4.1.2 • Public • Published

Ember FastBoot App Server

The FastBoot App Server is an application server for hosting Ember FastBoot apps. It manages downloading the Ember app, starting multiple HTTP server processes, and detecting when new versions of the application have been deployed.

FastBoot allows Ember apps to be rendered on the server, to support things like search crawlers and clients without JavaScript. For more information about FastBoot, see FastBoot website.


The App Server is designed to be flexible and extensible enough to run in whatever environment you want to use to host FastBoot apps. In particular, you can provide a custom:

  • Downloader, to control how app builds gets downloaded
  • Notifier, to control how new versions of the build are detected
  • HTTP Server, to use whatever stack you prefer for serving HTTP requests in Node.js


FastBoot App Server requires Node.js v10 or later.

Quick Start

Put the following in a fastboot-server.js file:

const FastBootAppServer = require('fastboot-app-server');


let server = new FastBootAppServer({
  distPath: 'dist',
  gzip: true, // Optional - Enables gzip compression.
  host: '', // Optional - Sets the host the server listens on.
  port: 4000, // Optional - Sets the port the server listens on (defaults to the PORT env var or 3000).
  buildSandboxGlobals(defaultGlobals) { // Optional - Make values available to the Ember app running in the FastBoot server, e.g. "MY_GLOBAL" will be available as "GLOBAL_VALUE"
    return Object.assign({}, defaultGlobals, { GLOBAL_VALUE: MY_GLOBAL });
  log: true, // Optional - Specifies whether the server should use its default request logging. Useful for turning off default logging when providing custom logging middlewares
  chunkedResponse: true // Optional - Opt-in to chunked transfer encoding, transferring the head, body and potential shoeboxes in separate chunks. Chunked transfer encoding should have a positive effect in particular when the app transfers a lot of data in the shoebox.


Configure distPath to point to the dist directory you upload to your server. (See Application Builds below.)

Run the server file:

$ PORT=8000 node fastboot-server.js

This will start an HTTP server on port 8000. To stop the server, type Ctrl-C.

NOTE: If you want to continue running ember serve in development, name the file fastboot-server.js instead.

Application Builds

When you build an Ember.js app via ember build, it will build the app for production and, by default, put the resulting files in your application's dist directory.


Because Node.js is single-threaded, you must run multiple processes to take advantage of multi-core systems. FastBoot App Server takes advantage of Node's clustering support out of the box, automatically spawning one worker HTTP server per core. You can override this via options.workerCount.

The app server will automatically spawn a new worker if one dies while handling a request. When a new application deploy is detected, workers will automatically reload with the newest version.

Custom HTTP Server

You can customize HTTP server (add middlewares, subdomains, etc.), either directly:

// start.js
const FastBootAppServer = require('fastboot-app-server');
const ExpressHTTPServer = require('fastboot-app-server/src/express-http-server');

const httpServer = new ExpressHTTPServer(/* {options} */);
const app =;
app.use('/api', apiRoutes);
let server = new FastBootAppServer({
  httpServer: httpServer


or extend the provided HTTP server and override any methods you need:

// my-custom-express-server.js
const FastBootAppServer = require('fastboot-app-server');
const ExpressHTTPServer = require('fastboot-app-server/src/express-http-server');

class MyCustomExpressServer extends ExpressHTTPServer {
  serve(middleware) {
    // put your custom code here, don't forget to add fastboot etc.
// start.js
const MyCustomExpressServer = require('./my-custom-express-server');
const httpServer = new MyCustomExpressServer(/* {options} */);
let server = new FastBootAppServer({
  httpServer: httpServer


Pre and Post FastBoot middleware hooks

If you need something less than a custom server and just want to run some middleware before or after FastBoot runs, the server provides hooks for you to do so:

// Custom Middlewares
function modifyRequest(req, res, next) { /* do pre-fastboot stuff to `req` */ };
function handleErrors(err, req, res, next) { /* do error recovery stuff */ };

const server = FastBootAppServer({
  beforeMiddleware: function (app) { app.use(modifyRequest); },
  afterMiddleware: function (app) { app.use(handleErrors); }


We provide simple log output by default, but if you want more logging control, you can disable the simple logger using the log: false option, and provide a custom middleware that suits your logging needs:

let server = new FastBootAppServer({
  log: false,
  beforeMiddleware: function(app) {
    let logger = function(req, res, next) {
      console.log('Hello from custom request logger');




You can point the app server at a static path that you manage, but that means taking responsibility for uploading builds to each server whenever you want to deploy a new version.

Instead, you can provide the app server with a downloader, an adapter that knows how to download the current version of your application.

For example, to use the S3 downloader that downloads a zip file from AWS S3:

const S3Downloader = require('fastboot-s3-downloader');
const FastBootAppServer = require('fastboot-app-server');

let downloader = new S3Downloader({
  bucket: 'S3_BUCKET',
  key: 'S3_KEY'

let server = new FastBootAppServer({
  downloader: downloader


Available Downloaders

Writing a Downloader

To write your own downloader, construct an object that conforms to the following interface:


Returns a promise that resolves to the path to the downloaded dist directory (which does not have to be named dist).

Note that download() may be called more than once in the lifetime of an application, if a new version is deployed. Make sure your downloader cleans up after itself to avoid running out of disk space.


Once the FastBoot App Server is up and running, it will happily chug away until the server dies or it reaches the inevitable heat death of the universe. Before that happens, presumably, you may want to deploy a new version of your application.

Notifiers are responsible for detecting when a new version of an app has been deployed and reloading the app server.

For example, here's how to use the S3 notifier, which polls the last modified date of a file on S3 to detect new versions:

const S3Notifier = require('fastboot-s3-notifier');
const FastBootAppServer = require('fastboot-app-server');

let notifier = new S3Notifier({
  bucket: S3_BUCKET,
  key: S3_KEY

let server = new FastBootAppServer({
  notifier: notifier


Available Notifiers

Writing a Notifier

To write your own notifier, construct an object that conforms to the following interface:


The subscribe() method on your notifier is passed a notify function. If you detect that a new version of your app has been deployed (whether via polling or a push notification), call this function to trigger a reload.

Basic Authentication

You can enable Basic Authentication by providing username and password options:

const FastBootAppServer = require('fastboot-app-server');

let server = new FastBootAppServer({
  username: 'tomster',
  password: 'zoey'

Scraper Issues

Twitter and LinkedIn

As of 2019-06-06, Twitter and LinkedIn's scrapers have a hard time extracting your site's metadata for sharing if chunkedResponse is set to true in your server.js file. Set chunkedResponse: false if your meta tags are in place but the Twitter card validator shows "Card not found" or LinkedIn's Post Inspector shows a 500 error.



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