Experimental support for Ember Engines
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This Ember addon implements the functionality described in the Ember Engines RFC. Engines allow multiple logical applications to be composed together into a single application from the user's perspective.

This addon must be installed in any ember-cli projects that function as either consumers or providers of engines. The following functionality is supported:

  • Routable engines which can be mounted at specific routes in a routing map, and which can contain routes of their own.
  • Route-less engines, which can be rendered in a template using the {{mount}} keyword.
  • Sharing of dependencies from parents (applications or other engines) to contained engines. Shared dependencies are currently limited to services and route paths.
  • Lazy loading of engines.

The following functionality will soon be supported:

  • Route serializer modules that isolate serialization logic from the rest of the route definition.

Support for the following concepts is under consideration:

  • Namespaced access to engine resources from applications.
  • Sharing of dependencies other than services and route paths.
  • Passing configuration attributes from an engine's parent.

Important Note about Compatibility and Stability

This addon should be considered experimental. But engines are production ready and many of the APIs are fully stable.

The master branch of this addon is being developed against the master branch of Ember and Ember-CLI, and should be considered unstable. If you're planning to deploy to production, please use one of the stable releases.

v0.5.15 or higher is compatible with FastBoot 1.0+.

v0.5 of this addon is compatible with v2.12.x of both Ember and Ember-CLI.

v0.4 of this addon is compatible with v2.10.x of both Ember and Ember-CLI.

v0.3 of this addon is compatible with v2.8.x of Ember. This is the first version of Ember in which the required hooks for engines are available without a feature flag.

Introduction Video

Introduction to Ember Engines at Global Ember Meetup


From your Ember CLI project's root directory, run the following:

ember install ember-engines

Install the appropriate version of Ember as noted above.

Providing Engines

Creating Engines

Engines can be created as separate addon projects or in-repo addons.

Separate addon projects can be created with the addon command:

ember addon <engine-name>

Note: As described in the RFC, ember-cli will hopefully support an engine command to get started more easily with engine projects.

In order to create an engine within an existing application's project, run the in-repo-engine generator:

ember g in-repo-engine <engine-name>

Don't forget to install ember-engines and the appropriate version of Ember in your project, as described above.

Lazy Loading Engines

You must also declare in your Engine's index.js file whether or not the engine should be lazy loaded. Until lazy loading is supported, this should be set to false:

const EngineAddon = require('ember-engines/lib/engine-addon');
module.exports = EngineAddon.extend({
  name: 'ember-blog',
  lazyLoading: {
    enabled: false

Routable Engines

Routable engines should declare their route map in a routes.js file within your engine's addon directory. For example:

import buildRoutes from 'ember-engines/routes';

export default buildRoutes(function() {

  this.route('post', { path: 'post/:id' }, function() {
    this.route('comments', function() {
      this.route('comment', { path: ':id' });

Routable engines interact with the parent application's router as if they are an extension of the parent application. A routable engine's application route will be mounted wherever specified by the parent's route map (its "mountpoint").

Route-less Engines

Route-less engines should define an engine.js as described above. Neither router.js nor routes.js should be defined. Route-less engines will be rendered as their application template (templates/application.hbs).

Declaring Dependencies

Your engine should declare any dependencies that it expects from its parent. Dependencies must be declared in the engine definition.

For example, the following engine requires a store service from its parent:

import Engine from 'ember-engines/engine';
import Resolver from 'ember-resolver';
import loadInitializers from 'ember-load-initializers';
import config from './config/environment';

const { modulePrefix } = config;

const Eng = Engine.extend({
  dependencies: {
    services: [

loadInitializers(Eng, modulePrefix);

export default Eng;

Currently, only services and route paths (see below) can be shared across the parent/engine boundary.

Linking To External Routes

Linking to routes outside of an Engine's isolated context is currently supported by defining "external routes" as dependencies of your Engine.

