Denise shares a cautionary tale of service design gone wrong.
Sunday, June 6th, 2021
Thursday, April 22nd, 2021
Tuesday, March 23rd, 2021
Service worker weirdness in Chrome
I think I’ve found some more strange service worker behaviour in Chrome.
It all started when I was checking out the very nice new redesign of WebPageTest. I figured while I was there, I’d run some of my sites through it. I passed in a URL from The Session. When the test finished, I noticed that the “screenshot” tab said that something was being logged to the console. That’s odd! And the file doing the logging was the service worker script.
I fired up Chrome (which isn’t my usual browser), and started navigating around The Session with dev tools open to see what appeared in the console. Sure enough, there was a failed
fetch attempt being logged. The only time my service worker script logs anything is in the
catch clause of fetching pages from the network. So Chrome was trying to fetch a web page, failing, and logging this error:
The service worker navigation preload request failed with a network error.
But all my pages were loading just fine. So where was the error coming from?
After a lot of spelunking and debugging, I think I’ve figured out what’s happening…
First of all, I’m making use of navigation preloads in my service worker. That’s all fine.
Secondly, the website is a progressive web app. It has a manifest file that specifies some metadata, including
start_url. If someone adds the site to their home screen, this is the URL that will open.
Thirdly, Google recently announced that they’re tightening up the criteria for displaying install prompts for progressive web apps. If there’s no network connection, the site still needs to return a
200 OK response: either a cached copy of the URL or a custom offline page.
So here’s what I think is happening. When I navigate to a page on the site in Chrome, the service worker handles the navigation just fine. It also parses the manifest file I’ve linked to and checks to see if that start URL would load if there were no network connection. And that’s when the error gets logged.
I only noticed this behaviour because I had specified a query string on my start URL in the manifest file. Instead of a
start_url value of
/, I’ve set a
start_url value of
/?homescreen. And when the error shows up in the console, the URL being fetched is
Crucially, I’m not seeing a warning in the console saying “Site cannot be installed: Page does not work offline.” So I think this is all fine. If I were actually offline, there would indeed be an error logged to the console and that
start_url request would respond with my custom offline page. It’s just a bit confusing that the error is being logged when I’m online.
I thought I’d share this just in case anyone else is logging errors to the console in the
catch clause of fetches and is seeing an error even when everything appears to be working fine. I think there’s nothing to worry about.
Update: Jake confirmed my diagnosis and agreed that the error is a bit confusing. The good news is that it’s changing. In Chrome Canary the error message has already been updated to:
DOMException: The service worker navigation preload request failed due to a network error. This may have been an actual network error, or caused by the browser simulating offline to see if the page works offline: see https://w3c.github.io/manifest/#installability-signals
Thursday, March 18th, 2021
This is terrific! Jeremy shows how you can implement a fairly straightforward service worker for performance gains, but then really kicks it up a notch with a recipe for turning a regular website into a speedy single page app without framework bloat.
Thursday, March 11th, 2021
It’s official. The extremely niche browser behaviour I documented is a bug.
When service workers met framesets
Oh boy, do I have some obscure browser behaviour for you!
To set the scene…
I’ve been writing here in my online journal for almost twenty years. The official anniversary will be on September 30th. But this website has been even online longer than that, just in a very different form.
Here’s the first version of adactio.com.
Like a tour guide taking you around the ruins of some lost ancient civilisation, let me point out some interesting features:
- Observe the
.shtmlfile extension. That means it was once using Apache’s server-side includes, a simple way of repeating chunks of markup across pages. Scientists have been trying to reproduce the wisdom of the ancients using modern technology ever since.
- See how the layout is
100vh? Well, this was long before viewport units existed. In fact there is no CSS at all on that page. It’s one big
tableelement with 100% width and 100% height.
