Tags: offline



Sunday, January 15th, 2017

Modernizing our Progressive Enhancement Delivery | Filament Group, Inc., Boston, MA

Scott runs through the latest improvements to the Filament Group website. There’s a lot about HTTP2, but also a dab of service workers (using a similar recipe to my site).

Wednesday, January 11th, 2017

Making Resilient Web Design work offline

I’ve written before about taking an online book offline, documenting the process behind the web version of HTML5 For Web Designers. A book is quite a static thing so it’s safe to take a fairly aggressive offline-first approach. In fact, a static unchanging book is one of the few situations that AppCache works for. Of course a service worker is better, but until AppCache is removed from browsers (and until service worker is supported across the board), I’m using both. I wouldn’t recommend that for most sites though—for most sites, use a service worker to enhance it, and avoid AppCache like the plague.

For Resilient Web Design, I took a similar approach to HTML5 For Web Designers but I knew that there was a good chance that some of the content would be getting tweaked at least for a while. So while the approach is still cache-first, I decided to keep the cache fairly fresh.

Here’s my service worker. It starts with the usual stuff: when the service worker is installed, there’s a list of static assets to cache. In this case, that list is literally everything; all the HTML, CSS, JavaScript, and images for the whole site. Again, this is a pattern that works well for a book, but wouldn’t be right for other kinds of websites.

The real heavy lifting happens with the fetch event. This is where the logic sits for what the service worker should do everytime there’s a request for a resource. I’ve documented the logic with comments:

// Look in the cache first, fall back to the network
  // CACHE
  // Did we find the file in the cache?
      // If so, fetch a fresh copy from the network in the background
      // NETWORK
          // Stash the fresh copy in the cache
  // If the file wasn't in the cache, make a network request
      // Stash a fresh copy in the cache in the background
  // If the request is for an image, show an offline placeholder
  // If the request is for a page, show an offline message

So my order of preference is:

  1. Try the cache first,
  2. Try the network second,
  3. Fallback to a placeholder as a last resort.

Leaving aside that third part, regardless of whether the response is served straight from the cache or from the network, the cache gets a top-up. If the response is being served from the cache, there’s an additional network request made to get a fresh copy of the resource that was just served. This means that the user might be seeing a slightly stale version of a file, but they’ll get the fresher version next time round.

Again, I think this acceptable for a book where the tweaks and changes should be fairly minor, but I definitely wouldn’t want to do it on a more dynamic site where the freshness matters more.

Here’s what it usually likes like when a file is served up from the cache:

  .then( responseFromCache => {
  // Did we find the file in the cache?
  if (responseFromCache) {
      return responseFromCache;

I’ve introduced an extra step where the fresher version is fetched from the network. This is where the code can look a bit confusing: the network request is happening in the background after the cached file has already been returned, but the code appears before the return statement:

  .then( responseFromCache => {
  // Did we find the file in the cache?
  if (responseFromCache) {
      // If so, fetch a fresh copy from the network in the background
          // NETWORK
          .then( responseFromFetch => {
              // Stash the fresh copy in the cache
              .then( cache => {
                  cache.put(request, responseFromFetch);
      return responseFromCache;

It’s asynchronous, see? So even though all that network code appears before the return statement, it’s pretty much guaranteed to complete after the cache response has been returned. You can verify this by putting in some console.log statements:

.then( responseFromCache => {
  if (responseFromCache) {
          .then( responseFromFetch => {
              console.log('Got a response from the network.');
              .then( cache => {
                  cache.put(request, responseFromFetch);
      console.log('Got a response from the cache.');
      return responseFromCache;

Those log statements will appear in this order:

Got a response from the cache.
Got a response from the network.

That’s the opposite order in which they appear in the code. Everything inside the event.waitUntil part is asynchronous.

Here’s the catch: this kind of asynchronous waitUntil hasn’t landed in all the browsers yet. The code I’ve written will fail.

But never fear! Jake has written a polyfill. All I need to do is include that at the start of my serviceworker.js file and I’m good to go:

// Import Jake's polyfill for async waitUntil

I’m also using it when a file isn’t found in the cache, and is returned from the network instead. Here’s what the usual network code looks like:

  .then( responseFromFetch => {
    return responseFromFetch;

I want to also store that response in the cache, but I want to do it asynchronously—I don’t care how long it takes to put the file in the cache as long as the user gets the response straight away.

Technically, I’m not putting the response in the cache; I’m putting a copy of the response in the cache (it’s a stream, so I need to clone it if I want to do more than one thing with it).

