prember = Pre Render Ember

This Ember addon allows you to pre-render any list of URLs into static HTML files at build time. It has no opinions about how you generate the list of URLs.

Quick Start

Add these packages to your app:

ember install ember-cli-fastboot
ember install prember

And configure some URLs that you would like to prerender:

// In ember-cli-build.js
let app = new EmberApp(defaults, {
  prember: {
    urls: [
      '/',
      '/about',
      '/contact'
    ]
  }
});

When you do ember build --environment=production, your built app will include fastboot-rendered HTML in the following files:

/index.html
/about/index.html
/contact/index.html

Explanation

When you build a normal ember app (ember build --environment=production) you get a structure something like this:

dist/
├── assets
│   ├── my-app-0d31988c08747007cb982909a0b2c9db.css
│   ├── my-app-bdaaa766a1077911a7dae138cbd9e39d.js
│   ├── vendor-553c722f80bed2ea90c42b2c6a54238a.js
│   └── vendor-9eda64f0de2569c64ba0d33f08940fbf.css
├── crossdomain.xml
├── index.html
└── robots.txt

To serve this app to users, you just need to configure a webserver to use index.html in response to all URLs that don't otherwise map to files (because the Ember app will boot and take care of the routing).

If you add ember-cli-fastboot to your app, it augments your build with a few things that are needed to run the app within node via fastboot:

dist/
├── assets
│   ├── assetMap.json
│   ├── my-app-0d31988c08747007cb982909a0b2c9db.css
│   ├── my-app-a72732b0d2468246920fa5401610caf4.js
│   ├── my-app-fastboot-af717865dadf95003aaf6903aefcd125.js
│   ├── vendor-553c722f80bed2ea90c42b2c6a54238a.js
│   └── vendor-9eda64f0de2569c64ba0d33f08940fbf.css
├── crossdomain.xml
├── index.html
├── package.json
└── robots.txt

You can still serve the resulting app in the normal way, but to get the benefits of server-side rendering you would probably serve it from a fastboot server that knows how to combine the JS files and the index.html file and generate unique output per URL. The downside of this is that your fastboot server is now in the critical path, which increases your ops complexity and is necessarily slower than serving static files.

prember starts with an app that's already capable of running in fastboot and augments it further. You configure it with a source of URLs to prerender, and it uses Fastboot to visit each one during the build process, saving the resulting HTML files:

dist/
├── _empty.html            <--------- A copy of the original index.html
├── about
│   └── index.html         <--------- Pre-rendered content
├── assets
│   ├── assetMap.json
│   ├── my-app-0d31988c08747007cb982909a0b2c9db.css
│   ├── my-app-a72732b0d2468246920fa5401610caf4.js
│   ├── my-app-fastboot-af717865dadf95003aaf6903aefcd125.js
│   ├── vendor-553c722f80bed2ea90c42b2c6a54238a.js
│   └── vendor-9eda64f0de2569c64ba0d33f08940fbf.css
├── contact
│   └── index.html         <--------- Pre-rendered content
├── crossdomain.xml
├── index.html             <--------- Rewritten with pre-rendered content
├── package.json
└── robots.txt

The resulting application can be served entirely statically, like a normal Ember app. But it has the fast-first-paint and SEO benefits of a Fastboot-rendered application for all of the URLs that you pre-rendered.

Configuring Your Webserver

Your webserver needs to do two things correctly for this to work:

1. It should use a file like about/index.html to respond to URLs like /about. This is a pretty normal default behavior.
2. It should use _empty.html to respond to unknown URLs (404s). In a normal Ember app, you would configure index.html here instead, but we may have already overwritten index.html with content that only belongs on the homepage, not on every route. This is why prember gives you a separate _empty.html file with no prerendered content.

Options

You pass options to prember by setting them in ember-cli-build.js:

// In ember-cli-build.js
let app = new EmberApp(defaults, {
  prember: {
    urls: [
      '/',
      '/about',
      '/contact'
    ]
  }
});

The supported options are:

• urls: this can be an array or a promise-returning function that resolves to an array. How you generate the list of URLs is up to you, there are many valid strategies. See next section about using a custom url discovery function.
• enabled: defaults to environment === 'production' so that prember only runs during production builds.
• indexFile: defaults to "index.html". This is the name we will give to each of the files we create during pre-rendering.
• emptyFile: defaults to "_empty.html". This is where we will put a copy of your empty index.html as it was before any pre-rendering.

Using a custom URL discovery function

If you pass a function as the urls option, prember will invoke it like:

let listOfUrls = await yourUrlFunction({ distDir, visit });

distDir is the directory containing your built application. This allows your function to inspect the build output to discover URLs.

visit is an asynchronous function that takes a URL string and resolves to a response from a running fastboot server. This lets your function crawl the running application to discover URLs.

For an example of both these strategies in action, see ./node-tests/url-tester.js in this repo's test suite.

Using prember in development

In addition to the enabled option, you can temporarily turn prember on by setting the environment variable PREMBER=true, like:

PREMBER=true ember serve

However, by default ember-cli doesn't understand that it should use a file like about/index.html to respond to a URL like /about. So you should do:

ember install prember-middleware

It's harmless to keep prember-middleware permanently installed in your app, it has no impact on your production application.

When running in development, you will see console output from ember-cli that distinguishes whether a given page was handled by prember vs handled on-the-fly by fastboot:

prember: serving prerendered static HTML for /about       <--- served by prember
2017-10-27T05:25:02.161Z 200 OK /some-other-page          <--- served by fastboot

Using prember from an addon

Addon authors may declare urls for prember during compilation. To do so, you will want to:

• Add prember-plugin to your addon's package.json keywords array;
  • Consider also using package.json's ember-addon object to configure your addon to run before: 'prember'
• Define a urlsForPrember(distDir, visit) function in your addon's main file;
  • This function shares an interface with the "custom URL discovery" function, as defined above; and
• Advise your addon's users to install & configure prember in the host application.

Deployment

You shouldn't need to do much special -- just make sure the html files get copied along with all your other files.

If you're using ember-cli-deploy-s3, you just need to customize the filePattern setting so it includes .html files. For example:

    ENV.s3 = {
      bucket: 'cardstack.com',
      region: 'us-east-1',
      filePattern: '**/*.{js,css,png,gif,ico,jpg,map,xml,txt,svg,swf,eot,ttf,woff,woff2,otf,html}'
      allowOverwrite: true
    };

Compared to other addons

There are other ways to pre-render content:

• ember-prerender depends on having a real browser to do prerendering, which is heavy and complex. It's old and unmaintained.
• ember-cli-prerender uses Fastboot like we do, but it is not integrated with the build pipeline (so it's harder to make it Just Work™ with things like ember-cli-deploy) and it has stronger opinions about what URLs it will discover, including blueprint-driven sitemap configuration.
• ember-cli-staticboot is quite similar to this addon, and I didn't realize it existed before I started making this one. I do think prember does a better job of integrating the static build output with the existing ember app in a way that requires the minimal webserver configuration.