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Now THAT’S What I Name Service Employee! – A Record Aside

The Service Employee API is the Dremel of the net platform. It gives extremely broad utility whereas additionally yielding resiliency and higher efficiency. In the event you’ve not used Service Employee but—and also you couldn’t be blamed in that case, as it hasn’t seen large adoption as of 2020—it goes one thing like this:

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  1. On the preliminary go to to a web site, the browser registers what quantities to a client-side proxy powered by a comparably paltry quantity of JavaScript that—like a Net Employee—runs by itself thread.
  2. After the Service Employee’s registration, you may intercept requests and determine how to answer them within the Service Employee’s fetch() occasion.

What you determine to do with requests you intercept is a) your name and b) is determined by your web site. You’ll be able to rewrite requests, precache static belongings throughout set up, present offline performance, and—as can be our eventual focus—ship smaller HTML payloads and higher efficiency for repeat guests.

Getting out of the woods#section2

Weekly Timber is a shopper of mine that gives logging providers in central Wisconsin. For them, a quick web site is significant. Their enterprise is situated in Waushara County, and like many rural stretches in america, community high quality and reliability isn’t nice.

A screenshot of a wireless coverage map for Waushara County, Wisconsin with a color overlay. Most of the overlay is colored tan, which represents areas of the county which have downlink speeds between 3 and 9.99 megabits per second. There are sparse light blue and dark blue areas which indicate faster service, but are far from being the majority of the county.
Determine 1. A wi-fi protection map of Waushara County, Wisconsin. The tan areas of the map point out downlink speeds between 3 and 9.99 Mbps. Pink areas are even slower, whereas the pale and darkish blue areas are quicker.

Wisconsin has farmland for days, nevertheless it additionally has loads of forests. Once you want an organization that cuts logs, Google might be your first cease. How briskly a given logging firm’s web site is could be sufficient to get you wanting elsewhere should you’re left ready too lengthy on a crappy community connection.

I initially didn’t consider a Service Employee was essential for Weekly Timber’s web site. In any case, if issues have been loads quick to begin with, why complicate issues? Alternatively, realizing that my shopper providers not simply Waushara County, however a lot of central Wisconsin, even a barebones Service Employee might be the type of progressive enhancement that provides resilience within the locations it could be wanted most.

The primary Service Employee I wrote for my shopper’s web site—which I’ll seek advice from henceforth because the “commonplace” Service Employee—used three well-documented caching methods:

  1. Precache CSS and JavaScript belongings for all pages when the Service Employee is put in when the window’s load occasion fires.
  2. Serve static belongings out of CacheStorage if out there. If a static asset isn’t in CacheStorage, retrieve it from the community, then cache it for future visits.
  3. For HTML belongings, hit the community first and place the HTML response into CacheStorage. If the community is unavailable the following time the customer arrives, serve the cached markup from CacheStorage.

These are neither new nor particular methods, however they supply two advantages:

  • Offline functionality, which is helpful when community situations are spotty.
  • A efficiency increase for loading static belongings.

That efficiency increase translated to a 42% and 48% lower within the median time to First Contentful Paint (FCP) and Largest Contentful Paint (LCP), respectively. Higher but, these insights are based mostly on Actual Person Monitoring (RUM). Which means these beneficial properties aren’t simply theoretical, however an actual enchancment for actual folks.

A screenshot of request/response timings in Chrome's developer tools. It depicts a service worker on a page serving a static asset from CacheStorage in roughly 23 milliseconds.
Determine 2. A breakdown of request/response timings depicted in Chrome’s developer instruments. The request is for a static asset from CacheStorage. As a result of the Service Employee doesn’t have to entry the community, it takes about 23 milliseconds to “obtain” the asset from CacheStorage.

This efficiency increase is from bypassing the community solely for static belongings already in CacheStorage—notably render-blocking stylesheets. The same profit is realized once we depend on the HTTP cache, solely the FCP and LCP enhancements I simply described are compared to pages with a primed HTTP cache with out an put in Service Employee.

In the event you’re questioning why CacheStorage and the HTTP cache aren’t equal, it’s as a result of the HTTP cache—not less than in some circumstances—should still contain a visit to the server to confirm asset freshness. Cache-Management’s immutable flag will get round this, however immutable doesn’t have nice help but. A protracted max-age worth works, too, however the mixture of Service Employee API and CacheStorage provides you much more flexibility.

Particulars apart, the takeaway is that the best and most well-established Service Employee caching practices can enhance efficiency. Doubtlessly greater than what well-configured Cache-Management headers can present. Even so, Service Employee is an unimaginable know-how with way more prospects. It’s doable to go farther, and I’ll present you ways.

