Tags: goingoffline

49

sparkline

Friday, December 20th, 2019

The Layers Of The Web

The opening presentation from the Beyond Tellerrand conference held in Berlin in November 2019.

Guten Morgen. All right. I’m just going to get started because I’ve got a lot to talk about and I’m very, very excited to be here.

I’m excited to talk about the web. I’ve been thinking a lot about the web. You know, I think a lot about the web all the time, but this year, in particular, thinking about where the web came from; asking myself where the web came from, which is kind of a dumb question because it’s pretty obvious where the web came from.

It came from this guy. This is Tim Berners-Lee and he is the creator of the World Wide Web. It was 30 years ago, March 1989, that he wrote a proposal while he was at CERN, a very dull-looking proposal called “Information Management: A Proposal” that had incomprehensible diagrams trying to explain what he had in mind. But a supervisor, Mike Sendall, saw the potential and scrawled across the top, “Vague but exciting.”

Tim Berners-Lee starts working on this idea he has for a global hypertext system and he starts creating the world’s first web browser and the world’s first web server, which is this NeXT machine which is in the Science Museum in London, a lovely machine, the NeXT box.

I was in the neighbourhood so I just had to come by and say hello…

I have a great affection for it because, earlier this year, I was very honored to be invited to CERN, along with this bunch of hackers, to take part in a project related to the 30th anniversary of that proposal. I will show you a video that explains the project.

So, we came to CERN this week in order to create some sort of modern-day interpretation of the very first web browser.

—Kimberly Blessing

Well, the project is to restore the first browser which was developed by the inventor of the Web, and the idea is to create an experience for the people who could not use the web in its early days to have an idea how it felt to use the web at that time.

—Martin Akolo Chiteri

I think the biggest difficulty was to make the browser work in the NeXT machine that we had.

—Angela Ricci

We really needed to work with an original NeXT box in order to really understand what that experience was like in order to be able to write some code and replicate that experience.

—Kimberly Blessing

My role is code, so generating the code to create the interactive aspect of the World Wide web browser, recreated browser. It’s very much writing JavaScript to kind of create all the NeXT operating system UI, making requests to servers to go and get the HTML and massage the HTML back into a format that looks good in the World Wide Web browser; and making sure we end up with a URL that goes into production that someone can visit and see their own webpages. The tangible software is what I’m responsible for, so I have to make sure it all gets done. Otherwise, we have no browser to look at, basically.

—Remy Sharp

We got together a few years back to do a similar sort of hack project here at CERN which was creating the world’s second-ever web browser, which was the Line Mode browser. We had a lot of fun with it and it’s a great bunch of people from all over the world. It’s been really great to get back together and it’s always amazing to be here at CERN, to be at not just the birthplace of the Web, but the most important place on the planet for science.

Yeah, it’s been a lot of fun. I kind of don’t want it to be over because we are in our element, hacking away, having fun, and just soaking up the atmosphere, and we are getting to chat with people who were there 30 years ago, Jean-Francois Groff and Robert Cailliau, these people who were involved in the creation of the World Wide web. To me, that’s amazing to be surrounded by so much World Wide Web history.

The plan is that this will go online and anyone will be able to access it because it’s on the web, and that’s the beautiful thing about the web is that anyone can visit a website, and so everyone will have the opportunity to try using the world’s first web browser and see what modern webpages would look like if they were passed through this first web browser.

—Jeremy Keith

Well, spoiler alert. The project was a success and you can, indeed, look at your websites in a recreation of the first-ever web browser. This is the URL. It’s worldwideweb.cern.ch.

Success, that was good. But as you could probably tell from that video, Remy was the one basically making this all happen. He was the one writing the JavaScript to recreate this in a modern browser. This is the first-ever web page viewed in the first-ever web browser.

As you gathered, again, I was really fascinated by the history of the Web, like, where did it come from, and the people who were there at the time and getting to pick their brains. I spent most of my time working on the accompanying website to go with this project. I was creating this timeline.

Timeline

Because this was to mark the 30th anniversary of this proposal, I thought, well, we could easily look at what has happened in the last 30 years: websites, web servers, formats, standards - all that stuff. But I thought it would be fascinating to look at the previous 30 years as well and try and figure out the things that were happening that influenced Tim Berners-Lee in terms of hypertext, networks, computing, and all this stuff.

But I’d kind of had given myself this arbitrary cut-off point of 30 years to make this nice symmetry of it being the 30th anniversary of the World Wide Web. I could go further back. I could start asking, well, what happened before 30 years ago? What were the biggest influences on Tim Berners-Lee and the World Wide Web?

Now, if you were to ask Tim Berners-Lee himself who his biggest influencers were, he would give you a straight-up answer. He will say his biggest influencers were Conway Berners-Lee and Mary Lee Woods, his father and mother, which is fair enough. Normally, when you ask people who their influences are, they say, “Oh, my parents. They gave me a loving environment. They kindled my curiosity,” and all that stuff. I’m sure that’s true but, in this case, it was also a big influence in a practical sense in that both Mary and Conway worked on the Ferranti Mark 1. That’s where they met. They were programmers. Tim Berners-Lee’s parents were programmers on the Ferranti Mark 1, a very early computer. This is in the 1950s in Britain.

Okay, this feels like a good origin story for the web, right? They were working on this early computer.

But it’s an early computer; it’s not the first computer. Maybe I need to go back further. How far back do I go to find the first computer?

Time machine.

Is this the first computer, the Antikythera mechanism? You can see this in a museum in Athens. This was recovered from a shipwreck. It was recovered at the start of the 20th Century, but it dates back thousands of years, a mechanism for predicting the position of stars and planets. It does calculations. It is a calculating device. Not a programmable computer as such, though.

Worshipping the reliquary of Babbage’s brain

If you’re thinking about the origins of the idea of a programmable computer, I think we could start to look at this gentleman, Charles Babbage. This is half of Charles Babbage’s brain, which is in the Science Museum in London along with that original NeXT box that the World Wide web was created on. The other half is in the Computing History Museum in California.

Charles Babbage lived in the 19th Century, and kind of got a lot of seed funding from the U.K. government to build a device, the Difference Engine, which would do calculations. Later on, he scrapped that and started working on the Analytical Engine which would be even better — a 2.0 version. It never got finished, by the way, but it was a really amazing idea because you could see the architecture of like a central processing unit, but it was still fundamentally a calculator, a calculating machine.

The breakthrough in terms of programming maybe came from Charles Babbage’s collaborator. This is Ada Lovelace. She was translating documents by an Italian mathematician about Difference Engines and calculations. She realized that—hang on—if we’re doing operations on numbers, what if those numbers could stand for other concepts, non-numerical like words or thoughts? Then we could do operations on things other than numbers, which is exactly what we do today in modern computing.

If you use a word processor, you’re not processing words; you’re operating on ones and zeros. If you use a graphics program, you’re not actually moving pixels around; you’re operating on ones and zeroes. This idea of how anything could stand in for ones and zeros for numbers kind of started with Ada Lovelace.

But, as I said, the Difference Engine and the Analytical Engine, they never got finished, and this was kind of a dead-end. It turns out, they weren’t an influence.

Later on, for example, this genius who was definitely responsible for the first working computers, Alan Turing, he wasn’t aware of the work of Babbage and Lovelace, which is a shame. He was kind of working in isolation.

He came up with the idea of the universal machine, the Turing Machine. Give it an infinitely long tape and enough state, enough time, you could calculate literally anything, which is pretty much what computers are.

He was working at Bletchley Park breaking the code for the Enigma machines, and that leads to the creation of what I think would be the first programmable computer. This is Colossus at Bletchley Park. This was created by a colleague of Turing, Tommy Flowers.

It is programmable. It’s using valves, but it’s absolutely programmable. It was top secret, so even for years after the war, this was not known about. In the history books, even to this day, you’ll often see ENIAC listed as the first programmable computer, but I think that honor goes to Tommy Flowers and Colossus.

By the way, Alan Turing, after the war, after 1945, he did go on to work and keep on working in the field of computing. In fact, he worked as a consultant at Ferranti. He was working on the Ferranti Mark 1, the same computer where Tim Berners-Lee’s parents met when they were programmers.

As I say, that was after the war ended in 1945. Now, we can’t say that the work at Bletchley Park was responsible for winning the war, but we could probably say that it’s certainly responsible for shortening the war. If it weren’t for the work done by the codebreakers at Bletchley Park, the war might not have finished in 1945.

1945 is the year that this gentleman wrote a piece that was certainly influential on Tim Berners-Lee. This is Vannevar Bush, a scientist, a thinker. In 1945, he published a piece in the Atlantic Monthly under the heading, “A Scientist Looks at Tomorrow,” he publishes, “As We May Think.”

In this piece, he describes an imaginary device. It’s a mechanical device inside a desk, and the operator is allowed to work on reams and reams of microfilm and to connect ideas together, make these associative trails. This is kind of like hypertext before the word hypertext has been coined. Vannevar Bush calls this device the Memex. That’s published in 1945.

Memex

Also, in 1945, this young man has been drafted into the U.S. Navy and he’s shipping out to the Pacific. His name is Douglas Engelbart. Literally as the ship is leaving the harbor to head to the Pacific, word comes through that the war is over.

Now, he still gets shipped out to the Pacific. He’s in the Philippines. But now, instead of fighting against the Japanese, he’s lounging around in a hut on stilts reading magazines and that’s where he reads “As We May Think,” by Vannevar Bush.

Fast-forward years later; he’s trying to decide what to do with his life other than settle down, get married, have a job, you know, that kind of thing. He thinks, “No, no, I want to make the world a better place.” He realizes that computers could be the way to do this if they could implement something very much like the Memex. Instead of a mechanical device, what if computers could create the Memex, this hypertext system? He devotes his life to this and effectively invents the field of Human Computer Interaction.

On December 9th, 1968, he demonstrates what he’s been working at. This is in San Francisco, and he demonstrates bitmap screens. He demonstrates real-time collaboration on documents, working hypertext …and also he invents the mouse for the demo.