- So if there’s no CSS, where is the
border-radiuscoming from? Let me introduce you to an old friend—the non-animated GIF. It’s got just enough transparency (though not proper alpha transparency) to fake rounded corners between two solid colours.
if (navigator.appName == "Netscape")
Note that these are not iframes, they are frames. Different thing. You could create single page apps long before Ajax was a twinkle in Jesse James Garrett’s eye.
If you view source, you’ll see a React-like component system. Each
frameset component contains
frame components that are isolated from one another. They’re like web components. Each frame has its own (non-shadow) DOM. That’s because each frame is actually a separate web page. If you right-click on any of the frames, your browser should give the option to view the framed document in its own tab or window.
Now for the part where modern and ancient technologies collide…
If you’re looking at the frameset URL in Firefox or Safari, everything displays as it should in all its ancient glory. But if you’re looking in Google Chrome and you’ve visited adactio.com before, something very odd happens.
Each frame of the frameset displays my custom offline page. The only way that could be served up is through my service worker script. You can verify this by opening the framest URL in an incognito window—everything works fine when no service worker has been registered.
I have no idea why this is happening. My service worker logic is saying “if there’s a request for a web page, try fetching it from the network, otherwise look in the cache, otherwise show an offline page.” But if those page requests are initiated by a
frame element, it goes straight to showing the offline page.
Is this a bug? Or perhaps this is the correct behaviour for some security reason? I have no idea.
I wonder if anyone has ever come across this before. It’s a very strange combination of factors:
- a domain served over HTTPS,
- that registers a service worker,
- but also uses framesets and frames.
I could submit a bug report about this but I fear I would be laughed out of the bug tracker.
Still …the World Wide Web is remarkable for its backward compatibility. This behaviour is unusual because browser makers are at pains to support existing content and never break the web.
Technically a modern website (one that registers a service worker) shouldn’t be using deprecated technology like frames. But browsers still need to be able support those old technologies in order to render old websites.
This situation has only arisen because the same domain—adactio.com—is host to a modern website and a really old one.
Maybe Chrome is behaving strangely because I’ve built my online home on ancient burial ground.
It’s all to do with navigation preloads and the value of
event.preloadResponse, which I believe is only supported in Chrome which would explain the differences between browsers.
According to this post by Jake:
event.preloadResponse is a promise that resolves with a response, if:
- Navigation preload is enabled.
- The request is a GET request.
- The request is a navigation request (which browsers generate when they’re loading pages, including iframes).
event.preloadResponseis still there, but it resolves with
Notice that iframes are mentioned, but not frames.
My code was assuming that if
event.preloadRepsonse exists in my block of code for responding to page requests, then there’d be a response. But if the request was initiated from a frameset, it is a request for a page and
event.preloadRepsonse does exist …but it’s undefined.
I’ve updated my code now to check this assumption (and fall back to
This may technically still be a bug though. Shouldn’t a page loaded from a frameset count as a navigation request?
Thursday, October 15th, 2020
This in an intriguing promise (there’s no code yet):
A PWA typically requires writing a service worker, an app manifest and a ton of custom code. Progressier flattens the learning curve. Just add it to your html template — you’re done.
Tuesday, September 29th, 2020
This is a great way to use a service worker to circumvent censorship:
After the visitor opens the website once over a VPN, the service worker is downloaded and installed. The VPN can then be disabled, and the service worker will take over to request content from non-blocked servers, effectively acting as a proxy.
Thursday, August 20th, 2020
Saturday, August 1st, 2020
So, why would you want to use a service worker? Here are some cool things you can do with it.
Chris lists some of the ways a service worker can enhance user experience.
Monday, July 20th, 2020
Thoughts on user experience design and service design, prompted by the Clearleft podcast:
I especially enjoyed the latest episode about a topic that has become a bit of a hyped buzzword over the last few years: Service design.
Rich with anecdotes and stories, the episode started with an investigation: What is service design, anyway?
Saturday, July 18th, 2020
A Chrome-only API for adding offline content to an index that can be exposed in Android’s “downloads” list. It just shipped in the lastest version of Chrome.
