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anchorStatus quo

The app is a mobile ticket counter for rail tickets. The basic flow through the app is as follows: a user enters an departure and arrival station and specifies the dates and passengers of the journey. After a search is initiated the results are shown, a specific trip is chosen, details like seating preference and passenger details are entered, the payment is processed and at the end, the details of the booked trip are shown and the purchased tickets can be downloaded.

At the moment Ember.js v2.14 is used and the app has 39 routes, 28 services, 77 components, 24 models which result in the following asset sizes:

app.js627.03 KB (117.22 KB gzipped)
vendor.js1.51 MB (342.87 KB gzipped)
app.css104.31 KB (19.25 KB gzipped)

Taken from the Ember Engine RFC:

Engines allow multiple logical applications to be composed together into a single application from the user's perspective.

As users need to fill in the search form - specify what kind of ticket they are looking for and for which connection - before they can even proceed to the next step, there is no reason to load all of the code that supports the subsequent booking flow on application startup. All of that code can be loaded lazily once the user proceeds to the next step by actually initiating a search or even while they are filling out the login form.

anchorExtract common functionality used in app into an addon

One fundamental design principle of engines is that they are isolated from the hosting app. It is possible to pass in services from the hosting app but apart from that, engines don't have access to anything of the app which mounts the engine. In order for components, helpers and styles to be accessible from both the host app and the engine, those common elements need to be put into an addon, which then the app and the engine depend on.

We're using an in repo addon for that:

ember generate in-repo-addon common

After the addon is created, we want to move common components and helpers into the addon. Let's look at the loading-indicator component: it shows a loading animation of 3 dots, which is used in the host app and the engine:

ember generate component loading-indicator --in-repo common

# move existing component definitions to common in-repo addon
git mv app/components/loading-indicator.js \

git mv app/templates/components/loading-indicator.hbs \

# add re-export of the addons' component so it is added to the app directory
git add lib/common/app/components/loading-indicator.js

Since the component is now located within the addon folder, we need to modify lib/common/addon/components/loading-indicator.js so the correct layout is used:

import layout from '../templates/components/loading-indicator';

export default Component.extend({
  // this is needed so the template within addon/templates/components is used

  // previous code of the component

The application uses ember-cli-sass for its styles and defines a number of variables for colors, sizes etc. In order for those to be accessible from both the host app and the engine, these style definitions need to be moved into the common addon as well:

git mv app/styles/vars.scss \

git mv app/styles/colors.scss \

git mv app/styles/mixins/button.scss \

git mv app/styles/components/loading-indicator.scss \

Apart from this, we also need to create a file which includes all the styles from the common components and helpers. By this all the styles from the common addon are included in the hosting app:

// lib/common/app/styles/common.scss

@import "common/components/loading-indicator";

Within the app we import the style definitions for the common addon, so all the styles for the components are included and we can use the variables, colors and mixins defined in the common addon.

// app/styles/app.scss

@import "common";

After all that is done, we now have all common components and helpers, as well as common style definitions moved into the addon. The next step is to finally create the engine, which will contain most of the application code.

anchorMove functionality into engine

An engine is basically an Ember.js addon. Since the engine won't be reused within another application, we decided to go with the in-repo solution for the engine as well:

ember generate in-repo-addon booking-flow

After that, setting up the in-repo engine according to the guides is pretty straight forward. At the end of that we have an engine, located at lib/booking-flow, so now it's time to move relevant routes, components, templates, ... out of app/ into it.

The first thing we want to do is to depend on the common in-repo addon, so we can re-use the common elements within the engine. Let's take a look at lib/booking-flow/package.json:

  "name": "booking-flow",

  "dependencies": {
    "ember-cli-htmlbars": "*",
    "ember-cli-babel": "*"

  "ember-addon": {
    "paths": ["../common"]

After that we can start to move all the routes, components, services which are only used within the booking-flow engine into the corresponding folders within lib/booking-flow/addon/. The nice thing about Ember Engines is that they don't introduce any new concepts in terms of location of the files. So a simple git mv does the trick.

The booking-flow addon should use the same style definitions as the hosting app, so we'd like to import the common style definitions within the booking-flow engines styles. For the imports to work properly, we need to add the path to the common addon to the sassOptions of the engine:

// lib/booking-flow/index.js
const EngineAddon = require('ember-engines/lib/engine-addon');

module.exports = EngineAddon.extend({
  name: 'booking-flow',

  lazy: {
    enabled: false

  sassOptions: {
    includePaths: ['lib/common/app/styles']

By this we can import the variables, colors and mixins in the engines' style definitions. And since we namespaced the files in the common addon under the common/ folder, we get distinct import paths:

// lib/booking-flow/addon/styles/components/booking-button.scss

@import "common/colors";
@import "common/mixins/button";

.booking-button {
  @include button-rounded-mixin;

  color: $color-primary;
  border-color: $color-secondary;

So now we have moved all the non-essential logic not needed at the beginning of the app into an in-repo engine. As a next optimization, let's make use of a nifty feature of Ember Engines…

anchorMake it lazy

After we extracted the common addon and the booking-flow engine, we are ready to load the engine lazily to actually reduce the amount of JavaScript that needs to be loaded, parsed and compiled to boot up the application. This is as hard work as switching a boolean:

// lib/booking-flow/index.js
const EngineAddon = require('ember-engines/lib/engine-addon');

module.exports = EngineAddon.extend({
  name: 'booking-flow',

  lazy: {
    // that was easy
    enabled: true

  sassOptions: {
    includePaths: ['lib/common/app/styles']

And et voilà: we now have 3 new, separate assets, which are loaded on demand once we navigate into a route within the engine. Since the initial assets only contain the essential logic needed for the search, they have shrunken in size as well:

app.js627.03 KB (117.22 KB gzipped)375.01 KB (78.23 KB gzipped)
vendor.js1.51 MB (342.87 KB gzipped)1.6 MB (364.5 KB gzipped)
app.css104.31 KB (19.25 KB gzipped)58.65 KB (12.17 KB gzipped)
booking-flow.js290.47 KB (48.48 KB gzipped)
booking-flow-vendor.js117 B (127 B gzipped)
booking-flow.css45.24 KB (7.97 KB gzipped)


We created an in-repo addon for commonly used components, helpers and style definitions. After that, everything which is not needed for the initial search screen has been moved into an in-repo engine. This helped us reduce the size of the initially served assets.

anchorIf you're facing challenges with Ember.js and need a helping hand, reach out!

Contact us!

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