We have a pointing device called a mouse, a standard keyboard, and a special key set we have here. And we are going to go for a picture down on our laboratory in Menlo Park and pipe it up. It’ll show you, from another point of view, more about how that mouse works.

Come in, Menlo Park. Okay, there’s Don Anders’ hand in Menlo Park. In a second, we’ll see the screen that he’s working and the way the tracking spot moves in conjunction with movements of that mouse. I don’t know why we call it a mouse sometimes. I apologize; it started that way and we never did change it.

—Douglas Engelbart

This was ground-breaking. The mother of all demos, it came to be known as. This was a big influence on Tim Berners-Lee.

At this point, we’ve entered the time cone of those 30 years before the proposal that Tim Berners-Lee made, which is good because this is the moment where I like to branch off from this timeline and sort of turn it around.

The question I’m sure nobody is asking—because you saw there was a video link-up there; Douglas Engelbart is in San Francisco, and he has a video link-up with Menlo Park to demonstrate real-time collaboration with computers—the question nobody is asking is, who is operating the video camera in Menlo Park?

Well, I’ll tell you the answer to that question that nobody is asking. The man operating the video camera in Menlo Park is this man. His name is Stuart Brand. Now, Stuart Brand has spent most of the ‘60s doing what you would do in the ‘60s; he was dropping acid. This was all kosher. This was before it was illegal.

He was on the Merry Pranksters bus with Ken Kesey and, on one particular acid trip, he literally saw the Earth curving away and realizing that, yeah, we’re all on one planet, man! And he started a campaign with badges called, “Why haven’t we seen a photograph of the whole Earth yet?” I like the “yet” part in there like it’s a conspiracy that we haven’t seen a photograph of the whole Earth.

He was kind of onto something here, realizing that seeing our planet as a whole planet from space could be a consciousness-changing thing much like LSD is a consciousness-changing thing. Sure enough, people did talk about the effect it had when we got photographs like Earthrise from Apollo 8, and he used those pictures when he published the Whole Earth Catalog, which was a series of books.

The Whole Earth Catalog was basically like Wikipedia before the internet. It was this big manual of how to do everything. The idea was, if you were running a commune, living in a commune, you needed to know about technology, and agriculture, and weather, and all the stuff, and you could find that in the Whole Earth Catalog.

He was quite an influential guy, Stuart Brand. You probably heard the Steve Jobs commencement speech where he quotes Stuart Brand, “Stay hungry, stay foolish,” all that stuff.

Stuart Brand also did a lot of writing. After Douglas Engelbart’s demo, he started to see that this computer thing was something else. He literally said computers are the new LSD, so he starts really investigating computing and computers.

He writes this great article in “Rolling Stone” magazine in 1972 about space war, one of the first games you could play on the screen. But he has a wide range of interests. He kind of kicked off the environmental movement in some ways.

At one point, he writes a book about architecture. He writes a book called “How Buildings Learn.” There’s a television series that goes with it as well. This is a classic book (the definition of a classic book being a book that everyone has heard of and nobody has read).

In this book, he starts looking at the work of a British architect called Frank Duffy. Frank Duffy has this idea about architecture he calls shearing layers. The way that Frank Duffy puts it is that a building, properly conceived, consists of several layers of longevity, so kind of different rates of change.

He diagrams this out in terms of a building, and you see that you’ve got the site that the building is on that’s moving at a geological timescale, right? That should be around for thousands of years, we would hope.

Then you’ve got the actual structure that could stand for centuries.

Then you get into the infrastructure inside. You know, the plumbing and all that, you probably want to swap out every few decades.

Basically, until you get down to the stuff inside a room, the furniture that you can move around on a daily basis. You’ve got all these timescales moving from fast to slow as you move inwards into the house.

What I find fascinating about this idea of these different layers as well is the way that each layer depends on the layer below. Like, you can’t have the structure of a building without first having a site to put it on. You can’t move furniture around inside a room until you’ve made the room using the walls and the doors, right? This idea of shearing layers is kind of fascinating, and we’re going to get back to it.

Something else that Stuart Brand went on to do; he was one of the co-founders of the Long Now Foundation. Anybody here part of the Long Now Foundation? Any members of the Long Now Foundation? Ah… It’s a great organization. It’s literally dedicated to long-term thinking. It was founded by Stuart Brand and Danny Hillis, the computer scientist, and Brian Eno, the musician and producer. Like I said, dedicated to long-term thinking. This is my membership card made out of a durable metal because it’s got to last for thousands of years.

If you go on the website of the Long Now Foundation, you’ll notice that the years are made up of five digits, so instead of 2019, it will be 02019. Well, you know, you’ve got to solve the Y10K problem. They’re dedicated to long-term thinking, to trying to think in the longer now.

Admiring the Clock Of The Long Now

One of the most famous projects is the clock of the Long Now. This is a clock that will tell time for 10,000 years. Brian Eno has done the chimes. They’re generative. It’ll never chime the same way twice. It chimes once a century. This is a scale model that’s in the Science Museum in London along with half of Charles Babbage’s brain and the original NeXT machine that Tim Berners-Lee created World Wide Web on.

This is just a scale model. The full-sized clock is going to be inside a mountain in west Texas. You’ll be able to visit it. It’ll be like a pilgrimage. Construction is underway. I hope to visit the clock one day.

Stuart Brand collected his thoughts. It’s a really fascinating project when you think about, how do you design something to last 10,000 years? How do you communicate over 10,000 years? One of those tricky design problems almost like the Voyager Golden Record or the Yucca Mountain waste disposal. How do you communicate to future generations? You can’t rely on language. You can’t rely on semiotics. Anyway, he collected a lot of his thoughts into this book called “The Clock of the Long Now,” subtitled “Time and Responsibility: The Ideas Behind the World’s Slowest Computer.” He’s thinking about time. That’s when he comes back to shearing layers and these different layers of rates of change; different layers of time.

Stuart Brand abstracts the idea of shearing layers into something called “pace layers.” What if it’s not just architecture? What if any kind of system has these different rates of change, these layers?

He diagrams this out in terms of the human species, so think of humans. We have these different layers that we operate at.

At the lowest, slowest level, there is our nature, literally, like what makes us human in terms of our DNA. That doesn’t change for tens of thousands of years. Physiologically, there’s no difference between a caveman and an astronaut, right?

Then you’ve got culture, which cumulates of centuries, and the tribal identities we have around things like nations, language, and things like that.

Governance, models of governance, so not governments but governance, as in the way we choose to run things, whether that’s a feudal society, or a monarchy, or representative democracy, right? Those things do change, but not too fast, hopefully.

Infrastructure: you’ve got to keep up with the times, you know? This needs to move at a faster pace, again.

Commerce: much more fast-moving. Commerce needs to — you’re getting into the faster timescales there.

Then he puts fashion at the top. By fashion, he means anything that is supposed to be new and exciting, so that includes pop music, for example. The whole idea with fashion is that it’s there to try stuff out and discard it very quickly.

“What about this?” “No.” “What about that?” “Try this.” “No, try that.”

The good stuff, the stuff that kind of sticks to the wall, will maybe find its way down to the longer-lasting layers. Maybe a really good pop song from fashion ends up becoming part of culture, over time.

Here’s the way that Stuart Brand describes pace layers. He says:

Fast learns; slow remembers. Fast proposals, and slow disposes. Fast is discontinuous; slow is continuous. Fast and small instructs slow and big by a crude innovation and by occasional revolution, but slow and big controls small and fast by constraint and constancy.

He says:

Fast gets all of our attention but slow has all the power.

Pace layers is one of those ideas that, once you see it, you can’t unsee it. You know when you want to make someone’s life a misery, you just teach them about typography. Now they can’t unsee all the terrible kerning in the world. I can’t unsee pace layers. I see them wherever I look.

Does anyone remember this book, UX designers in the room, “The Elements of User Experience,” by Jesse James Garrett? It’s old now. We’re going back in the way but, in it, he’s got this diagram about the different layers to a user experience. You’ve got the strategy below that finally ends up with an interface at the top.

I look at this, and I go, “Oh, right. It’s pace layers. It’s literally pace layers.” Each layer depending on the layer below, the slower layers at the bottom, the faster-moving things at the top.

With this mindset that pace layers are everywhere, I thought, “Can I map out the web in terms of pace layers, the technology stack of the web?” I’m going to give it a go.

At the lowest stack, the slowest moving, I would say there’s the internet itself, as in TCP/IP, the transmission control protocol, Internet protocol created by Bob Kahn and Vint Cerf in the ‘70s and pretty much unchanged since then, deliberately dumb, deliberately simple. All it does is move packets around. Pretty much unchanged.

On top of that, you get the other protocols that use TCP/IP, like in the case of the web, the hypertext transfer protocol. Now, this has changed over time. We now have HTTP/2. But it hasn’t been rapid change. It’s been gradual. Again, that kind of feels right. It feels good that HTTP isn’t constantly changing underneath us too much.

Then we serve up over HTTP are URLs. I wish that URLs were down here. I wish that URLs were everlasting, never changing. But, unfortunately, I must acknowledge that that’s not true. Links die. We have to really work hard to keep them alive. I think we should work hard to keep them alive.

What do you put at those URLs? At the simplest level, it’s supposed to be plain text. But this is the web, so let’s say structured text. This is going to be HTML, the hypertext markup language, which Tim Berners-Lee came up with when he created the World Wide Web. I say, “Came up with.” He basically stole it from SGML that scientists at CERN were already using and sprinkled in one or two new tags, as they were calling it back then.

There were maybe like 20-something tags in HTML when Tim Berners-Lee created the web. Now we’ve got over 100 elements, as we call them. But I feel like I’ve been able to keep up with the pace of change. I mean, the vast big kind of growth spurt with HTML was probably HTML5. That’s been a while back now. It’s definitely change that I can keep on top of.