I’m not a fan of browser-specific non-standards but you can treat this as an enhancement—implementing it doesn’t harm non-supporting browsers and you can use feature detection to test for it.
How do we tell our visitors our sites work offline? How do we tell our visitors that they don’t need an app because it’s no more capable than the URL they’re on right now?
Remy expands on his call for ideas on branding websites that work offline with a universal symbol, along the lines of what we had with RSS.
What I’d personally like to see as an outcome: some simple iconography that I can use on my own site and other projects that can offer ambient badging to reassure my visitor that the URL they’re visiting will work offline.
Wednesday, July 15th, 2020
This is an interesting push by Remy to try to figure out a way we can collectively indicate to users that a site works offline.
Well, seeing as browsers have completely dropped the ball on any kind of ambient badging, it’s fair enough that we take matters into our own hands.
Service design on the Clearleft podcast
If you’re subscribed to the Clearleft podcast there’s a new episode winging its way across the airwaves to alight in your podcast software of choice.
This episode is all about service design. More precisely, it’s about me trying to understand what service design is. I don’t think I’m alone in being unsure of its meaning.
So in some ways, this is similar to the first episode, which involved a lot me asking “What exactly is a design system anyway?” But for the service design episode, rather than using interviews as my source material, I’ve dug into the archives of UX London. There are past talks on Clearleft’s Vimeo channel. I made plenty of use of presentations by Kerry Bodine, Jamin Hegeman, and Lou Downe.
That worked out well, but I felt there was still something missing from the episode. It needed a good story to wrap things up. So I cornered Rich for a chat about a project Clearleft worked on for Brighton council. That did the trick!
Again, there’s not much of me in this one. I’m there to thread the narrative together but my voice is not the one doing the explaining or the story-telling.
The episode ended up being almost half an hour long. Like I said before, rather than trying to squeeze each episode into a predefined timeslot, each episode will be as long as needs to be. And this one needed the time for Rich to tell his story.
Ooh, and I even tried adding in some sound effects during that part! It probably just sounds cheesy, but I’m still trying to figure out what works and what doesn’t.
Anyway, have a listen to this episode and see what you think. It’s got dead badgers, Downton Abbey, icebergs, and airplanes. Service design really does encompass a lot!
Monday, April 20th, 2020
If you dodged an accessibility lawsuit because you have physical locations, what does it mean when those physical locations close?
As movie theaters, restaurant ordering, college courses, and more move to online-first delivery, the notion of a corresponding brick-and-mortar venue falls away. If the current pandemic physical distancing measures stretch into the next year as many think, then this blip becomes the de facto new normal.
Wednesday, April 8th, 2020
80 geocoding service plans to choose from.
I’m going to squirrel this one away for later—I’ve had to switch geocoding providers in the past, so I have a feeling that this could come in handy.
Thursday, March 26th, 2020
Apple’s attack on service workers
Apple aren’t the best at developer relations. But, bad as their communications can be, I’m willing to cut them some slack. After all, they’re not used to talking with the developer community.
John Wilander wrote a blog post that starts with some excellent news: Full Third-Party Cookie Blocking and More. Safari is catching up to Firefox and disabling third-party cookies by default. Wonderful! I’ve had third-party cookies disabled for a few years now, and while something occassionally breaks, it’s honestly a pretty great experience all around. Denying companies the ability to track users across sites is A Good Thing.
In the same blog post, John said that client-side cookies will be capped to a seven-day lifespan, as previously announced. Just to be clear, this only applies to client-side cookies. If you’re setting a cookie on the server, using PHP or some other server-side language, it won’t be affected. So persistent logins are still doable.