You specify what external things your Engine wants to link to by providing an array of names like so:

// ember-blog/addon/engine.js
export default Engine.extend({
  // ...
  dependencies: {
    externalRoutes: [

The Engine's consumer is then responsible for defining where those things are located via a route path:

// dummy/app/app.js
import Application from '@ember/application';
import Resolver from './resolver';
import loadInitializers from 'ember-load-initializers';
import config from './config/environment';

const App = Application.extend({
  modulePrefix: config.modulePrefix,
  podModulePrefix: config.podModulePrefix,

  engines: {
    emberBlog: {
      dependencies: {
        externalRoutes: {
          home: 'home.index',
          settings: 'settings.blog.index'

You can then use those external routes either programmatically or within a template like so:

{{#link-to-external 'home'}}Go home{{/link-to-external}}
// ember-blog/addon/some-route.js

For further documentation on this subject, view the Engine Linking RFC.

Lazy-Loading Routing Considerations

When routing into an Engine that is lazily loaded there are some special considerations and subtle differences from how routing works in a normal Ember application.

Serialization of URLs

Since the links to your Engine are constructed before the Engine itself is loaded, you need to make sure the application has the necessary code to serialize data into the URLs. To that end, you need to replace any Route#serialize functions with route serializers, as defined in the Route Serializers RFC.

For example, if you had a Post route defined like so:

import Route from '@ember/routing/route';

export default Route.extend({
  serialize(model) {
    return { post_id: model.id };

You would need to remove that function and inline it into your routes.js map, which is loaded pre-emptively with the application:

function serializePost(model) {
  return { post_id: model.id };

export default buildRoutes(function() {
  this.route('post', { serialize: serializePost });

Note that route serializers are unique to Engines and won't work in normal applications. In a normal Ember application you should continue to use Route#serialize.

Loading / Error Substates

The loading and error substates work in a similar fashion to substates in a normal Ember app. The only difference is that lazily loaded Engines will enter a loading state while the assets for the Engine are loaded and can enter an error state when an asset fails to load.

Accessing Engine Configuration Settings

As in an application, you can provide configuration settings for your engine in config/environment.js. You can access these settings in a couple different ways.

The simplest method is to import these settings:

// addon/engine.js
import config from './config/environment';


Configuration settings are also registered with the key config:environment and can be looked up given an engine instance. For example:

// addon/instance-initializers/hello-instance.js
export function initialize(engineInstance) {
  let config = engineInstance.resolveRegistration('config:environment');
  console.log('modulePrefix', config.modulePrefix);

export default {
  name: 'hello-instance',
  initialize: initialize

Built Engine Output

Eager Engines

Eager engines are built approximately the same as existing addons. Differences are limited to consolidating the namespace of app code inside of an engine into the engine's namespace instead of the host application.

Beyond that it adds in a configuration module for the engine, and nothing else. It is a remarkably straightforward process.

Lazy Engines

Lazy engines are built in the same way as eager engines, but their assets are not combined back into the host application's vendor.js file. This means that they are run through a separate and unique build process from what a default addon will go through, though it reaches out to the upstream implementation in Ember CLI where possible.

A lazy engine's output (lazy-engine) looks like this:

├── assets
│   ├── host-application.css
│   ├── host-application.js
│   ├── vendor.css
│   └── vendor.js
├── crossdomain.xml
├── engines-dist
│   └── lazy-engine
│       ├── assets
│       │   ├── engine-vendor.css
│       │   ├── engine-vendor.js
│       │   ├── engine.css
│       │   └── engine.js
│       └── public-asset.jpg
├── index.html
└── robots.txt


The routes.js file and anything it imports must be present at boot time of the host application. It will be bundled into the host application's vendor.js file. This location should be considered undefined behavior and should not be relied upon as it may change in the future.

Its module name inside of the host application will be lazy-engine/routes. Any imports will also be in the lazy-engine module path.


Assets in this folder don't make sense and will be ignored as they break the isolation guarantees of engines.