Then we have CSS, the presentation layer. That feels like it’s been moving at a nice clip lately. I feel like we’ve been getting a lot of cool stuff in CSS, like Flexbox and Grid, and all this new stuff that browsers are shipping. Still, I feel like, yeah, yeah, this is good. It’s right that we get lots of CSS pretty rapidly. It’s not completely overwhelming.

Then there’s the JavaScript ecosystem. I specifically say the “JavaScript ecosystem” as opposed to the “JavaScript language” because the JavaScript language is being developed at a nice pace. I feel like it’s going at a good speed of standardization. But the ecosystem, the frameworks, the libraries, the build tools, all of that stuff, that feels like, “You know what? Try this. No, try that. What about this? What about that? Oh, you’re still using that framework? No, no, we stopped using that last week. Oh, you’re still using that build tool? No, no, no, that’s so … we’ve moved on.”

I find this very overwhelming. Can I get a show of hands of anybody else who feels overwhelmed by this rate of change? All right. Keep your hands up. Keep your hands up and just look around. I want you to see you are not alone. You are not alone.

But I tell you what; after mapping these layers out into the pace layer diagram, I realized, wait a minute. The JavaScript layer, the fashion layer, if you will, it’s supposed to be like that. It’s supposed to be trying stuff out. Throw this at the wall. No, throw that at the wall. How about this? How about that?

It’s true that the good stuff does stick. Like if I think back to the first uses of JavaScript—okay, I’m showing my age, but—when JavaScript first came along, we’d use it for things like image rollovers or form validation, right? These days, if I wanted to do an image rollover—you mouseover something and it changes its appearance—I wouldn’t use JavaScript. I’d use CSS because we’ve got :hover.

If I was doing a form validation, like, “Oh, has that field actually been filled in?” because it’s required and, “Does that field actually look like an email address?” because it’s supposed to be an email address, I wouldn’t even use JavaScript. I would use HTML; input type="email" required. Again, the good stuff moves down into the sort of slower layers. Fast learns; slow remembers.

The other thing I realized when I diagrammed this out was that, “Huh, this kind of maps to how I approach building on the web.” I pretty much take this for granted that it’s going to be on the Internet. There’s not much I can do about that. Then I start thinking about URLs like URL-first design, the information architecture of a site. I think it’s underrated. I think people should create a URL-first design. URL design, in general, I think it’s a really good place to start if you’re building a product or a service.

Then, about your content in terms of structure. What is the most important thing on this page? That should be an H1. Is this a paragraph? Is it a list? Thinking about the structure first and then going on to think about the appearance which is definitely the way you want to go if you’re making something responsive. Think about the structure first and then the appearance and all these different form factors.

Then finally, add in behavior with JavaScript. Whatever HTML and CSS can’t do, that’s what I will use JavaScript for to kind of enhance it from there.

This maps really nicely to how I personally approach building things on the web. But, it is a testament to the flexibility of the World Wide Web that, if you don’t want to build in this way, you don’t have to.

If you want, you could build like this. JavaScript is a really powerful language. If you wanted to do navigations and routing in JavaScript, you can. If you want to inject all your content into the page using JavaScript, you can. CSS in JS? You can. Right? I mean, this is pretty much the architecture of a single-page app. It’s on the internet and everything is in JavaScript. The internet is a delivery mechanism for a chunk of JavaScript that does everything: the markup, the CSS, the routing.

This isn’t how I approach building on the web. I kind of ask myself why this doesn’t feel quite right to me. I think it’s because of the way it kind of turns everything into a single point of failure, which is the JavaScript, rather than spreading out those points of failure. We’re on the Internet and, as long as the JavaScript runs okay, the user gets everything. It turns what you’re building into a binary proposition that either it doesn’t work at all or it works great. Those are your own two options.

Now I’ll point out that, in another medium, this would make complete sense. Like if you’re building a native app. If you build an iOS app and I’ve got an iOS device, I get 100% of what you’ve designed and built. But if you’re building an iOS app and I have an Android device, I get zero percent of what you’ve designed and built because you can’t install an iOS app on an Android device. Either it works great or it doesn’t work at all; 100% or zero percent.

The web doesn’t have to be like that. If you build in that layered way on the web, then maybe I don’t get 100% of what you’ve designed and built but I don’t get zero percent, either. I’ll get something somewhere along the way, hopefully, closer to working great. It goes from not working at all to just about working. It works fine and works well; it works great.

You’re building up these layers of experience, the idea being that nobody gets left behind. Everybody gets something regardless of their device, their network, their browser. Everyone is not going to get the same experience, but everybody gets something. That feels very true to the original sort of founding ideas of the web and it maps so nicely to our technical stack on the web, the fact that you can start to think about things like URLs-first and think about the structure, then the presentation, and then the behavior.

I’m not the only one who likes thinking in this layered kind of way when it comes to the web. I’m going to quote my friend Ethan Marcotte. He says:

I like designing in layers. I love looking at the design of a page, a pattern, whatever, and thinking about how it will change if, say, fonts aren’t available, or JavaScript doesn’t work, or someone doesn’t see the design as you or I might and is having the page read allowed to them.

That’s a really good point that when you build in the layered way, you’re building in the resilience that something can fall back to a layer a little further down.

This brings up something I’ve mentioned here before at Beyond Tellerrand, which is that, when we’re evaluating technologies, the question we tend to ask is, how well does it work? That’s an absolutely valid question. You’re about to try a new tool, a new framework, a new standard. You ask yourself; how well does it work?

I think the more important question to ask is:

How well does it fail?

What happens if that piece of technology fails? That’s why I like this layered approach because this fails really well. JavaScript’s no longer a single point of failure. Neither is CSS, frankly. If the CSS never loaded, the user still gets something.

Now, this brings up an idea, a principle that definitely influenced Tim Berners-Lee. It was at the heart of his design principles for the World Wide Web. It’s called the Principle of Least Power that states, “Choose the least powerful language suitable for a given purpose,” which sounds really counterintuitive. Why would I choose the least powerful language to do something?

It’s kind of down to the fact that there’s a trade-off. With power, you get a fragility, right? Maybe something that is really powerful isn’t as universal as something simpler. It makes sense to figure out the simplest technology you can use to achieve a task.

I’ll give an example from my friend Derek Featherstone. He says:

In the web front-end stack—HTML, CSS, JavaScript, and ARIA—if you can solve a problem with a simpler solution lower in the stack, you should. It’s less fragile, more foolproof, and just works.

Again, he’s talking about the resilience you get by building in a layered way and choosing the least powerful technology.

It’s like a classic example being ARIA. The first rule of ARIA is, don’t use ARIA if you don’t have to. Rather than using a div and then adding the event handlers and the ARIA roles to make it look like a button, just use a button. Use the simpler technology lower in the stack.

Now, I get pushback on this because people will tell me, “Well, that’s fine if you’re building something simple, but I’m not building something simple. I’m building something complex.” Everyone likes to think they’re building something complex, right? Everyone is convinced they’re working on really hard things, which makes sense. That’s human nature.

If you’re at a cocktail party and someone says, “What do you do?” and you describe your work and they say, “Oh, okay. That sounds really easy,” you’d be offended, right?

If you’re at a party and someone says, “What do you do?” you describe your work and they go, “Wow, God, that sounds hard,” you’d be like, “Yeah! Yeah, it is hard. What I do is hard.”

I think we gravitate to this, especially when someone markets it as, “This is a serious tool for serious, complex sites.” I’m like, “That’s me. I’m working on a serious, complex site.”

I don’t think the reality is quite like that. Reality is just messier. There’s nothing quite that simple. Very few things are really that complex.

Everything kind of exists on this continuum somewhere along the way. Even the simplest website has some form of interaction, something appy about it.

Those are those other two terms people use when talking about simple and complex is website and web app, as if you can divide the entirety of the whole World Wide Web into two categories: websites and web apps.

Again, that just doesn’t make sense to me. I think the truth is, things are messier and schmooshier between this continuum of websites and web apps. I don’t get why we even need the separate word. It’s all web stuff.

Though, there is this newer term, “Progressive Web App,” that I kind of like.

Who has heard of Progressive Web Apps? All right.

Who thinks they have a good handle on what a Progressive Web App is?

Okay. See, that’s a lot fewer hands, which is totally understandable because, if you start googling, “What is a Progressive Web App?” you get these Zen-like articles. “It’s a state of mind.” “It’s about rich, native-like interactions, man.”

No. No, it’s not. Worse still, you read, “Oh, a Progressive Web App is a Single PageApp.” I was like, no, you’ve lost me there. No, it’s not. Or least it can be, but any website can be a Progressive Web App. You can elevate a website to be a Progressive Web App.

I don’t mean in some sort of Zen-like fashion. I mean using technologies, three particular technologies.

  1. You make sure that website is running HTTPS,
  2. you have a web app manifest that’s a JSON file with metadata, and
  3. you have a service worker that gets installed on the user’s device.

That’s it. These three technologies turn a website into a Progressive Web App — no mystery about it.

The tricky bit is that service worker part. It’s kind of a weird thing because it’s JavaScript but it’s JavaScript that gets installed on the user’s device and acts like a proxy. It intercepts network requests and can do different things like grab things from the cache instead of going to the network.

I’m not going to go into how it works because I’ve written plenty about that in this book “Going Offline,” so if you want to know the code, you can go read the book.

I will say that when I first came across service workers, it totally did my head in because this is my mental model of the web. We’ve got the stack of technologies that we’re building on top of, each layer depending on the layer below. Then service workers come along and say, “Well, actually, you could have a website like this,” where the lowest layer, the network, the Internet goes away and the website still works. Mind is blown!

It took me a while to get my head around that. The service worker file is on the user’s device and, if they’ve got no Internet connect, it can still make decisions and serve up something like a custom offline page.

Here’s a website I run called huffduffer.com. It’s for making your own podcast out of found sounds. If you’re offline or the website is down, which happens, and you visit Huffduffer, you get this offline page saying, “Sorry, you’re offline.” Not very useful, but it’s branded like the site, okay? It’s almost like the way you have a custom 404 page. Now you can have a custom offline page that matches your site. It’s a small thing, but it can be handy.

