.md "When to install undici vs using Node.js built-in fetch")
+  * [Migrating from Undici 7 to 8](/docs/best-practices/migrating-from-v7-to-v8.md "Upgrade applications and libraries from Undici v7 to v8")
   * [Proxy](/docs/best-practices/proxy.md "Connecting through a proxy")
   * [Client Certificate](/docs/best-practices/client-certificate.md "Connect using a client certificate")
   * [Writing Tests](/docs/best-practices/writing-tests.md "Using Undici inside tests")