JavaScript assets in this folder will be processed as per normal addon behavior except that they will end up inside of the engine.js file. Their module definition will be rooted to the engine name.

For example, /addon/routes/application.js will result in a JavaScript module named lazy-engine/routes/application inside of the /dist/engines-dist/lazy-engine/engine.js file.


Templates will be compiled by your engine but they must include ember-cli-htmlbars inside of dependencies in the engine's package.json.

As an example, /addon/templates/application.hbs will result in a JavaScript module named lazy-engine/templates/application inside of the /dist/engines-dist/lazy-engine/engine.js file.


CSS files will be built similarly to how they are processed inside of typical adddons. Typical addon behavior is as follows:

  1. All nested addons are processed. Each of them may return a style tree. By default these style trees only contain the contents of addon/styles/addon.css. The contents of the addon/styles/addon.css file is moved inside of the Broccoli tree to ${addon-name}.css. This can be modified if the addon specifies a custom treeForStyle hook.
  2. All top-level addons (those directly depended upon by the host) have all of addon/styles/**/*.css included into the host's vendor.css file. For example addon/styles/foo.css will appear in the output Broccoli tree at foo.css.
  3. If you name a CSS file in one of the top-level addons the same as an addon name (e.g. addon name is alpha), any top-level addon which has a CSS file of the same name as that addon (alpha.css) and is provided by an addon lexicographically after it (zeta) will clobber the contents of alpha/addon/styles/addon.css (from anywhere in the dependency graph) with zeta/addon/styles/alpha.css. (This is also a possible consequence of DAG topsorting.)

Lazy engines will use a variation of this approach:

  1. The engine itself will be treated as if it is a top-level dependency. This means that addon/styles/**/*.css will end up inside of engine.css.
  2. Child addons of a lazy engine will be treated as if they are top-level addons. This means that they will have their treeForStyle hook executed and the result of that hook will be merged into engine-vendor.css in DAG/lexicographic order.
  3. Nested lazy engine boundaries will not be crossed when calculating the child treeForStyle hook.


Assets appearing in the public folder will appear at the root of the engine output with no transformation. For example /public/public-asset.jpg appears at the root level of the /dist/engines-dist/lazy-engine/ output folder. Assets in this folder have no default behavior and you are responsible for any custom behavior.

Asset Manifest

Further, the engine must enumerate its primary assets (JS and CSS) in order to be loaded by the asset loading service. That will be generated at /dist/asset-manifest.json at build time. It will also by default be inserted into a meta tag config inside of the host application's index.html.

Nested Eager Engines

Nested eager engines will be built into their host engine or application. Modules will be deduplicated within the engine boundary and with the host application.

Nested Lazy Engines

Nested lazy engines will be promoted to /dist/engines-dist/ folder in the build output. Module deduplication will only be done with the host application.

Consuming Engines

Engines that are published as separate addons should be installed like any other addon:

ember install <engine-name>

As mentioned above, engines can also exist as in-repo addons, in which case you just need to ensure that this addon (ember-engines) has been installed in your main project.

Route-less Engines

Route-less engines can be rendered in a template using the {{mount}} keyword.

Using {{mount}} in Templates

Route-less engines can be mounted in templates using the {{mount}} keyword. For example, the following template renders the ember-chat engine:

{{mount "ember-chat"}}

Currently, the engine name is the only argument that can be passed to {{mount}}.

Routable Engines

Mounting Engines in your Route Map

Routable engines should be mounted in your router's route map using the mount() method. For example:

import Route from '@ember/routing/route'
import config from './config/environment';

const Router = Router.extend({
  location: config.locationType

Router.map(function() {
  this.route('blogs', function() {
    // Mount the main blog at /blogs/ember-blog

    // Mount the hr blog at /blogs/hr-blog
    this.mount('ember-blog', { as: 'hr-blog' });

    // Mount the admin blog at /blogs/special-admin-blog-here
    this.mount('ember-blog', { as: 'admin-blog', path: '/special-admin-blog-here' });

export default Router;

The above example mounts three different instances of the ember-blog engine within the blogs route.