We ran this conference in Brighton for two years, Ampersand. It’s a web typography conference. That also has a very simple offline page that just says, “Sorry. You’re offline,” but then it has the bare minimum information you need about the conference like where is this conference happening; what time does it start?

You can imagine a restaurant website having this, an offline page that tells you, “Here is the address. Here are the opening hours.” I would like it if restaurant websites had that information when you’re online as well, but…

You can also have fun with this, like Trivago. Their site relies on search, so there’s not much you can do when you’re offline, so they give you a game to play, the offline maze to keep you entertained.

That’s kind of at the simplest level of what you could do, a custom offline page.

Then at the other level, I’ve written this book called “Resilient web Design.” A lot of the ideas I’m talking about here are in this book. The book is a website. You go to the website and you read the book. That’s it. It’s free. You just go to resilientwebdesign.com. I mean free. I don’t ask for your email address and I’m not tracking any information at all.

This is how it looks when you’re online, and then this is how it looks when you’re offline. It is exactly the same. In fact, the moment you visit the website, it basically downloads the whole book.

Now, that’s the extreme example. Most websites, you wouldn’t want to do that because you kind of want the HTML to be fresh. This is never going to get updated. I’m done with this so I’m totally fine with, you go straight to the cache; never even go to the network. It’s absolutely offline first. You’re probably going to want something in between those two extremes.

On my own website, adactio.com, if you’re browsing around the website and you’re reading things, that’s all fine. Then what if you lose your internet connection? You get the custom offline page that says, “Sorry, you’re offline,” but then it also shows you the things you’ve previously visited.

You can revisit any of these pages. These have all been cached, so you can cache things as people are browsing around the site. That’s a nice little pattern that a lot of websites could benefit from. It only suffers from the fact that all I can show you is stuff you’ve already seen. You have to have already visited these pages for them to show up in this list. Another pattern that I think is maybe better from a user experience point is when you put the control in the hands of the user.

This website, archive.dconstruct.org, this is what it sounds like. It’s an archive. It’s conference talks.

We ran a conference called dConstruct for 10 years from 2005 to 2015. Breaking news; we’re bringing it back for a one-off event next year, September 2020.

Anyway, all the talks from ten years are online here as audio files. You can browse around and listen to these talks.

You’ll also see that there’s this option to save for offline, exposed on the interface. What that does it is doesn’t just save the page offline; it also saves the audio offline. Then, when you’re an airplane or at the bottom of the ocean or whatever, you can then listen to the things you explicitly asked to be saved offline. It’s effectively a podcast player in the browser.

You see there’s a lot of things you can do. There are kind of a lot of layers you can build upon once you have a service worker.

At the very least, you can do caching because that’s the stuff we do anyway, like put this file in the cache, your CSS, your JavaScript, your icons, whatever.

Then think about, well, maybe I should have a custom offline page, even if it’s just for the branding reasons of having that nice page, just like we have a custom 404 page.

Then you start thinking, well, I want the adding to home screen experience to be good, so you’ve got the web app manifest.

You implement one of those patterns there allowing the users to save things offline, maybe.

Also, push notifications are now possible thanks to service workers. It used to be, if you wanted to make someone’s life a misery, you had to build a native app to give them push notifications all day long. Now you can make someone’s life a misery on the web too.

There’s even more advanced APIs like background sync where the website can talk to the web server even when that website isn’t open in the browser and sync up information — super powerful stuff.

Now, the support for something like service worker and the cache API is almost universal at this point. The support for stuff like background sync notifications is spottier, not universal, and that’s okay because, as long as you’re adding these things in layers. Then it’s fine if something doesn’t have universal support, right? It’s making something work great but, if someone doesn’t get that, it still works good. It’s all about building in that layered way.

Now you’re probably thinking, “Ah-ha! I’ve hoisted him by his own petard because service workers use JavaScript. That means they rely on JavaScript. You’ve made JavaScript a single point of failure!” Exactly what I was complaining about with Single Page Apps, right?

There’s a difference. With a Single Page App, you’re relying on JavaScript. The user gets absolutely nothing if JavaScript doesn’t work.

In the case of service workers, you literally cannot make a website that relies on a service worker. You have to make a website that works first without a service worker and then add the service worker on top, because, think about it; the first time anybody visits the website, even if their browser supports service workers, the service worker is not installed. So you have to build in layers.

I think this is why it appeals to me so much. The design of service workers is a layered design. You have to have something that works first, and then you elevate it. You improve the user experience using these technologies but you don’t rely on it. It’s not a single point of failure. It’s an enhancement.

That means you can take any website. Somebody’s homepage; a book that’s online; this archived stuff ; something that is more appy, sure, and make it work pretty much like a native app. It can appear full screen, add to the home screen, be indistinguishable from native apps so that the latest and greatest browsers and devices get the best experience. They’re making full use of the newest technologies.

But, as well as these things working in the latest and greatest browsers, they still work in the first web browser ever created. You can still look at these things in that very first web browser that Tim Berners-Lee created at worldwideweb.cern.ch.

WorldWideWeb

It’s like it is an unbroken line over 30 years on the web. We’re not talking about the Long Now when we’re talking about 30 years but, in terms of technology, that does feel special.

You can also look at the world’s first webpage in the first-ever web browser but, almost more amazingly, you can look at the world’s first web page at its original URL in a modern web browser and it still works.

We managed to make the web so much better with new APIs, new technologies, without breaking it, without breaking that backward compatibility. There’s something special about that. There’s something special about the web if you build in layers.

I’m encouraging you to think in terms of layers and use the layers of the web.

Thank you.

Monday, December 16th, 2019

Liveblogging An Event Apart 2019

I was at An Event Apart in San Francisco last week. It was the last one of the year, and also my last conference of the year.

I managed to do a bit of liveblogging during the event. Combined with the liveblogging I did during the other two Events Apart that I attended this year—Seattle and Chicago—that makes a grand total of seventeen liveblogged presentations!

  1. Slow Design for an Anxious World by Jeffrey Zeldman
  2. Designing for Trust in an Uncertain World by Margot Bloomstein
  3. Designing for Personalities by Sarah Parmenter
  4. Generation Style by Eric Meyer
  5. Making Things Better: Redefining the Technical Possibilities of CSS by Rachel Andrew
  6. Designing Intrinsic Layouts by Jen Simmons
  7. How to Think Like a Front-End Developer by Chris Coyier
  8. From Ideation to Iteration: Design Thinking for Work and for Life by Una Kravets
  9. Move Fast and Don’t Break Things by Scott Jehl
  10. Mobile Planet by Luke Wroblewski
  11. Unsolved Problems by Beth Dean
  12. Making Research Count by Cyd Harrell
  13. Voice User Interface Design by Cheryl Platz
  14. Web Forms: Now You See Them, Now You Don’t! by Jason Grigsby
  15. The Weight of the WWWorld is Up to Us by Patty Toland
  16. The Mythology of Design Systems by Mina Markham
  17. The Technical Side of Design Systems by Brad Frost

For my part, I gave my talk on Going Offline. Time to retire that talk now.

Here’s what I wrote when I first gave the talk back in March at An Event Apart Seattle:

I was quite nervous about this talk. It’s very different from my usual fare. Usually I have some big sweeping arc of history, and lots of pretentious ideas joined together into some kind of narrative arc. But this talk needed to be more straightforward and practical. I wasn’t sure how well I would manage that brief.

I’m happy with how it turned out. I had quite a few people come up to me to say how much they appreciated how I was explaining the code. That was very nice to hear—I really wanted this talk to be approachable for everyone, even though it included plenty of JavaScript.

The dates for next year’s Events Apart have been announced, and I’ll be speaking at three of them:

The question is, do I attempt to deliver another practical code-based talk or do I go back to giving a high-level talk about ideas and principles? Or, if I really want to challenge myself, can I combine the two into one talk without making a Frankenstein’s monster?

Come and see me at An Event Apart in 2020 to find out.

Thursday, November 21st, 2019

Request with Intent: Caching Strategies in the Age of PWAs – A List Apart

Aaron outlines some sensible strategies for serving up images, including using the Cache API from your service worker script.

Friday, October 25th, 2019

Offline Page Descriptions | Erik Runyon

Here’s a nice example of showing pages offline. It’s subtly different from what I’m doing on my own site, which goes to show that there’s no one-size-fits-all recipe when it comes to offline strategies.

Monday, October 14th, 2019

Paris Web 2019 - 10 octobre après-midi - Amphithéâtre - YouTube

Here’s the livestream of the talk I gave at Paris Web—Going Offline, complete with French live-captioning and simultaneous interpretation in .

Thursday, October 3rd, 2019

Blog service workers and the chicken and the egg

This is a great little technique from Remy: when a service worker is being installed, you make sure that the page(s) the user is first visiting get added to a cache.

Saturday, September 21st, 2019

Going offline with microformats

For the offline page on my website, I’ve been using a mixture of the Cache API and the localStorage API. My service worker script uses the Cache API to store copies of pages for offline retrieval. But I used the localStorage API to store metadata about the page—title, description, and so on. Then, my offline page would rifle through the pages stored in a cache, and retreive the corresponding metadata from localStorage.

It all worked fine, but as soon as I read Remy’s post about the forehead-slappingly brilliant technique he’s using, I knew I’d be switching my code over. Instead of using localStorage—or any other browser API—to store and retrieve metadata, he uses the pages themselves! Using the Cache API, you can examine the contents of the pages you’ve stored, and get at whatever information you need:

I realised I didn’t need to store anything. HTML is the API.

Refactoring the code for my offline page felt good for a couple of reasons. First of all, I was able to remove a dependency—localStorage—and simplify the JavaScript. That always feels good. But the other reason for the warm fuzzies is that I was able to use data instead of metadata.

Many years ago, Cory Doctorow wrote a piece called Metacrap. In it, he enumerates the many issues with metadata—data about data. The source of many problems is when the metadata is stored separately from the data it describes. The data may get updated, without a corresponding update happening to the metadata. Metadata tends to rot because it’s invisible—out of sight and out of mind.