  .then( responseFromFetch => {
    // Stash a fresh copy in the cache in the background
    let responseCopy = responseFromFetch.clone();
      .then( cache => {
          cache.put(request, responseCopy);
    return responseFromFetch;

That all seems to be working well in browsers that support service workers. For legacy browsers, like Mobile Safari, there’s the much blunter caveman logic of an AppCache manifest.

Here’s the JavaScript that decides whether a browser gets the service worker or the AppCache:

if ('serviceWorker' in navigator) {
  // If service workers are supported
} else if ('applicationCache' in window) {
  // Otherwise inject an iframe to use appcache
  var iframe = document.createElement('iframe');
  iframe.setAttribute('src', '/appcache.html');
  iframe.setAttribute('style', 'width: 0; height: 0; border: 0');

Either way, people are making full use of the offline nature of the book and that makes me very happy indeed.

Thursday, December 8th, 2016

Hey, Hey, Cloud Four is a PWA! - Cloud Four

Jason talks through the service worker strategy for his company website.

Sunday, December 4th, 2016

Installing web apps on phones (for real)

Henrik points to some crucial information that slipped under the radar at the Chrome Dev Summit—the Android OS is going to treat progressive web apps much more like regular native apps. This is kind of a big deal.

It’s a good time to go all in on the web. I can’t wait to see what the next few years bring. Personally, I feel like the web is well poised to replace the majority of apps we now get from app stores.

Service Worker, what are you? - Mariko Kosaka

This is a fun—and accurate—explanation of service workers.

There’s definitely something “alien” about a service worker—it’s kind of like a virus that gets installed on the user’s device. I’ve taken to describing it as “a man-in-the-middle attack on your own website” which makes sound a bit scarier than is necessary.

Monday, November 21st, 2016

My biggest takeaway from the second Offline Camp in Santa Margarita, CA — plus toast!

J. Renée Beach writes on Ev’s blog about three things to consider when planning for offline experiences:

  • Freshness,
  • Reach, and
  • Assurance.

How will you express to your users that the content is up to date, safe and available across their network?

Friday, October 28th, 2016

Working with mobile technology - Digital Service Manual - GOV.UK

Excellent guidelines from GDS on providing services that work well on mobile. The watchwords are:

  • responsive design,
  • progressive enhancement,
  • open data, and
  • emerging technology (service workers, notifications, etc.).

Native and hybrid apps are rarely justified.

Tuesday, October 18th, 2016


Laurie Voss has written a thoughtful article called Web development has two flavors of graceful degradation in response to Nolan Lawson’s recent article. But I’m afraid I don’t agree with Laurie’s central premise:

…web app development and web site development are so different now that they probably shouldn’t be called the same thing anymore.

This is an idea I keep returning to, and each time I do, I find that it just isn’t that simple. There are very few web thangs that are purely interactive without any content, and there are also very few web thangs that are purely passive without any interaction. Instead, it’s a spectrum. Quite often, the position on that spectrum changes according to the needs of the user at any particular time—are Twitter and Flicker web sites while I’m viewing text and images, but then transmogrify into web apps the moment I want add, update, or delete a piece of text or an image?

In any case, the more interesting question than “is something a web site or a web app?” is the question “why?” Why does it matter? In my experience, the answer to that question generally comes down to the kind of architectural approach that a developer will take.

That’s exactly what Laurie dives into in his post. For web apps, use one architectural approach—for web sites, use a different architectural approach. To summarise:

  • in a web app, front-load everything and rely on client-side JavaScript for all subsequent interaction,
  • in a web site, optimise for many page loads, and make sure you don’t rely on client-side JavaScript.

I’m oversimplifying here, but the general idea is:

  • build web apps with the single page app architecture,
  • build web sites with progressive enhancement.

That’s sensible advice, but I’m worried that it could lead to a tautological definition of what constitutes a web app:

  1. This is a web app so it’s built as a single page app.
  2. Why do you define it as a web app?
  3. Because it’s built as a single page app.

The underlying question of what makes something a web app is bypassed by the architectural considerations …but the architectural considerations should be based on that underlying question. Laurie says:

If you are developing an app, the user ideally loads the app exactly once — whether it’s over a slow connection or not.

And similarly:

But if you are developing a web site consisting of many discrete pages, the act of loading goes from a single event to the most common event.

I completely agree that the architectural approach of single page apps is better suited to some kinds of web thangs more than others. It’s a poor architectural choice for a content-based site like nasa.gov, for example. Progressive enhancement would make more sense there.

But I don’t think that the architectural choices need to be in opposition. It’s entirely possible to reconcile the two. It’s not always easy—and the further along that spectrum you are, the tougher it gets—but it’s doable. You can begin with progressive enhancement, and then build up to a single page app architecture for more capable browsers.