A greater, quicker Service Employee#section3

The online loves itself some “innovation,” which is a phrase we equally like to throw round. To me, true innovation isn’t once we create new frameworks or patterns solely for the good thing about builders, however whether or not these innovations profit individuals who find yourself utilizing no matter it’s we slap up on the net. The precedence of constituencies is a factor we must respect. Customers above all else, at all times.

The Service Employee API’s innovation house is appreciable. How you’re employed inside that house can have an enormous impact on how the net is skilled. Issues like navigation preload and ReadableStream have taken Service Employee from nice to killer. We will do the next with these new capabilities, respectively:

  • Scale back Service Employee latency by parallelizing Service Employee startup time and navigation requests.
  • Stream content material in from CacheStorage and the community.

Furthermore, we’re going to mix these capabilities and pull out another trick: precache header and footer partials, then mix them with content material partials from the community. This not solely reduces how a lot information we obtain from the community, nevertheless it additionally improves perceptual efficiency for repeat visits. That’s innovation that helps everybody.

Grizzled, I flip to you and say “let’s do that.”

Laying the groundwork#section4

If the thought of mixing precached header and footer partials with community content material on the fly looks like a Single Web page Utility (SPA), you’re not far off. Like an SPA, you’ll want to use the “app shell” mannequin to your web site. Solely as a substitute of a client-side router plowing content material into one piece of minimal markup, you must consider your web site as three separate elements:

  • The header.
  • The content material.
  • The footer.

For my shopper’s web site, that appears like this:

A screenshot of the Weekly Timber website color coded to delineate each partial that makes up the page. The header is color coded as blue, the footer as red, and the main content in between as yellow.
Determine 3. A coloration coding of the Weekly Timber web site’s completely different partials. The Footer and Header partials are saved in CacheStorage, whereas the Content material partial is retrieved from the community except the consumer is offline.

The factor to recollect right here is that the person partials don’t must be legitimate markup within the sense that each one tags must be closed inside every partial. The one factor that counts within the ultimate sense is that the mix of those partials should be legitimate markup.

To start out, you’ll have to precache separate header and footer partials when the Service Employee is put in. For my shopper’s web site, these partials are served from the /partial-header and /partial-footer pathnames:

self.addEventListener("set up", occasion => {
  const cacheName = "fancy_cache_name_here";
  const precachedAssets = (
    "/partial-header",  // The header partial
    "/partial-footer",  // The footer partial
    // Different belongings price precaching

  occasion.waitUntil(caches.open(cacheName).then(cache => {
    return cache.addAll(precachedAssets);
  }).then(() => {
    return self.skipWaiting();

Each web page should be fetchable as a content material partial minus the header and footer, in addition to a full web page with the header and footer. That is key as a result of the preliminary go to to a web page gained’t be managed by a Service Employee. As soon as the Service Employee takes over, then you definitely serve content material partials and assemble them into full responses with the header and footer partials from CacheStorage.

In case your website is static, this implies producing a complete different mess of markup partials that you could rewrite requests to within the Service Employee’s fetch() occasion. In case your web site has a again finish—as is the case with my shopper—you need to use an HTTP request header to instruct the server to ship full pages or content material partials.

The onerous half is placing all of the items collectively—however we’ll do exactly that.

Placing all of it collectively#section5

Writing even a primary Service Employee might be difficult, however issues get actual difficult actual quick when assembling a number of responses into one. One cause for that is that so as to keep away from the Service Employee startup penalty, we’ll have to arrange navigation preload.

Implementing navigation preload#section6

Navigation preload addresses the issue of Service Employee startup time, which delays navigation requests to the community. The very last thing you wish to do with a Service Employee is maintain up the present.

Navigation preload should be explicitly enabled. As soon as enabled, the Service Employee gained’t maintain up navigation requests throughout startup. Navigation preload is enabled within the Service Employee’s activate occasion:

self.addEventListener("activate", occasion => {
  const cacheName = "fancy_cache_name_here";
  const preloadAvailable = "navigationPreload" in self.registration;

  occasion.waitUntil(caches.keys().then(keys => {
    return Promise.all((
      keys.filter(key => {
        return key !== cacheName;
      }).map(key => {
        return caches.delete(key);
      preloadAvailable ? self.registration.navigationPreload.allow() : true

As a result of navigation preload isn’t supported all over the place, we now have to do the standard characteristic test, which we retailer within the above instance within the preloadAvailable variable.

Moreover, we have to use Promise.all() to resolve a number of asynchronous operations earlier than the Service Employee prompts. This contains pruning these outdated caches, in addition to ready for each purchasers.declare() (which tells the Service Employee to claim management instantly somewhat than ready till the following navigation) and navigation preload to be enabled.

A ternary operator is used to allow navigation preload in supporting browsers and keep away from throwing errors in browsers that don’t. If preloadAvailable is true, we allow navigation preload. If it isn’t, we cross a Boolean that gained’t have an effect on how Promise.all() resolves.