In fact, that’s always been at the heart of one of the core principles behind microformats. Instead of duplicating information—once as data and again as metadata—repurpose the visible data; mark it up so its meta-information is directly attached to the information itself.

So if you have a person’s contact details on a web page, rather than repeating that information somewhere else—in the head of the document, say—you could instead attach some kind of marker to indicate which bits of the visible information are contact details. In the case of microformats, that’s done with class attributes. You can mark up a page that already has your contact information with classes from the h-card microformat.

Here on my website, I’ve marked up my blog posts, articles, and links using the h-entry microformat. These classes explicitly mark up the content to say “this is the title”, “this is the content”, and so on. This makes it easier for other people to repurpose my content. If, for example, I reply to a post on someone else’s website, and ping them with a webmention, they can retrieve my post and know which bit is the title, which bit is the content, and so on.

When I read Remy’s post about using the Cache API to retrieve information directly from cached pages, I knew I wouldn’t have to do much work. Because all of my posts are already marked up with h-entry classes, I could use those hooks to create a nice offline page.

The markup for my offline page looks like this:

<h1>Offline</h1>
<p>Sorry. It looks like the network connection isn’t working right now.</p>
<div id="history">
</div>

I’ll populate that “history” div with information from a cache called “pages” that I’ve created using the Cache API in my service worker.

I’m going to use async/await to do this because there are lots of steps that rely on the completion of the step before. “Open this cache, then get the keys of that cache, then loop through the pages, then…” All of those thens would lead to some serious indentation without async/await.

All async functions have to have a name—no anonymous async functions allowed. I’m calling this one listPages, just like Remy is doing. I’m making the listPages function execute immediately:

(async function listPages() {
...
})();

Now for the code to go inside that immediately-invoked function.

I create an array called browsingHistory that I’ll populate with the data I’ll use for that “history” div.

const browsingHistory = [];

I’m going to be parsing web pages later on, so I’m going to need a DOM parser. I give it the imaginative name of …parser.

const parser = new DOMParser();

Time to open up my “pages” cache. This is the first await statement. When the cache is opened, this promise will resolve and I’ll have access to this cache using the variable …cache (again with the imaginative naming).

const cache = await caches.open('pages');

Now I get the keys of the cache—that’s a list of all the page requests in there. This is the second await. Once the keys have been retrieved, I’ll have a variable that’s got a list of all those pages. You’ll never guess what I’m calling the variable that stores the keys of the cache. That’s right …keys!

const keys = await cache.keys();

Time to get looping. I’m getting each request in the list of keys using a for/of loop:

for (const request of keys) {
...
}

Inside the loop, I pull the page out of the cache using the match() method of the Cache API. I’ll store what I get back in a variable called response. As with everything involving the Cache API, this is asynchronous so I need to use the await keyword here.

const response = await cache.match(request);

I’m not interested in the headers of the response. I’m specifically looking for the HTML itself. I can get at that using the text() method. Again, it’s asynchronous and I want this promise to resolve before doing anything else, so I use the await keyword. When the promise resolves, I’ll have a variable called html that contains the body of the response.

const html = await response.text();

Now I can use that DOM parser I created earlier. I’ve got a string of text in the html variable. I can generate a Document Object Model from that string using the parseFromString() method. This isn’t asynchronous so there’s no need for the await keyword.

const dom = parser.parseFromString(html, 'text/html');

Now I’ve got a DOM, which I have creatively stored in a variable called …dom.

I can poke at it using DOM methods like querySelector. I can test to see if this particular page has an h-entry on it by looking for an element with a class attribute containing the value “h-entry”:

if (dom.querySelector('.h-entry h1.p-name') {
...
}

In this particular case, I’m also checking to see if the h1 element of the page is the title of the h-entry. That’s so that index pages (like my home page) won’t get past this if statement.

Inside the if statement, I’m going to store the data I retrieve from the DOM. I’ll save the data into an object called …data!

const data = new Object;

Well, the first piece of data isn’t actually in the markup: it’s the URL of the page. I can get that from the request variable in my for loop.

data.url = request.url;

I’m going to store the timestamp for this h-entry. I can get that from the datetime attribute of the time element marked up with a class of dt-published.

data.timestamp = new Date(dom.querySelector('.h-entry .dt-published').getAttribute('datetime'));

While I’m at it, I’m going to grab the human-readable date from the innerText property of that same time.dt-published element.

data.published = dom.querySelector('.h-entry .dt-published').innerText;

The title of the h-entry is in the innerText of the element with a class of p-name.

data.title = dom.querySelector('.h-entry .p-name').innerText;

At this point, I am actually going to use some metacrap instead of the visible h-entry content. I don’t output a description of the post anywhere in the body of the page, but I do put it in the head in a meta element. I’ll grab that now.

data.description = dom.querySelector('meta[name="description"]').getAttribute('content');

Alright. I’ve got a URL, a timestamp, a publication date, a title, and a description, all retrieved from the HTML. I’ll stick all of that data into my browsingHistory array.

browsingHistory.push(data);

My if statement and my for/in loop are finished at this point. Here’s how the whole loop looks:

for (const request of keys) {
  const response = await cache.match(request);
  const html = await response.text();
  const dom = parser.parseFromString(html, 'text/html');
  if (dom.querySelector('.h-entry h1.p-name')) {
    const data = new Object;
    data.url = request.url;
    data.timestamp = new Date(dom.querySelector('.h-entry .dt-published').getAttribute('datetime'));
    data.published = dom.querySelector('.h-entry .dt-published').innerText;
    data.title = dom.querySelector('.h-entry .p-name').innerText;
    data.description = dom.querySelector('meta[name="description"]').getAttribute('content');
    browsingHistory.push(data);
  }
}

That’s the data collection part of the code. Now I’m going to take all that yummy information an output it onto the page.

First of all, I want to make sure that the browsingHistory array isn’t empty. There’s no point going any further if it is.

if (browsingHistory.length) {
...
}

Within this if statement, I can do what I want with the data I’ve put into the browsingHistory array.

I’m going to arrange the data by date published. I’m not sure if this is the right thing to do. Maybe it makes more sense to show the pages in the order in which you last visited them. I may end up removing this at some point, but for now, here’s how I sort the browsingHistory array according to the timestamp property of each item within it:

browsingHistory.sort( (a,b) => {
  return b.timestamp - a.timestamp;
});

Now I’m going to concatenate some strings. This is the string of HTML text that will eventually be put into the “history” div. I’m storing the markup in a string called …markup (my imagination knows no bounds).

let markup = '<p>But you still have something to read:</p>';

I’m going to add a chunk of markup for each item of data.

browsingHistory.forEach( data => {
  markup += `
<h2><a href="${ data.url }">${ data.title }</a></h2>
<p>${ data.description }</p>
<p class="meta">${ data.published }</p>
`;
});

With my markup assembled, I can now insert it into the “history” part of my offline page. I’m using the handy insertAdjacentHTML() method to do this.

document.getElementById('history').insertAdjacentHTML('beforeend', markup);

Here’s what my finished JavaScript looks like:

<script>
(async function listPages() {
  const browsingHistory = [];
  const parser = new DOMParser();
  const cache = await caches.open('pages');
  const keys = await cache.keys();
  for (const request of keys) {
    const response = await cache.match(request);
    const html = await response.text();
    const dom = parser.parseFromString(html, 'text/html');
    if (dom.querySelector('.h-entry h1.p-name')) {
      const data = new Object;
      data.url = request.url;
      data.timestamp = new Date(dom.querySelector('.h-entry .dt-published').getAttribute('datetime'));
      data.published = dom.querySelector('.h-entry .dt-published').innerText;
      data.title = dom.querySelector('.h-entry .p-name').innerText;
      data.description = dom.querySelector('meta[name="description"]').getAttribute('content');
      browsingHistory.push(data);
    }
  }
  if (browsingHistory.length) {
    browsingHistory.sort( (a,b) => {
      return b.timestamp - a.timestamp;
    });
    let markup = '<p>But you still have something to read:</p>';
    browsingHistory.forEach( data => {
      markup += `
<h2><a href="${ data.url }">${ data.title }</a></h2>
<p>${ data.description }</p>
<p class="meta">${ data.published }</p>
`;
    });
    document.getElementById('history').insertAdjacentHTML('beforeend', markup);
  }
})();
</script>

I’m pretty happy with that. It’s not too long but it’s still quite readable (I hope). It shows that the Cache API and the h-entry microformat are a match made in heaven.

If you’ve got an offline strategy for your website, and you’re using h-entry to mark up your content, feel free to use that code.

If you don’t have an offline strategy for your website, there’s a book for that.

Thursday, August 1st, 2019

Navigation preloads in service workers

There’s a feature in service workers called navigation preloads. It’s relatively recent, so it isn’t supported in every browser, but it’s still well worth using.

Here’s the problem it solves…

If someone makes a return visit to your site, and the service worker you installed on their machine isn’t active yet, the service worker boots up, and then executes its instructions. If those instructions say “fetch the page from the network”, then you’re basically telling the browser to do what it would’ve done anyway if there were no service worker installed. The only difference is that there’s been a slight delay because the service worker had to boot up first.

  1. The service worker activates.
  2. The service worker fetches the file.
  3. The service worker does something with the response.

It’s not a massive performance hit, but it’s still a bit annoying. It would be better if the service worker could boot up and still be requesting the page at the same time, like it would do if no service worker were present. That’s where navigation preloads come in.

  1. The service worker activates while simultaneously requesting the file.
  2. The service worker does something with the response.

Navigation preloads—like the name suggests—are only initiated when someone navigates to a URL on your site, either by following a link, or a bookmark, or by typing a URL directly into a browser. Navigation preloads don’t apply to requests made by a web page for things like images, style sheets, and scripts. By the time a request is made for one of those, the service worker is already up and running.