The engine mounted with this.mount('ember-blog') will have a root path of /blogs/ember-blog and its root route can be referenced as ember-blog.

The engine mounted with this.mount('ember-blog', { as: 'hr-blog' }) will have a root path of /blogs/hr-blog and its root route can be referenced as hr-blog.

The engine mounted with this.mount('ember-blog', { as: 'admin-blog', path: '/special-admin-blog-here' }) will have a root path of /blogs/special-admin-blog-here and its root route can be referenced as admin-blog.

Note: The above example is not very practical currently without a method to configure individual instances of ember-blog.

Providing Dependencies to Engines

Applications or engines that contain an engine must provide mappings that fulfill the dependencies required by that engine.

For example, the following engine expects its parent to provide store and session services:

import Engine from 'ember-engines/engine';
import Resolver from 'ember-resolver';

export default Engine.extend({
  modulePrefix: 'ember-blog',


  dependencies: {
    services: [

An application that contains this engine must explicitly fulfill these dependencies. For example:

import Application from '@ember/application';
import Resolver from './resolver';
import loadInitializers from 'ember-load-initializers';
import config from './config/environment';

const App = Application.extend({
  modulePrefix: config.modulePrefix,
  podModulePrefix: config.podModulePrefix,

  engines: {
    emberBlog: {
      dependencies: {
        services: [
          {'session': 'user-session'}

loadInitializers(App, config.modulePrefix);

export default App;

Note that the app's store service is directly mapped to the engine's store service, while the app's user-session service is mapped to the engine's session service.

Also note that multiple engines can be configured per parent application/engine, and that each engine name should be camelCased (emberBlog instead of ember-blog).

Unit/Integration testing for in repo-engines

To test components declared inside an in-repo engine, you need to set a custom resolver with the engine's prefix.

Assuming you have an in-repo engine called appointments-manager and it has a component date-picker. The following would be the setup to test such component from the host app:

// host-app/tests/integration/components/date-picker-test.js

import { moduleForComponent, test } from 'ember-qunit';
import hbs from 'htmlbars-inline-precompile';
import engineResolverFor from 'ember-engines/test-support/engine-resolver-for';

const resolver = engineResolverFor('appointments-manager');

moduleForComponent('date-picker', 'Integration | Component | Date picker', {
  integration: true,

test('renders text', function(assert) {

  assert.equal(this.$().text().trim(), 'una fecha');

Note: you could create a helper and then use it like Resolver from ../helpers/appointments-manager/resolver

Testing for standalone engines

If you have a lazy engine, you'll need to edit your tests/test-helper.js like this:

import Application from '../app';
import config from '../config/environment';
import { setApplication } from '@ember/test-helpers';
import { start } from 'ember-qunit';
import preloadAssets from 'ember-asset-loader/test-support/preload-assets';
import manifest from '<app-name>/config/asset-manifest';


preloadAssets(manifest).then(start); // This ensures all engine resources are loaded before the tests

This should be enough to make integration & acceptance tests work. For unit tests, you'll need to use a custom resolver, as described in Unit/Integration testing for standalone engines.

Demo Projects

  • ember-engines-demo - an example of a parent application (consumer).
    • ember-chat-engine - an example of a route-less engine that is an in-repo addon.
  • ember-blog-engine - an example of a routable engine that is a separate addon project.



  • git clone this repository
  • npm install


Running Tests

  • npm test (Runs ember try:testall to test your addon against multiple Ember versions)
  • ember test
  • ember test --server


  • ember build

For more information on using ember-cli, visit http://www.ember-cli.com/.


Copyright 2015-2018 Dan Gebhardt and Robert Jackson. MIT License (see LICENSE.md for details).

Current Tags

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