Initial Miniflare v3 docs (#581)

* Initial tre docs

* Update migration guide

* fixup! Update migration guide

---------

Co-authored-by: bcoll <[email protected]>

Author: penalosa

docs/src/content/get-started/migrating.md

@@ -2,6 +2,176 @@
 order: 3
 ---
 
+# ⬆️ Migrating from Version 2
+
+Refer to
+[the Miniflare and workerd blog post](https://blog.cloudflare.com/miniflare-and-workerd/)
+for the Miniflare v3 announcement.
+
+## Missing Features
+
+Several features from Miniflare v2 are not supported in Miniflare v3's initial
+release. However, they are on the roadmap, and will be added soon:
+
+- Step-through debugging
+- Automatically triggering scheduled events via CRON schedules, or manually
+  triggering them via `/.mf/scheduled` or `/cdn-cgi/mf/scheduled` (manually
+  triggering events is supported via the `--test-scheduled` Wrangler flag and
+  visiting `/__scheduled`)
+- Starting an HTTPS server
+
+## CLI Changes
+
+Miniflare v3 no longer includes a standalone CLI. To get the same functionality,
+you will need to switch over to using
+[Wrangler](https://developers.cloudflare.com/workers/wrangler/). Wrangler v3
+uses Miniflare v3 by default. To use Wrangler, run:
+
+```sh
+$ npx wrangler@3 dev
+```
+
+If there are features from the Miniflare CLI you would like to see in Wrangler,
+please open an issue on
+[GitHub](https://github.com/cloudflare/workers-sdk/issues/new/choose).
+
+## API Changes
+
+We have tried to keep Miniflare v3’s API close to Miniflare v2 where possible,
+but many options and methods have been removed or changed with the switch to the
+open-source `workerd` runtime. See the
+[GitHub `README` for the new API docs](https://github.com/cloudflare/miniflare/blob/tre/packages/miniflare/README.md).
+
+### Changed Options
+
+#### `bindings`
+
+Values passed to the `bindings` option must now be JSON-serialisable. Consider
+using the `serviceBindings` option if you need to bind custom functions.
+
+### Removed Options
+
+#### `wranglerConfigPath` and `wranglerConfigEnv`
+
+Miniflare no longer handles Wrangler’s configuration. To programmatically start
+up a Worker with Wrangler configuration, use the
+[`unstable_dev()`](https://developers.cloudflare.com/workers/wrangler/api/#unstable_dev)
+API.
+
+#### `packagePath`
+
+Specify your script using the `scriptPath` option instead.
+
+#### `upstream`
+
+Always pass the full upstream URL when calling `dispatchFetch()`.
+
+#### `watch`
+
+Miniflare’s API is primarily intended for testing use cases, where file watching
+isn’t too important. This option was here to enable Miniflare’s CLI which has
+now been removed. If you need to watch files, consider using a separate file
+watcher and calling `setOptions()` with your original config on change.
+
+#### `logUnhandledRejections`
+
+Unhandled rejections can be handled in Workers with
+[`addEventListener("unhandledrejection")`](https://community.cloudflare.com/t/2021-10-21-workers-runtime-release-notes/318571)
+
+#### `globalAsyncIO`, `globalTimers`, `globalRandom`, and `inaccurateCpu`
+
+These options are not supported by
+[`workerd`](https://github.com/cloudflare/workerd), the open source Cloudflare
+Workers runtime, and so can't be supported in Miniflare.
+
+#### `actualTime`
+
+Miniflare will always return the current time.
+
+#### `https`, `httpsKey`, `httpsKeyPath`, `httpsCert`, `httpsCertPath`, `httpsPfx`, `httpsPfxPath`, and `httpsPassphrase`
+
+Miniflare does not currently support starting HTTPS servers. These options may
+be added back in a future release.
+
+#### `metaProvider`
+
+The `cf` object and `X-Forwarded-Proto`/`X-Real-IP` headers can be specified
+when calling `dispatchFetch()` instead
+
+#### `cFetch`
+
+Renamed to `cf`.
+
+#### `crons`
+
+This is not currently supported by
+[`workerd`](https://github.com/cloudflare/workerd), the open source Cloudflare
+Workers runtime, but will be in a future release.
+
+#### `durableObjectAlarms`
+
+Durable Object alarms are always enabled in Miniflare v3.
+
+#### `globals`
+
+`globals` is not supported by
+[`workerd`](https://github.com/cloudflare/workerd), the open source Cloudflare
+Workers runtime.
+
+#### `mounts`
+
+Miniflare v3 does not have the concept of parent/child Workers. Instead, all
+Workers are at the same level. The same functionality can be achieved using the
+`workers` option. Note some options are always top-level, and some options are
+per-Worker.
+
+### Changed Methods
+
+#### `setOptions()`
+
+Calling `setOptions()` now requires a full configuration object, not a partial
+patch.
+
+### Removed Methods
+
+#### `reload()`
+
+Call `setOptions()` with the original configuration object instead.
+
+#### `getMount()`
+
+Miniflare v3 does not have the concept of parent/child Workers. Instead, all
+Workers are at the same level. Refer to the
+[`3.0.0-next.1` release notes](https://github.com/cloudflare/miniflare/releases/tag/v3.0.0-next.1)
+for an example multi-Worker configuration.
+
+#### `createServer()` and `startServer()`
+
+These methods are now redundant. Miniflare v3 always starts an HTTP server.
+
+#### `startScheduled()` and `dispatchScheduled()`
+
+[`workerd`](https://github.com/cloudflare/workerd) does not support triggering
+scheduled events yet, but will in an upcoming release.
+
+#### `dispatchQueue()`
+
+Use the `queue()` method on
+[service bindings](https://developers.cloudflare.com/workers/platform/bindings/about-service-bindings/)
+or
+[queue producer bindings](https://developers.cloudflare.com/queues/platform/configuration/#producer).
+
+#### `Response#waitUntil()`
+
+[`workerd`](https://github.com/cloudflare/workerd) does not support waiting for
+all `waitUntil()`ed promises yet.
+
+#### `getGlobalScope()`, `getBindings()`, `getModuleExports()`, `getKVNamespace()`, `getR2Bucket()`, `getCaches()`, and `getDurableObjectNamespace()`
+
+These methods returned objects from inside the Workers sandbox. Since Miniflare
+now uses [`workerd`](https://github.com/cloudflare/workerd), which runs in a
+different process, these methods can no longer be supported.
+
 # ⬆️ Migrating from Version 1
 
 Miniflare 2 includes [breaking changes](/get-started/changelog#_2-0-0). This