To enable navigation preloads, call the enable() method on registration.navigationPreload during the activate event in your service worker script. But first do a little feature detection to make sure registration.navigationPreload exists in this browser:

if (registration.navigationPreload) {
  addEventListener('activate', activateEvent => {
    activateEvent.waitUntil(
      registration.navigationPreload.enable()
    );
  });
}

If you’ve already got event listeners on the activate event, that’s absolutely fine: addEventListener isn’t exclusive—you can use it to assign multiple tasks to the same event.

Now you need to make use of navigation preloads when you’re responding to fetch events. So if your strategy is to look in the cache first, there’s probably no point enabling navigation preloads. But if your default strategy is to fetch a page from the network, this will help.

Let’s say your current strategy for handling page requests looks like this:

addEventListener('fetch', fetchEvent => {
  const request = fetchEvent.request;
  if (request.headers.get('Accept').includes('text/html')) {
    fetchEvent.respondWith(
      fetch(request)
      .then( responseFromFetch => {
        // maybe cache this response for later here.
        return responseFromFetch;
      })
      .catch( fetchError => {
        return caches.match(request)
        .then( responseFromCache => {
          return responseFromCache || caches.match('/offline');
        });
      })
    );
  }
});

That’s a fairly standard strategy: try the network first; if that doesn’t work, try the cache; as a last resort, show an offline page.

It’s that first step (“try the network first”) that can benefit from navigation preloads. If a preload request is already in flight, you’ll want to use that instead of firing off a new fetch request. Otherwise you’re making two requests for the same file.

To find out if a preload request is underway, you can check for the existence of the preloadResponse promise, which will be made available as a property of the fetch event you’re handling:

fetchEvent.preloadResponse

If that exists, you’ll want to use it instead of fetch(request).

if (fetchEvent.preloadResponse) {
  // do something with fetchEvent.preloadResponse
} else {
  // do something with fetch(request)
}

You could structure your code like this:

addEventListener('fetch', fetchEvent => {
  const request = fetchEvent.request;
  if (request.headers.get('Accept').includes('text/html')) {
    if (fetchEvent.preloadResponse) {
      fetchEvent.respondWith(
        fetchEvent.preloadResponse
        .then( responseFromPreload => {
          // maybe cache this response for later here.
          return responseFromPreload;
        })
        .catch( preloadError => {
          return caches.match(request)
          .then( responseFromCache => {
            return responseFromCache || caches.match('/offline');
          });
        })
      );
    } else {
      fetchEvent.respondWith(
        fetch(request)
        .then( responseFromFetch => {
          // maybe cache this response for later here.
          return responseFromFetch;
        })
        .catch( fetchError => {
          return caches.match(request)
          .then( responseFromCache => {
            return responseFromCache || caches.match('/offline');
          });
        })
      );
    }
  }
});

But that’s not very DRY. Your logic is identical, regardless of whether the response is coming from fetch(request) or from fetchEvent.preloadResponse. It would be better if you could minimise the amount of duplication.

One way of doing that is to abstract away the promise you’re going to use into a variable. Let’s call it retrieve. If a preload is underway, we’ll assign it to that variable:

let retrieve;
if (fetchEvent.preloadResponse) {
  retrieve = fetchEvent.preloadResponse;
}

If there is no preload happening (or this browser doesn’t support it), assign a regular fetch request to the retrieve variable:

let retrieve;
if (fetchEvent.preloadResponse) {
  retrieve = fetchEvent.preloadResponse;
} else {
  retrieve = fetch(request);
}

If you like, you can squash that into a ternary operator:

const retrieve = fetchEvent.preloadResponse ? fetchEvent.preloadResponse : fetch(request);

Use whichever syntax you find more readable.

Now you can apply the same logic, regardless of whether retrieve is a preload navigation or a fetch request:

addEventListener('fetch', fetchEvent => {
  const request = fetchEvent.request;
  if (request.headers.get('Accept').includes('text/html')) {
    const retrieve = fetchEvent.preloadResponse ? fetchEvent.preloadResponse : fetch(request);
    fetchEvent.respondWith(
      retrieve
      .then( responseFromRetrieve => {
        // maybe cache this response for later here.
       return responseFromRetrieve;
      })
      .catch( fetchError => {
        return caches.match(request)
        .then( responseFromCache => {
          return responseFromCache || caches.match('/offline');
        });
      })
    );
  }
});

I think that’s the least invasive way to update your existing service worker script to take advantage of navigation preloads.

Like I said, preload navigations can give a bit of a performance boost if you’re using a network-first strategy. That’s what I’m doing here on adactio.com and on thesession.org so I’ve updated their service workers to take advantage of navigation preloads. But on Resilient Web Design, which uses a cache-first strategy, there wouldn’t be much point enabling navigation preloads.

Jeff Posnick made this point in his write-up of bringing service workers to Google search:

Adding a service worker to your web app means inserting an additional piece of JavaScript that needs to be loaded and executed before your web app gets responses to its requests. If those responses end up coming from a local cache rather than from the network, then the overhead of running the service worker is usually negligible in comparison to the performance win from going cache-first. But if you know that your service worker always has to consult the network when handling navigation requests, using navigation preload is a crucial performance win.

Oh, and those browsers that don’t yet support navigation preloads? No problem. It’s a progressive enhancement. Everything still works just like it did before. And having a service worker on your site in the first place is itself a progressive enhancement. So enabling navigation preloads is like a progressive enhancement within a progressive enhancement. It’s progressive enhancements all the way down!

By the way, if all of this service worker stuff sounds like gibberish, but you wish you understood it, I think my book, Going Offline, will prove quite valuable.

Tuesday, July 2nd, 2019

The trimCache function in Going Offline …again

It seems that some code that I wrote in Going Offline is haunted. It’s the trimCache function.

First, there was the issue of a typo. Or maybe it’s more of a brainfart than a typo, but either way, there’s a mistake in the syntax that was published in the book.

Now it turns out that there’s also a problem with my logic.

To recap, this is a function that takes two arguments: the name of a cache, and the maximum number of items that cache should hold.

function trimCache(cacheName, maxItems) {

First, we open up the cache:

caches.open(cacheName)
.then( cache => {

Then, we get the items (keys) in that cache:

cache.keys()
.then(keys => {

Now we compare the number of items (keys.length) to the maximum number of items allowed:

if (keys.length > maxItems) {

If there are too many items, delete the first item in the cache—that should be the oldest item:

cache.delete(keys[0])

And then run the function again:

.then(
    trimCache(cacheName, maxItems)
);

A-ha! See the problem?

Neither did I.

It turns out that, even though I’m using then, the function will be invoked immediately, instead of waiting until the first item has been deleted.

Trys helped me understand what was going on by making a useful analogy. You know when you use setTimeout, you can’t put a function—complete with parentheses—as the first argument?

window.setTimeout(doSomething(someValue), 1000);

In that example, doSomething(someValue) will be invoked immediately—not after 1000 milliseconds. Instead, you need to create an anonymous function like this:

window.setTimeout( function() {
    doSomething(someValue)
}, 1000);

Well, it’s the same in my trimCache function. Instead of this:

cache.delete(keys[0])
.then(
    trimCache(cacheName, maxItems)
);

I need to do this:

cache.delete(keys[0])
.then( function() {
    trimCache(cacheName, maxItems)
});

Or, if you prefer the more modern arrow function syntax:

cache.delete(keys[0])
.then( () => {
    trimCache(cacheName, maxItems)
});

Either way, I have to wrap the recursive function call in an anonymous function.

Here’s a gist with the updated trimCache function.

What’s annoying is that this mistake wasn’t throwing an error. Instead, it was causing a performance problem. I’m using this pattern right here on my own site, and whenever my cache of pages or images gets too big, the trimCaches function would get called …and then wouldn’t stop running.

I’m very glad that—witht the help of Trys at last week’s Homebrew Website Club Brighton—I was finally able to get to the bottom of this. If you’re using the trimCache function in your service worker, please update the code accordingly.

Management regrets the error.

Friday, June 7th, 2019

Jeremy Keith: Going offline - YouTube

Here’s the opening keynote I gave at Frontend United in Utrecht a few weeks back.

Wednesday, May 8th, 2019

Timing out

Service workers are great for creating a good user experience when someone is offline. Heck, the book I wrote about service workers is literally called Going Offline.

But in some ways, the offline experience is relatively easy to handle. It’s a binary situation; either you’re online or you’re offline. What’s more challenging—and probably more common—is the situation that Jake calls Lie-Fi. That’s when technically you’ve got a network connection …but it’s a shitty connection, like one bar of mobile signal. In that situation, because there’s technically a connection, the user gets a slow frustrating experience. Whatever code you’ve got in your service worker for handling offline situations will never get triggered. When you’re handling fetch events inside a service worker, there’s no automatic time-out.

But you can make one.

That’s what I’ve done recently here on adactio.com. Before showing you what I added to my service worker script to make that happen, let me walk you through my existing strategy for handling offline situations.

Service worker strategies

Alright, so in my service worker script, I’ve got a block of code for handling requests from fetch events:

addEventListener('fetch', fetchEvent => {
        const request = fetchEvent.request;
    // Do something with this request.
});

I’ve got two strategies in my code. One is for dealing with requests for pages:

if (request.headers.get('Accept').includes('text/html')) {
    // Code for handling page requests.
}

By adding an else clause I can have a different strategy for dealing with requests for anything else—images, style sheets, scripts, and so on:

if (request.headers.get('Accept').includes('text/html')) {
    // Code for handling page requests.
} else {
    // Code for handling everthing else.
}

For page requests, I’m going to try to go the network first:

fetchEvent.respondWith(
    fetch(request)
    .then( responseFromFetch => {
        return responseFromFetch;
    })

My logic is:

When someone requests a page, try to fetch it from the network.

If that doesn’t work, we’re in an offline situation. That triggers the catch clause. That’s where I have my offline strategy: show a custom offline page that I’ve previously cached (during the install event):

.catch( fetchError => {
    return caches.match('/offline');
})

Now my logic has been expanded to this:

When someone requests a page, try to fetch it from the network, but if that doesn’t work, show a custom offline page instead.