With navigation preload enabled, we have to write code in our Service Employee’s fetch() occasion handler to utilize the preloaded response:

self.addEventListener("fetch", occasion => {
  const { request } = occasion;

  // Static asset dealing with code omitted for brevity
  // ...

  // Verify if this can be a request for a doc
  if (request.mode === "navigate") {
    const networkContent = Promise.resolve(occasion.preloadResponse).then(response => {
      if (response) {
        addResponseToCache(request, response.clone());

        return response;

      return fetch(request.url, {
        headers: {
          "X-Content material-Mode": "partial"
      }).then(response => {
        addResponseToCache(request, response.clone());

        return response;
    }).catch(() => {
      return caches.match(request.url);

    // Extra to come back...

Although this isn’t the whole lot of the Service Employee’s fetch() occasion code, there’s quite a bit that wants explaining:

  1. The preloaded response is on the market in occasion.preloadResponse. Nevertheless, as Jake Archibald notes, the worth of occasion.preloadResponse can be undefined in browsers that don’t help navigation preload. Due to this fact, we should cross occasion.preloadResponse to Promise.resolve() to keep away from compatibility points.
  2. We adapt within the ensuing then callback. If occasion.preloadResponse is supported, we use the preloaded response and add it to CacheStorage through an addResponseToCache() helper operate. If not, we ship a fetch() request to the community to get the content material partial utilizing a customized X-Content material-Mode header with a worth of partial.
  3. Ought to the community be unavailable, we fall again to probably the most just lately accessed content material partial in CacheStorage.
  4. The response—no matter the place it was procured from—is then returned to a variable named networkContent that we use later.

How the content material partial is retrieved is hard. With navigation preload enabled, a particular Service-Employee-Navigation-Preload header with a worth of true is added to navigation requests. We then work with that header on the again finish to make sure the response is a content material partial somewhat than the complete web page markup.

Nevertheless, as a result of navigation preload isn’t out there in all browsers, we ship a unique header in these situations. In Weekly Timber’s case, we fall again to a customized X-Content material-Mode header. In my shopper’s PHP again finish, I’ve created some helpful constants:


// Is that this a navigation preload request?

// Is that this an express request for a content material partial?
outline("PARTIAL_MODE", isset($_SERVER("HTTP_X_CONTENT_MODE")) && stristr($_SERVER("HTTP_X_CONTENT_MODE"), "partial") !== false);

// If both is true, this can be a request for a content material partial
outline("USE_PARTIAL", NAVIGATION_PRELOAD === true || PARTIAL_MODE === true);


From there, the USE_PARTIAL fixed is used to adapt the response:


if (USE_PARTIAL === false) {


if (USE_PARTIAL === false) {


The factor to be hip to right here is that it’s best to specify a Range header for HTML responses to take the Service-Employee-Navigation-Preload (and on this case, the X-Content material-Mode header) into consideration for HTTP caching functions—assuming you’re caching HTML in any respect, which is probably not the case for you.

With our dealing with of navigation preloads full, we will then transfer onto the work of streaming content material partials from the community and stitching them along with the header and footer partials from CacheStorage right into a single response that the Service Employee will present.

Streaming partial content material and stitching collectively responses#section7

Whereas the header and footer partials can be out there virtually instantaneously as a result of they’ve been in CacheStorage for the reason that Service Employee’s set up, it’s the content material partial we retrieve from the community that would be the bottleneck. It’s due to this fact very important that we stream responses so we will begin pushing markup to the browser as rapidly as doable. ReadableStream can do that for us.

This ReadableStream enterprise is a mind-bender. Anybody who tells you it’s “straightforward” is whispering candy nothings to you. It’s onerous. After I wrote my very own operate to merge streamed responses and tousled a crucial step—which ended up not bettering web page efficiency, thoughts you—I modified Jake Archibald’s mergeResponses() operate to swimsuit my wants:

async operate mergeResponses (responsePromises) {
  const readers = responsePromises.map(responsePromise => {
    return Promise.resolve(responsePromise).then(response => {
      return response.physique.getReader();

  let doneResolve,

  const accomplished = new Promise((resolve, reject) => {
    doneResolve = resolve;
    doneReject = reject;

  const readable = new ReadableStream({
    async pull (controller) {
      const reader = await readers(0);

      attempt {
        const { accomplished, worth } = await reader.learn();

        if (accomplished) {

          if (!readers(0)) {


          return this.pull(controller);

      } catch (err) {
        throw err;
    cancel () {

  const headers = new Headers();
  headers.append("Content material-Kind", "textual content/html");

  return {
    response: new Response(readable, {

As typical, there’s quite a bit happening:

