Tags: serviceworkers

I’ve noticed a strange mindset amongst front-end/full-stack developers. At least it seems strange to me. But maybe I’m the one with the strange mindset and everyone else knows something I don’t.

It’s to do with trust and suspicion.

I’ve made no secret of the fact that I’m suspicious of third-party code and dependencies in general. Every dependency you add to a project is one more potential single point of failure. You have to trust that the strangers who wrote that code knew what they were doing. I’m still somewhat flabbergasted that developers regularly add dependencies—via npm or yarn or whatever—that then pull in even more dependencies, all while assuming good faith and competence on the part of every person involved.

It’s a touching expression of faith in your fellow humans, but I’m not keen on the idea of faith-based development.

I’m much more trusting of native browser features—HTML elements, CSS features, and JavaScript APIs. They’re not always perfect, but a lot of thought goes into their development. By the time they land in browsers, a whole lot of smart people have kicked the tyres and considered many different angles. As a bonus, I don’t need to install them. Even better, end users don’t need to install them.

And yet, the mindset I’ve noticed is that many developers are suspicious of browser features but trusting of third-party libraries.

When I write and talk about using service workers, I often come across scepticism from developers about writing the service worker code. “Is there a library I can use?” they ask. “Well, yes” I reply, “but then you’ve got to understand the library, and the time it takes you to do that could be spent understanding the native code.” So even though a library might not offer any new functionality—just a different idion—many developers are more likely to trust the third-party library than they are to trust the underlying code that the third-party library is abstracting!

Developers are more likely to trust, say, Bootstrap than they are to trust CSS grid or custom properties. Developers are more likely to trust React than they are to trust web components.

On the one hand, I get it. Bootstrap and React are very popular. That popularity speaks volumes. If lots of people use a technology, it must be a safe bet, right?

But if we’re talking about popularity, every single browser today ships with support for features like grid, custom properties, service workers and web components. No third-party framework can even come close to that install base.

And the fact that these technologies have shipped in stable browsers means they’re vetted. They’ve been through a rigourous testing phase. They’ve effectively got a seal of approval from each individual browser maker. To me, that seems like a much bigger signal of trustworthiness than the popularity of a third-party library or framework.

So I’m kind of confused by this prevalent mindset of trusting third-party code more than built-in browser features.

Is it because of the job market? When recruiters are looking for developers, their laundry list is usually third-party technologies: React, Vue, Bootstrap, etc. It’s rare to find a job ad that lists native browser technologies: flexbox, grid, service workers, web components.

I would love it if someone could explain why they avoid native browser features but use third-party code.

Until then, I shall remain perplexed.

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 /?homescreen.

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

Much better!

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 .shtml file 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 100vw and 100vh? Well, this was long before viewport units existed. In fact there is no CSS at all on that page. It’s one big table element with 100% width and 100% height.
• So if there’s no CSS, where is the border-radius coming 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.
• The management takes no responsibility for any trauma that might befall you if you view source. There you will uncover JavaScript from the dawn of time; ancient runic writing like if (navigator.appName == "Netscape")

Now if your constitution was able to withstand that, brace yourself for what happens when you click on either of the two links, deutsch or english.

You find yourself inside a frameset. You may also experience some disorienting “DHTML”—the marketing term given to any combination of JavaScript and positioning in the late ’90s.

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.

Update: Both Remy and Jake did some debugging and found the issue…

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).

Otherwise event.preloadResponse is still there, but it resolves with undefined.

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 fetch).

This may technically still be a bug though. Shouldn’t a page loaded from a frameset count as a navigation request?

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 script element and this script file to your pages.” That JavaScript file can then store a unique idenitifer on the client. This could be done with a cookie, with Local Storage, or with Indexed DB, for example. But I’m struggling to understand how a service worker script could be used in this way. I’d really like to see some examples of this actually happening.

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.

Nope:

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.