I think that’s going to get easier as frameworks adopt a more mixed approach. Almost all the major libraries are working on server-side rendering as a default. Ember is leading the way with FastBoot, and Angular Universal is following. Neither of them are doing it for reasons of progressive enhancement—they’re doing it for performance and SEO—but the upshot is that you can more easily build a web app that simultaneously uses progressive enhancement and a single-page app model.

I guess my point is that I don’t think we should get too locked into the idea of web apps and web sites requiring fundamentally different approaches, especially with the changes in the technologies we used to build them.

We’ve made the mistake in the past of framing problems as “either/or”, when in fact, the correct solution was “both!”:

  • you can either have a desktop site or a mobile site,
  • you can either have rich interactivity or accessibility,
  • you can either have a single page app or progressive enhancement.

We don’t have to choose. It might take more work, but we can have our web cake and eat it.

The false dichotomy that I’m most concerned about is the pernicious idea that offline functionality is somehow in opposition to progressive enhancement. Given the design of service workers, I find this proposition baffling.

This remark by Tom is the very definition of a false dichotomy:

People who say your site should work without JavaScript are actually hurting the people they think they’re helping.

He was also linking to Nolan’s article, which could indeed be read as saying that you should for offline instead of building with progressive enhancement. But I don’t think that’s what Nolan is saying (at least, I sincerely hope not). I think that Nolan is saying that we should prioritise the offline scenario over scenarios where JavaScript fails or isn’t available. That’s a completely reasonable thing to say. But the idea that we should build for the offline scenario instead of scenarios where JavaScript fails is absurdly reductionist. We don’t have to choose!

But I can certainly understand how developers might come to be believe that building a progressive web app is at odds with progressive enhancement. Having made a bunch of progressive web apps—Huffduffer, The Session, this site, I can testify that service workers work superbly as a layer on top of an existing site, but all the messaging around progressive web apps seems to fixated on the idea of the app-shell model (a small tweak to the single page app model, where a little bit of interface is available on the initial page load instead of requiring JavaScript for absolutely everything). Again, it’s entirely possible to reconcile the app-shell approach with server rendering and progressive enhancement, but nobody seems to be talking about that. Instead, all of the examples and demos are built with an assumption about JavaScript availability.

Assumptions are the problem. Whether it’s assumptions about screen size, assumptions about being able-bodied, assumptions about network connectivity, or assumptions about browser capabilities, I don’t think any assumptions are a safe bet. Now you might quite reasonably say that we have to make some assumptions when we’re building on the web, and you’d be right. But I think we should still aim to keep them to a minimum.

Tom’s tweet included a screenshot of this part of Nolan’s article:

As Benedict Evans has noted, the next billion people who are poised to come online will be using the internet almost exclusively through smartphones. And if Google’s plans with Android One are any indication, then we have a fairly good idea of what kind of devices the “next billion” will be using:

  • They’ll mostly be running Android.
  • They’ll have decent specs (1GB RAM, quad-core processors).
  • They’ll have an evergreen browser and WebView (Android 5+).
  • What they won’t have, however, is a reliable internet connection.

Those seem like a reasonable set of assumptions. But even there, things aren’t so simple. Will people really be using “an evergreen browser and WebView”? Millions of people use proxy browsers like Opera Mini, which means you can’t guarantee JavaScript availability beyond the initial page load. UC Browser—which can also run in proxy mode—is now the second most popular mobile browser in the world.

That’s just one nit-picky example, but what I’m getting at here is that it really isn’t safe to make any assumptions. When we must make assumptions, let’s try to make them a last resort.

And just to be clear here, I’m not saying that just because we can’t make assumptions about devices or browsers doesn’t mean that we can’t build rich interactive web apps that work offline. I’m saying that we can build rich interactive web apps that work offline and also work when JavaScript fails or isn’t supported.

You don’t have to choose between progressive enhancement and a single page app/progressive web app/app shell/other things with the word “app”.

Progressive enhancement is an architectural approach to building on the web. You don’t have to use it, but please try to remember that it is your choice to make. You can choose to build a web app using progressive enhancement or not—there is nothing inherent in the nature of the thing you’re building that precludes progressive enhancement.

Personally, I find progressive enhancement a sensible way to counteract any assumptions I might inadvertently make. Progressive enhancement increases the chances that the web site (or web app) I’m building is resilient to the kind of scenarios that I never would’ve predicted or anticipated.

That’s why I choose to use progressive enhancement …and build progressive web apps.

