Discussion of Progressive Web Apps (PWA) is catching fire in the JavaScript community, but amongst Rails developers... not so much.

Progressive Web App technology is still very young and rapidly churning; perhaps there's more reluctance on Rails teams to get onboard with new JavaScript APIs until they become more stable. Also, the Rails community hasn't had a clear path to integrating PWA technology—until now. In this post, we'll demonstrate how to turn your Rails app into a Progressive Web App using the serviceworker-rails gem.

What are Progressive Web Apps? Simply put, they are web applications that deliver mobile app-like experiences. For example, open web technologies are now making it possible for browser-based web apps to be:

• installable - add web apps to the Home Screen of a mobile device easily
• more reliable - provide a user experience when the device is offline or network responsiveness has degraded; sync user requests in the background when network capability is restored
• more engaging - notify users of activity even when they're not using the site

Sound good? Let's get started.

Your first Progressive Web App on Rails

Eric Elliot recently posted a thorough overview of Progessive Web App requirements that's worth a read. Here's summary of what's needed:

• HTTPS - any page that uses Progressive Web App technology needs to be served over SSL/TLS so "HTTPS everywhere" is recommended
• Web App Manifest - a text file with application metadata to support home screen installation
• Service Worker - a client-side JavaScript worker that can intercept network requests, modify responses, interact with local caches, sync data in the background, and enable push notifications

Set up HTTPS

To enable HTTPS on our website, we'll need to decide for ourselves how to set up our web server depending on our hosting provider and deployment needs. Tutorials for setting up Heroku and Digital Ocean may be a good place to start.

We'll also want to force SSL settings in our Rails application configuration for our remote environments, i.e., production.

# config/environments/production.rb

Rails.application.configure do
  # ...

  # Force all access to the app over SSL, use Strict-Transport-Security, and use secure cookies.
  config.force_ssl = true

  # ...
end

We should be sure to test out this behavior on a secondary remote environment before going live in production as we'll want to be aware of hiccups like mixed content warnings and hard-coded non-HTTPS urls in our application.

Add a manifest and Service Worker

For this next step, we'll assume we're using the Rails asset pipeline. This is both helpful and presents a problem:

The Rails asset pipeline makes a number of assumptions about what's best for deploying JavaScript, including asset digest fingerprints and long-lived cache headers - mostly to increase "cacheability". Rails also assumes a single parent directory, /public/assets, to make it easier to look up the file path for a given asset.

Service worker and manifest assets must play by different rules. Service workers may only be active from within the scope from which they are served. So if you try to register a service worker from a Rails asset pipeline path, like /assets/serviceworker-abcd1234.js, it will only be able to interact with requests and responses within /assets/**. This is not what we want.

To address this issue, I created the serviceworker-rails gem (source). This Rails plugin makes it easier to set up your app to serve service worker scripts and web app manifests at canonical urls while taking advantage of the transpilation and interpolation features the asset pipeline provides.

To get started with serviceworker-rails, we'll bundle it with our Rails app.

Add the gem to the Gemfile:

# Gemfile

gem "serviceworker-rails"

Bundle the app:

$ bundle

We'll use the built-in generator from service worker rails to add some starter JavaScript files to our project and the proper configuration.

$ rails g serviceworker:install

The generator will create the following files:

• config/initializers/serviceworker.rb - for configuring your Rails app
• app/assets/javascripts/serviceworker.js.erb - a blank Service Worker script with some example strategies
• app/assets/javascripts/serviceworker-companion.js - a snippet of JavaScript necessary to register your Service Worker in the browser
• app/assets/javascripts/manifest.json.erb - a starter web app manifest pointing to some default app icons provided by the gem
• public/offline.html - a starter offline page

It will also make the following modifications to existing files:

• Adds a sprockets directive to application.js to require serviceworker-companion.js
• Adds serviceworker.js and manifest.json to the list of compiled assets in config/initializers/assets.rb
• Injects tags into the head of app/views/layouts/application.html.erb for linking to the web app manifest

Of course, we could do this set up manually, but it may be helpful to run the automated install for our first attempt. If going manual, consult the serviceworker-rails README and my previous blog post on configuring the gem for more help during setup.

At this point, we've got all the boilerplate in place in our Rails app to begin adding Progessive Web App functionality. The great part is, we can pick and choose which features we want to add.

Here are few things you can try:

• Adding an offline page for your Rails app - by @rossta
• Sending Web Push notifications from Rails - by @rossta
• Instant loading Web Apps with a Service Worker application shell architecture - by Addy Osmani *

Going further

For more on grasping Service Worker fundamentals and developing offline solutions for the web, I highly recommend this free Udacity course:

• Offline Web Applications by Google

You'll want to understand the Service Worker life cycle, which Jake Archibald treats in great detail:

• The Service Worker Lifecycle

For some open-source abstractions for implementing Service Worker fetching and caching strategies, checkout out SW-Toolbox and SW-Precache from the Google Chrome team.

Of course, we've only covered the getting started part of our journey with Progressive Web Apps. I've left out a lot of fun parts like decided how to implement caching strategies or send push notifications. Here are some resources to check out to take your PWA skills to the next level:

• ServiceWorke.rs - a set of Service Worker recipes and demos from Mozilla
• Offline Cookbook - a great rundown of Service Worker snippets, including a number of caching strategies, by Jake Archibald
• Service Worker on Rails Sandbox - a demo Progressive Web App on Rails using the serviceworker-rails gem by @rossta
• Awesome Progressive Web Apps - kitchen sink of PWA resources
• Awesome Service Workers - kitchen sink of Service Worker resources

Rails 💜 PWA

Nothing about Rails is incongruent with Progressive Web App technology so there's no technical reason why we can't start introducing these features today. It's worth noting the choice to transition to PWA is completely orthogonal to whatever JavaScript MVC framework/module bundler/turbolinks decision you might be otherwise already tackling.

In the coming years, I believe it will become increasingly important to adopt PWA features to keep up with demand as more and more site visits will shift to mobile web.