So my overall code for dealing with requests for pages looks like this:

if (request.headers.get('Accept').includes('text/html')) {
    fetchEvent.respondWith(
        fetch(request)
        .then( responseFromFetch => {
            return responseFromFetch;
        })
        .catch( fetchError => {
            return caches.match('/offline');
        })
    );
}

Now I can fill in the else statement that handles everything else—images, style sheets, scripts, and so on. Here my strategy is different. I’m looking in my caches first, and I only fetch the file from network if the file can’t be found in any cache:

caches.match(request)
.then( responseFromCache => {
    return responseFromCache || fetch(request);
})

Here’s all that fetch-handling code put together:

addEventListener('fetch', fetchEvent => {
    const request = fetchEvent.request;
    if (request.headers.get('Accept').includes('text/html')) {
        fetchEvent.respondWith(
            fetch(request)
            .then( responseFromFetch => {
                return responseFromFetch;
            })
            .catch( fetchError => {
                return caches.match('/offline');
            })
        );
    } else {
        caches.match(request)
        .then( responseFromCache => {
            return responseFromCache || fetch(request);
        })
    }
});

Good.

Cache as you go

Now I want to introduce an extra step in the part of the code where I deal with requests for pages. Whenever I fetch a page from the network, I’m going to take the opportunity to squirrel it away in a cache. I’m calling that cache “pages”. I’m imaginative like that.

fetchEvent.respondWith(
    fetch(request)
    .then( responseFromFetch => {
        const copy = responseFromFetch.clone();
        try {
            fetchEvent.waitUntil(
                caches.open('pages')
                .then( pagesCache => {
                    return pagesCache.put(request, copy);
                })
            )
        } catch(error) {
            console.error(error);
        }
        return responseFromFetch;
    })

You’ll notice that I can’t put the response itself (responseFromCache) into the cache. That’s a stream that I only get to use once. Instead I need to make a copy:

const copy = responseFromFetch.clone();

That’s what gets put in the pages cache:

fetchEvent.waitUntil(
    caches.open('pages')
    .then( pagesCache => {
        return pagesCache.put(request, copy);
    })
)

Now my logic for page requests has an extra piece to it:

When someone requests a page, try to fetch it from the network and store a copy in a cache, but if that doesn’t work, show a custom offline page instead.

Here’s my updated fetch-handling code:

addEventListener('fetch', fetchEvent => {
    const request = fetchEvent.request;
    if (request.headers.get('Accept').includes('text/html')) {
        fetchEvent.respondWith(
            fetch(request)
            .then( responseFromFetch => {
                const copy = responseFromFetch.clone();
                try {
                    fetchEvent.waitUntil(
                        caches.open('pages')
                        .then( pagesCache => {
                            return pagesCache.put(request, copy);
                        })
                    )
                } catch(error) {
                    console.error(error);
                }
                return responseFromFetch;
            })
            .catch( fetchError => {
                return caches.match('/offline');
            })
        );
    } else {
        caches.match(request)
        .then( responseFromCache => {
            return responseFromCache || fetch(request);
        })
    }
});

I call this the cache-as-you-go pattern. The more pages someone views on my site, the more pages they’ll have cached.

Now that there’s an ever-growing cache of previously visited pages, I can update my offline fallback. Currently, I reach straight for the custom offline page:

.catch( fetchError => {
    return caches.match('/offline');
})

But now I can try looking for a cached copy of the requested page first:

.catch( fetchError => {
    caches.match(request)
    .then( responseFromCache => {
        return responseFromCache || caches.match('/offline');
    })
});

Now my offline logic is expanded:

When someone requests a page, try to fetch it from the network and store a copy in a cache, but if that doesn’t work, first look for an existing copy in a cache, and otherwise show a custom offline page instead.

I can also access this ever-growing cache of pages from my custom offline page to show people which pages they can revisit, even if there’s no internet connection.

So far, so good. Everything I’ve outlined so far is a good robust strategy for handling offline situations. Now I’m going to deal with the lie-fi situation, and it’s that cache-as-you-go strategy that sets me up nicely.

Timing out

I want to throw this addition into my logic:

When someone requests a page, try to fetch it from the network and store a copy in a cache, but if that doesn’t work, first look for an existing copy in a cache, and otherwise show a custom offline page instead (but if the request is taking too long, try to show a cached version of the page).

The first thing I’m going to do is rewrite my code a bit. If the fetch event is for a page, I’m going to respond with a promise:

if (request.headers.get('Accept').includes('text/html')) {
    fetchEvent.respondWith(
        new Promise( resolveWithResponse => {
            // Code for handling page requests.
        })
    );
}

Promises are kind of weird things to get your head around. They’re tailor-made for doing things asynchronously. You can set up two parameters; a success condition and a failure condition. If the success condition is executed, then we say the promise has resolved. If the failure condition is executed, then the promise rejects.

In my re-written code, I’m calling the success condition resolveWithResponse (and I haven’t bothered with a failure condition, tsk, tsk). I’m going to use resolveWithResponse in my promise everywhere that I used to have a return statement:

addEventListener('fetch', fetchEvent => {
    const request = fetchEvent.request;
    if (request.headers.get('Accept').includes('text/html')) {
        fetchEvent.respondWith(
            new Promise( resolveWithResponse => {
                fetch(request)
                .then( responseFromFetch => {
                    const copy = responseFromFetch.clone();
                    try {
                        fetchEvent.waitUntil(
                            caches.open('pages')
                            then( pagesCache => {
                                return pagesCache.put(request, copy);
                            })
                        )
                    } catch(error) {
                        console.error(error);
                    }
                    resolveWithResponse(responseFromFetch);
                })
                .catch( fetchError => {
                    caches.match(request)
                    .then( responseFromCache => {
                        resolveWithResponse(
                            responseFromCache || caches.match('/offline')
                        );
                    })
                })
            })
        );
    } else {
        caches.match(request)
        .then( responseFromCache => {
            return responseFromCache || fetch(request);
        })
    }
});

By itself, rewriting my code as a promise doesn’t change anything. Everything’s working the same as it did before. But now I can introduce the time-out logic. I’m going to put this inside my promise:

const timer = setTimeout( () => {
    caches.match(request)
    .then( responseFromCache => {
        if (responseFromCache) {
            resolveWithResponse(responseFromCache);
        }
    })
}, 3000);

If a request takes three seconds (3000 milliseconds), then that code will execute. At that point, the promise attempts to resolve with a response from the cache instead of waiting for the network. If there is a cached response, that’s what the user now gets. If there isn’t, then the wait continues for the network.

The last thing left for me to do is cancel the countdown to timing out if a network response does return within three seconds. So I put this in the then clause that’s triggered by a successful network response:

clearTimeout(timer);

I also add the clearTimeout statement to the catch clause that handles offline situations. Here’s the final code:

addEventListener('fetch', fetchEvent => {
    const request = fetchEvent.request;
    if (request.headers.get('Accept').includes('text/html')) {
        fetchEvent.respondWith(
            new Promise( resolveWithResponse => {
                const timer = setTimeout( () => {
                    caches.match(request)
                    .then( responseFromCache => {
                        if (responseFromCache) {
                            resolveWithResponse(responseFromCache);
                        }
                    })
                }, 3000);
                fetch(request)
                .then( responseFromFetch => {
                    clearTimeout(timer);
                    const copy = responseFromFetch.clone();
                    try {
                        fetchEvent.waitUntil(
                            caches.open('pages')
                            then( pagesCache => {
                                return pagesCache.put(request, copy);
                            })
                        )
                    } catch(error) {
                        console.error(error);
                    }
                    resolveWithResponse(responseFromFetch);
                })
                .catch( fetchError => {
                    clearTimeout(timer);
                    caches.match(request)
                    .then( responseFromCache => {
                        resolveWithResponse(
                            responseFromCache || caches.match('/offline')
                        );
                    })
                })
            })
        );
    } else {
        caches.match(request)
        .then( responseFromCache => {
            return responseFromCache || fetch(request)
        })
    }
});

That’s the JavaScript translation of this logic:

When someone requests a page, try to fetch it from the network and store a copy in a cache, but if that doesn’t work, first look for an existing copy in a cache, and otherwise show a custom offline page instead (but if the request is taking too long, try to show a cached version of the page).

For everything else, try finding a cached version first, otherwise fetch it from the network.

Pros and cons

As with all service worker enhancements to a website, this strategy will do absolutely nothing for first-time visitors. If you’ve never visited my site before, you’ve got nothing cached. But the more you return to the site, the more your cache is primed for speedy retrieval.

I think that serving up a cached copy of a page when the network connection is flaky is a pretty good strategy …most of the time. If we’re talking about a blog post on this site, then sure, there won’t be much that the reader is missing out on—a fixed typo or ten; maybe some additional webmentions at the end of a post. But if we’re talking about the home page, then a reader with a flaky network connection might think there’s nothing new to read when they’re served up a stale version.

What I’d really like is some way to know—on the client side—whether or not the currently-loaded page came from a cache or from a network. Then I could add some kind of interface element that says, “Hey, this page might be stale—click here if you want to check for a fresher version.” I’d also need some way in the service worker to identify any requests originating from that interface element and make sure they always go out to the network.

I think that should be doable somehow. If you can think of a way to do it, please share it. Write a blog post and send me the link.

But even without the option to over-ride the time-out, I’m glad that I’m at least doing something to handle the lie-fi situation. Perhaps I should write a sequel to Going Offline called Still Online But Only In Theory Because The Connection Sucks.

Friday, March 22nd, 2019

Benjamin Parry Offline Homebrewing

Two of my favourite things: indie web and service workers.

This makes me so happy. I remember saying when my book came out, that the best feedback I could possibly get would be readers making their websites work offline. The same can be said for the talk of the book.

Thursday, March 7th, 2019

Going Offline—the talk of the book

I gave a new talk at An Event Apart in Seattle yesterday morning. The talk was called Going Offline, which the eagle-eyed amongst you will recognise as the title of my most recent book, all about service workers.