Sunday, October 16th, 2016

The Service Worker Lifecycle  |  Web  |  Google Developers

Jake goes into the details of what exactly is happening when a service worker is installed or replaced.

This is easily the most complex part of working with service workers, and I think I’m beginning to wrap my head around it, but the good news is that, for the most part, you don’t really need to know the ins and outs of this to get started (and dev tools are now making it easier to nuke from orbit if this begins to bite).

Tuesday, September 27th, 2016

Offline content with service workers · MadebyMike

This is a really great step-by-step walkthrough of adding a service worker to a website. Mike mentions the gotchas he encountered along the way, and describes how he incrementally levelled up the functionality.

If you’ve been going through a similar process, please write it down and share it like this!

Wednesday, September 21st, 2016

Progressive Web Apps Simply Make Sense - Cloud Four

Progressive Web Apps versus native is the wrong question because every step on the path to a Progressive Web App makes sense on its own, irrespective of what a company does with their native apps.

Not all of your customers are going to have your app installed. For those who visit via the web, providing them with a better experience will make them happier and generate more revenue for your business.

It’s really that simple.

SpeedCurve | PWA Performance

Steve describes a script you can use on WebPageTest to simulate going offline so you can test how your progressive web app performs.

Monday, September 19th, 2016

An intro to progressive web apps | 8th Light

A nice introduction to progressive web apps. There’s a little bit of confusion about permissions—whether a site has been added to the home screen or not has no effect on the permissions granted to it (for things like push notifications)—but the wrap-up nails the advantages of using the web:

No more waiting to download an app, no more prompts for updating an app. From a developer perspective, it means we will be able to iterate a lot quicker. We don’t need to wait for app store approvals anymore, and we can deploy at our own leisure.

Another advantage that a progressive web app has over a native mobile app is that it is linkable, hence it is easier to share and, probably even more importantly, can be indexed by search engines. This makes discoverability of the app a lot better.

Cross-origin Service Workers: Experimenting with Foreign Fetch | Web Updates - Google Developers

This one is definitely for service worker nerds only. I’ve been trying to get my head around this idea of “foreign fetch” which allows third parties to install service workers—could be handy for sites with APIs like Huffduffer and The Session. This article does a good job of explaining the somewhat tangled process.

What, Exactly, Makes Something A Progressive Web App? | Infrequently Noted

Alex runs through the features that a progressive web app must have, should have, and would be nice to have.

In general, installability criteria are tightening. Today’s Good-To-Haves may become part of tomorrow’s baseline. The opposite is unlikely because at least one major browser has made a strong commitment to tightening up the rules for installability.

Right now, this is in the nice-to-have category:

Mobile-friendly, not mobile-only.

Personally, I’d put that in the must-have category, and not just for progressive web apps.

Anyway, read on for some advice on testing and tooling when it comes to evaluating progressive web apps.

lyzadanger/pragmatist-service-worker: Pragmatist’s Guide to Service Worker

Lyza put together some example code for her Smashing Conference talk on service workers. If you haven’t written a service worker before, these are really nice examples of how to grok it bit by bit.

Monday, September 12th, 2016

How Google And Others Are Plotting The Revenge Of The Web App | Fast Company | Business + Innovation

It’s always, um …”interesting” when a mainstream publication covers a topic from the web’s bikeshed. In this case, it’s progressive web apps, and—apart from the sensationalist headline—it’s actually not that bad at all.

Monday, September 5th, 2016

The Building Blocks Of Progressive Web Apps – Smashing Magazine

This is a really good overview of progressive web apps:

An ideal web app is a web page that has the best aspects of both the web and native apps. It should be fast and quick to interact with, fit the device’s viewport, remain usable offline and be able to have an icon on the home screen.

At the same time, it must not sacrifice the things that make the web great, such as the ability to link deep into the app and to use URLs to enable sharing of content. Like the web, it should work well across platforms and not focus solely on mobile. It should behave just as well on a desktop computer as in other form factors, lest we risk having another era of unresponsive m.example.com websites.

Tuesday, August 30th, 2016

Indicating offline | justmarkup

Some interesting interface ideas here for informing users when a service worker is doing its magic.

In the future users may expect a site to work offline after visiting again, but until this happens, I think it is a good idea to let the users know about this feature.

Monday, August 29th, 2016

How we made the RioRun progressive web app | Info | The Guardian

The devs at The Guardian walk through the process of building a progressive web app for the Olympics. There were some gotchas with the life cycle of service workers, but the pay-off was worth it:

Once you get there though, it’s quite magical when you load the page on a phone, switch it to airplane mode, reload, and continue using the app as though nothing was wrong.