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+- Start Date: 2019-04-08
+- Target Major Version: 3.x
+- Reference Issues: N/A
+- Implementation PR: N/A
+
+# Summary
+
+Re-design app bootstrapping and global API.
+
+- Global APIs that globally mutate Vue's behavior are now moved to **app instances** created the new `createApp` method, and their effects are now scoped to that app instance only.
+
+- Global APIs that are do not mutate Vue's behavior (e.g. `nextTick` and the APIs proposed in [Advanced Reactivity API](https://github.com/vuejs/rfcs/pull/22)) are now named exports as specified in [the Global API Treeshaking RFC](https://github.com/vuejs/rfcs/blob/treeshaking/active-rfcs/0000-global-api-treeshaking.md).
+
+# Basic example
+
+## Before
+
+``` js
+import Vue from 'vue'
+import App from './App.vue'
+
+Vue.config.ignoredElements = [/^app-/]
+Vue.use(/* ... */)
+Vue.mixin(/* ... */)
+Vue.component(/* ... */)
+Vue.directive(/* ... */)
+
+new Vue({
+  render: h => h(App)
+}).$mount('#app')
+```
+
+## After
+
+``` js
+import { createApp } from 'vue'
+import App from './App.vue'
+
+const app = createApp()
+
+app.config.ignoredElements = [/^app-/]
+app.use(/* ... */)
+app.mixin(/* ... */)
+app.component(/* ... */)
+app.directive(/* ... */)
+
+app.mount(App, '#app')
+```
+
+# Motivation
+
+Some of Vue's current global API and configurations permanently mutate global state. This leads to a few problems:
+
+- Global configuration makes it easy to accidentally pollute other test cases during testing. Users need to carefully store original global configuration and restore it after each test (e.g. resetting `Vue.config.errorHandler`). Some APIs (e.g. `Vue.use`, `Vue.mixin`) don't even have a way to revert their effects. This makes tests involving plugins particularly tricky.
+
+  - `vue-test-utils` has to implement a special API `createLocalVue` to deal with this
+
+- This also makes it difficult to share the same copy of `Vue` between multiple "apps" on the same page, but with different global configurations:
+
+  ``` js
+  // this affects both root instances
+  Vue.mixin({ /* ... */ })
+
+  const app1 = new Vue({ el: '#app-1' })
+  const app2 = new Vue({ el: '#app-2' })
+  ```
+
+# Detailed design
+
+Technically, Vue 2 doesn't have the concept of an "app". What we define as an app is simply a root Vue instance created via `new Vue()`. Every root instance created from the same `Vue` constructor shares the same global configuration.
+
+In this proposal we introduce a new global API, `createApp`:
+
+``` js
+import { createApp } from 'vue'
+
+const app = createApp()
+```
+
+Calling `createApp` returns an **app instance**. An app instance provides an **app context**. The entire component tree mounted by the app instance share the same app context, which provides the configurations that were previously "global" in Vue 2.x.
+
+## Global API Mapping
+
+An app instance exposes a subset of the current global APIs. The rule of thumb is any APIs that globally mutate Vue's behavior are now moved to the app instance. These include:
+
+- Global configuration
+  - `Vue.config` -> `app.config`
+    - `config.productionTip`: removed. ([details](#remove-config-productiontip))
+    - `config.ignoredElements` -> `config.isCustomElement`. ([details](#config-ignoredelements-config-iscustomelement))
+- Asset registration APIs
+  - `Vue.component` -> `app.component`
+  - `Vue.directive` -> `app.directive`
+- Behavior Extension APIs
+  - `Vue.mixin` -> `app.mixin`
+  - `Vue.use` -> `app.use`
+
+All other global APIs that do not globally mutate behavior are now named exports as proposed in [Global API Treeshaking](https://github.com/vuejs/rfcs/pull/19).
+
+## Mounting App Instance
+
+The app instance can mount a root component with the `mount` method. It works similarly to the 2.x `vm.$mount()` method and returns the mounted root component instance:
+
+``` js
+const rootInstance = app.mount(App, '#app')
+```
+
+The `mount` method can also accept props to be passed to the root component via the third argument:
+
+``` js
+app.mount(App, '#app', {
+  // props to be passed to root component
+})
+```
+
+## Provide / Inject
+
+An app instance can also provide dependencies that can be injected by any component inside the app:
+
+``` js
+// in the entry
+app.provide({
+  [ThemeSymbol]: theme
+})
+
+// in a child component
+export default {
+  inject: {
+    theme: {
+      from: ThemeSymbol
+    }
+  },
+  template: `<div :style="{ color: theme.textColor }" />`
+}
+```
+
+This is similar to using the `provide` option in a 2.x root instance.
+
+## Remove `config.productionTip`
+
+In 3.0, the "use production build" tip will only show up when using the "dev + full build" (the build that includes the runtime compiler and has warnings).
+
+For ES modules builds, since they are used with bundlers, and in most cases a CLI or boilerplate would have configured the production env properly, this tip will no longer show up.
+
+## `config.ignoredElements` -> `config.isCustomElement`
+
+This config option was introduced with the intention to support native custom elements, so the renaming better conveys what it does. The new option also expects a function which provides more flexibility than the old string / RegExp version:
+
+``` js
+// before
+Vue.config.ignoredElements = ['my-el', /^ion-/]
+
+// after
+const app = Vue.createApp()
+app.config.isCustomElement = tag => tag.startsWith('ion-')
+```
+
+**Important:** in 3.0, the check of whether an element is a component or not has been moved to the template compilation phase, therefore this config option is only respected when using the runtime compiler. If you are using the runtime-only build, `isCustomElement` must be passed to `@vue/compiler-dom` in the build setup instead - for example, via the [`compilerOptions` option in `vue-loader`](https://vue-loader.vuejs.org/options.html#compileroptions).
+
+- If `config.isCustomElement` is assigned to when using a runtime-only build, a warning will be emitted instructing the user to pass the option in the build setup instead;
+
+- This will be a new top-level option in the Vue CLI config.
+
+# Drawbacks
+
+## Plugin auto installation
+
+Many Vue 2.x libraries and plugins offer auto installation in their UMD builds, for example `vue-router`:
+
+``` html
+<script src="https://unpkg.com/vue"></script>
+<script src="https://unpkg.com/vue-router"></script>
+```
+
+Auto installation relies on calling `Vue.use` which is no longer available. This should be a relatively easy migration, and we can expose a stub for `Vue.use` that emits a warning instead.
+
+# Alternatives
+
+N/A
+
+# Adoption strategy
+
+- The transformation is straightforward (as seen in the basic example).
+- Moved methods can be replaced with stubs that emit warnings to guide migration.
+- A codemod can also be provided.
+- For `config.ingoredElements`, a compat shim can be easily provided.