Then, in an audacious example of burying the lede, towards the end of the blog post, John announces that a whole bunch of other client-side storage technologies will also be capped to seven days. Most of the technologies are APIs that, like cookies, can be used to store data: Indexed DB, Local Storage, and Session Storage (though there’s no mention of the Cache API). At the bottom of the list is this:
Service Worker registrations
Okay, let’s clear up a few things here (because they have been so poorly communicated in the blog post)…
The seven day timer refers to seven days of Safari usage, not seven calendar days (although, given how often most people use their phones, the two are probably interchangable). So if someone returns to your site within a seven day period of using Safari, the timer resets to zero, and your service worker gets a stay of execution. Lucky you.
This only applies to Safari. So if your site has been added to the home screen and your web app manifest has a value for the “display” property like “standalone” or “full screen”, the seven day timer doesn’t apply.
That piece of information was missing from the initial blog post. Since the blog post was updated to include this clarification, some people have taken this to mean that progressive web apps aren’t affected by the upcoming change. Not true. Only progressive web apps that have been added to the home screen (and that have an appropriate “display” value) will be spared. That’s a vanishingly small percentage of progressive web apps, especially on iOS. To add a site to the home screen on iOS, you need to dig and scroll through the share menu to find the right option. And you need to do this unprompted. There is no ambient badging in Safari to indicate that a site is installable. Chrome’s install banner isn’t perfect, but it’s better than nothing.
Just a reminder: a progressive web app is a website that
- runs on HTTPS,
- has a service worker,
- and a web manifest.
Adding to the home screen is something you can do with a progressive web app (or any other website). It is not what defines progressive web apps.
In any case, this move to delete service workers after seven days of using Safari is very odd, and I’m struggling to find the connection to the rest of the blog post, which is about technologies that can store data.
As I understand it, with the crackdown on setting third-party cookies, trackers are moving to first-party technologies. So whereas in the past, a tracking company could tell its customers “Add this
script element to your pages”, now they have to say “Add this
The best explanation I can come up with for this move by Apple is that it feels like the neatest solution. That’s neat as in tidy, not as in nifty. It is definitely not a nifty solution.
If some technologies set by a specific domain are being purged after seven days, then the tidy thing to do is purge all technologies from that domain. Service workers are getting included in that dragnet.
Now, to be fair, browsers and operating systems are free to clean up storage space as they see fit. Caches, Local Storage, Indexed DB—all of those are subject to eventually getting cleaned up.
So I was curious. Wanting to give Apple the benefit of the doubt, I set about trying to find out how long service worker registrations currently last before getting deleted. Maybe this announcement of a seven day time limit would turn out to be not such a big change from current behaviour. Maybe currently service workers last for 90 days, or 60, or just 30.
There was no time limit previously.
This is not a minor change. This is a crippling attack on service workers, a technology specifically designed to improve the user experience for return visits, whether it’s through improved performance or offline access.
I wouldn’t be so stunned had this announcement come with an accompanying feature that would allow Safari users to know when a website is a progressive web app that can be added to the home screen. But Safari continues to ignore the existence of progressive web apps. And now it will actively discourage people from using service workers.
If you’d like to give feedback on this ludicrous development, you can file a bug (down in the cellar in the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying “Beware of the Leopard”).
No doubt there will still be plenty of Apple apologists telling us why it’s good that Safari has wished service workers into the cornfield. But make no mistake. This is a terrible move by Apple.
I will say this though: given The Situation we’re all living in right now, some good ol’ fashioned Hot Drama by a browser vendor behaving badly feels almost comforting.
Monday, March 23rd, 2020
Performance matters …especially when the chips are down:
If you are in charge of a web site that provides even slightly important information, or important services, it’s time to get static. I’m thinking here of sites for places like health departments (and pretty much all government services), hospitals and clinics, utility services, food delivery and ordering, and I’m sure there are more that haven’t occurred to me. As much as you possibly can, get it down to static HTML and CSS and maybe a tiny bit of enhancing JS, and pare away every byte you can.
Thursday, February 6th, 2020
Tantek documents the features he wants his posting interface to have.