I was quite nervous about this talk. It’s very different from my usual fare. Usually I have some big sweeping arc of history, and lots of pretentious ideas joined together into some kind of narrative arc. But this talk needed to be more straightforward and practical. I wasn’t sure how well I would manage that brief.

I knew from pretty early on that I was going to show—and explain—some code examples. Those were the parts I sweated over the most. I knew I’d be presenting to a mixed audience of designers, developers, and other web professionals. I couldn’t assume too much existing knowledge. At the same time, I didn’t want to teach anyone to such eggs.

In the end, there was an overarching meta-theme to talk, which was this: logic is more important than code. In other words, figuring out what you’re trying to accomplish (and describing it clearly) is more important than typing curly braces and semi-colons. Programming is an act of translation. Before you can translate something, you need to be able to articulate it clearly in your own language first. By emphasising that point, I hoped to make the code less overwhelming to people unfamilar with it.

I had tested the talk with some of my Clearleft colleagues, and they gave me great feedback. But I never know until I’ve actually given a talk in front of a real conference audience whether the talk is any good or not. Now that I’ve given the talk, and received more feedback, I think I can confidentally say that it’s pretty damn good.

My goal was to explain some fairly gnarly concepts—let’s face it: service workers are downright weird, and not the easiest thing to get your head around—and to leave the audience with two feelings:

  1. This is exciting, and
  2. This is something I can do today.

I deliberately left time for questions, bribing people with free copies of my book. I got some great questions, and I may incorporate some of them into future versions of this talk (conference organisers, if this sounds like the kind of talk you’d like at your event, please get in touch). Some of the points brought up in the questions were:

  • Is there some kind of wizard for creating a typical service worker script for any site? I didn’t have a direct answer to this, but I have attempted to make a minimal viable service worker that could be used for just about any site. Mostly I encouraged the questioner to roll their sleeves up and try writing a bespoke script. I also mentioned the Workbox library, but I gave my opinion that if you’re going to spend the time to learn the library, you may as well spend the time to learn the underlying language.
  • What are some state-of-the-art progressive web apps for offline user experiences? Ooh, this one kind of stumped me. I mean, the obvious poster children for progressive webs apps are things like Twitter, Instagram, and Pinterest. They’re all great but the offline experience is somewhat limited. To be honest, I think there’s more potential for great offline experiences by publishers. I especially love the pattern on personal sites like Una’s and Sara’s where people can choose to save articles offline to read later—like a bespoke Instapaper or Pocket. I’d love so see that pattern adopted by some big publications. I particularly like that gives so much more control directly to the end user. Instead of trying to guess what kind of offline experience they want, we give them the tools to craft their own.
  • Do caches get cleaned up automatically? Great question! And the answer is mostly no—although browsers do have their own heuristics about how much space you get to play with. There’s a whole chapter in my book about being a good citizen and cleaning up your caches, but I didn’t include that in the talk because it isn’t exactly exciting: “Hey everyone! Now we’re going to do some housekeeping—yay!”
  • Isn’t there potential for abuse here? This is related to the previous question, and it’s another great question to ask of any technology. In short, yes. Bad actors could use service workers to fill up caches uneccesarily. I’ve written about back door service workers too, although the real problem there is with iframes rather than service workers—iframes and cookies are technologies that are already being abused by bad actors, and we’re going to see more and more interventions by ethical browser makers (like Mozilla) to clamp down on those technologies …just as browsers had to clamp down on the abuse of pop-up windows in the early days of JavaScript. The cache API could become a tragedy of the commons. I liken the situation to regulation: we should self-regulate, but if we prove ourselves incapable of that, then outside regulation (by browsers) will be imposed upon us.
  • What kind of things are in the future for service workers? Excellent question! If you think about it, a service worker is kind of a conduit that gives you access to different APIs: the Cache API and the Fetch API being the main ones now. A service worker is like an airport and the APIs are like the airlines. There are other APIs that you can access through service workers. Notifications are available now on desktop and on Android, and they’ll be coming to iOS soon. Background Sync is another powerful API accessed through service workers that will get more and more browser support over time. The great thing is that you can start using these APIs today even if they aren’t universally supported. Then, over time, more and more of your users will benefit from those enhancements.

If you attended the talk and want to learn more about about service workers, there’s my book (obvs), but I’ve also written lots of blog posts about service workers and I’ve linked to lots of resources too.

Finally, here’s a list of links to all the books, sites, and articles I referenced in my talk…

Books

Sites

Progressive Web Apps

Tuesday, December 18th, 2018

The 15 Web Design Books of 2018 You Can’t Afford to Miss

How lovely! Going Offline is in very good company in this list, and Oliver has some nice words to say about it:

Starting with no assumption of JavaScript knowledge, Jeremy explains the latest strategies, the ins and outs of fetching and caching, how to enhance your website’s performance, and more.

Extremely beginner-friendly and approachable, it can be read in half a day and will help you get Service Workers up and running in no time.

But all I want for Christmas is for Shopify to stop enabling Breitbart.

Thursday, November 8th, 2018

A Book Apart, Front-End Next Steps

If you buy this bundle of books, you get Going Offline in some very, very good company.

Sunday, September 23rd, 2018

Service workers in Samsung Internet browser

I was getting reports of some odd behaviour with the service worker on thesession.org, the Irish music website I run. Someone emailed me to say that they kept getting the offline page, even when their internet connection was perfectly fine and the site was up and running.

They didn’t mind answering my pestering follow-on questions to isolate the problem. They told me that they were using the Samsung Internet browser on Android. After a little searching, I found this message on a Github thread about using waitUntil. It’s from someone who works on the Samsung Internet team:

Sadly, the asynchronos waitUntil() is not implemented yet in our browser. Yes, we will implement it but our release cycle is so far. So, for a long time, we might not resolve the issue.

A-ha! That explains the problem. See, here’s the pattern I was using:

  1. When someone requests a file,
  2. fetch that file from the network,
  3. create a copy of the file and cache it,
  4. return the contents.

Step 1 is the event listener:

// 1. When someone requests a file
addEventListener('fetch', fetchEvent => {
  let request = fetchEvent.request;
  fetchEvent.respondWith(

Steps 2, 3, and 4 are inside that respondWith:

// 2. fetch that file from the network
fetch(request)
.then( responseFromFetch => {
  // 3. create a copy of the file and cache it
  let copy = responseFromFetch.clone();
  caches.open(cacheName)
  .then( cache => {
    cache.put(request, copy);
  })
  // 4. return the contents.
  return responseFromFetch;
})

Step 4 might well complete while step 3 is still running (remember, everything in a service worker script is asynchronous so even though I’ve written out the steps sequentially, you never know what order the steps will finish in). That’s why I’m wrapping that third step inside fetchEvent.waitUntil:

// 2. fetch that file from the network
fetch(request)
.then( responseFromFetch => {
  // 3. create a copy of the file and cache it
  let copy = responseFromFetch.clone();
  fetchEvent.waitUntil(
    caches.open(cacheName)
    .then( cache => {
      cache.put(request, copy);
    })
  );
  // 4. return the contents.
  return responseFromFetch;
})

If a browser (like Samsung Internet) doesn’t understand the bit where I say fetchEvent.waitUntil, then it will throw an error and execute the catch clause. That’s where I have my fifth and final step: “try looking in the cache instead, but if that fails, show the offline page”:

.catch( fetchError => {
  console.log(fetchError);
  return caches.match(request)
  .then( responseFromCache => {
    return responseFromCache || caches.match('/offline');
  });
})

Normally in this kind of situation, I’d use feature detection to check whether a browser understands a particular API method. But it’s a bit tricky to test for support for asynchronous waitUntil. That’s okay. I can use a try/catch statement instead. Here’s what my revised code looks like:

fetch(request)
.then( responseFromFetch => {
  let copy = responseFromFetch.clone();
  try {
    fetchEvent.waitUntil(
      caches.open(cacheName)
      .then( cache => {
        cache.put(request, copy);
      })
    );
  } catch (error) {
    console.log(error);
  }
  return responseFromFetch;
})

Now I’ve managed to localise the error. If a browser doesn’t understand the bit where I say fetchEvent.waitUntil, it will execute the code in the catch clause, and then carry on as usual. (I realise it’s a bit confusing that there are two different kinds of catch clauses going on here: on the outside there’s a .then()/.catch() combination; inside is a try{}/catch{} combination.)

At some point, when support for async waitUntil statements is universal, this precautionary measure won’t be needed, but for now wrapping them inside try doesn’t do any harm.

There are a few places in chapter five of Going Offline—the chapter about service worker strategies—where I show examples using async waitUntil. There’s nothing wrong with the code in those examples, but if you want to play it safe (especially while Samsung Internet doesn’t support async waitUntil), feel free to wrap those examples in try/catch statements. But I’m not going to make those changes part of the errata for the book. In this case, the issue isn’t with the code itself, but with browser support.

Thursday, August 30th, 2018

Offline Web Experiences with Jeremy Keith « CTRL+CLICK CAST

I had a great time chatting with Lea and Emily about service workers on this episode of their podcast—they’re such great hosts!

Here’s the huffduffed audio.

Thursday, August 16th, 2018

Going Offline by Jeremy Keith – a post by Marc Thiele

This is such a lovely, lovely review from Marc!

Jeremy’s way of writing certainly helps, as a specialised or technical book on a topic like Service Workers, could certainly be one, that bores you to death with dry written explanations. But Jeremy has a friendly, fresh and entertaining way of writing books. Sometimes I caught myself with a grin on my face…

Tuesday, August 14th, 2018

005: Service workers - Web Components Club

I strongly recommend that you read Going Offline by Jeremy Keith. Before his book, I found the concept of service workers quite daunting and convinced myself that it’s one of those things that I’ll have to set aside a big chunk of time to learn. I got through Jeremy’s book in a few hours and felt confident and inspired. This is because he’s very good at explaining concepts in a friendly, concise manner.

Wednesday, July 11th, 2018