  1. mergeResponses() accepts an argument named responsePromises, which is an array of Response objects returned from both a navigation preload, fetch(), or caches.match(). Assuming the community is on the market, this may at all times comprise three responses: two from caches.match() and (hopefully) one from the community.
  2. Earlier than we will stream the responses within the responsePromises array, we should map responsePromises to an array containing one reader for every response. Every reader is used later in a ReadableStream() constructor to stream every response’s contents.
  3. A promise named accomplished is created. In it, we assign the promise’s resolve() and reject() features to the exterior variables doneResolve and doneReject, respectively. These can be used within the ReadableStream() to sign whether or not the stream is completed or has hit a snag.
  4. The brand new ReadableStream() occasion is created with a reputation of readable. As responses stream in from CacheStorage and the community, their contents can be appended to readable.
  5. The stream’s pull() methodology streams the contents of the primary response within the array. If the stream isn’t canceled one way or the other, the reader for every response is discarded by calling the readers array’s shift() methodology when the response is absolutely streamed. This repeats till there aren’t any extra readers to course of.
  6. When all is finished, the merged stream of responses is returned as a single response, and we return it with a Content material-Kind header worth of textual content/html.

That is a lot less complicated should you use TransformStream, however relying on if you learn this, that is probably not an possibility for each browser. For now, we’ll have to stay with this strategy.

Now let’s revisit the Service Employee’s fetch() occasion from earlier, and apply the mergeResponses() operate:

self.addEventListener("fetch", occasion => {
  const { request } = occasion;

  // Static asset dealing with code omitted for brevity
  // ...

  // Verify if this can be a request for a doc
  if (request.mode === "navigate") {
    // Navigation preload/fetch() fallback code omitted.
    // ...

    const { accomplished, response } = await mergeResponses((


On the finish of the fetch() occasion handler, we cross the header and footer partials from CacheStorage to the mergeResponses() operate, and cross the outcome to the fetch() occasion’s respondWith() methodology, which serves the merged response on behalf of the Service Employee.

Are the outcomes well worth the trouble?#section8

It is a lot of stuff to do, and it’s difficult! You would possibly mess one thing up, or perhaps your web site’s structure isn’t well-suited to this actual strategy. So it’s vital to ask: are the efficiency advantages well worth the work? For my part? Sure! The artificial efficiency beneficial properties aren’t unhealthy in any respect:

A bar graph comparing First Contentful Paint and Largest Contentful Paint performance for the Weekly Timber website for scenarios in which there is no service worker, a "standard" service worker, and a streaming service worker that stitches together content partials from CacheStorage and the network. The first two scenarios are basically the same, while the streaming service worker delivers measurably better performance for both FCP and LCP—especially for FCP!
Determine 4. A bar chart of median FCP and LCP artificial efficiency information throughout varied Service Employee varieties for the Weekly Timber web site.

Artificial exams don’t measure efficiency for something besides the precise gadget and web connection they’re carried out on. Even so, these exams have been performed on a staging model of my shopper’s web site with a low-end Nokia 2 Android cellphone on a throttled “Quick 3G” connection in Chrome’s developer instruments. Every class was examined ten occasions on the homepage. The takeaways listed below are:

  • No Service Employee in any respect is barely quicker than the “commonplace” Service Employee with less complicated caching patterns than the streaming variant. Like, ever so barely quicker. This can be as a result of delay launched by Service Employee startup, nonetheless, the RUM information I’ll go over exhibits a unique case.
  • Each LCP and FCP are tightly coupled in situations the place there’s no Service Employee or when the “commonplace” Service Employee is used. It’s because the content material of the web page is fairly easy and the CSS is pretty small. The Largest Contentful Paint is normally the opening paragraph on a web page.
  • Nevertheless, the streaming Service Employee decouples FCP and LCP as a result of the header content material partial streams in instantly from CacheStorage.
  • Each FCP and LCP are decrease within the streaming Service Employee than in different circumstances.
A bar chart comparing the RUM median FCP and LCP performance of no service worker, a "standard" service worker, and a streaming service worker. Both the "standard" and streaming service worker offer better FCP and LCP performance over no service worker, but the streaming service worker excels at FCP performance, while only being slightly slower at LCP than the "standard" service worker.
Determine 5. A bar chart of median FCP and LCP RUM efficiency information throughout varied Service Employee varieties for the Weekly Timber web site.

The advantages of the streaming Service Employee for actual customers is pronounced. For FCP, we obtain an 79% enchancment over no Service Employee in any respect, and a 63% enchancment over the “commonplace” Service Employee. The advantages for LCP are extra refined. In comparison with no Service Employee in any respect, we understand a 41% enchancment in LCP—which is unimaginable! Nevertheless, in comparison with the “commonplace” Service Employee, LCP is a contact slower.

As a result of the lengthy tail of efficiency is vital, let’s take a look at the ninety fifth percentile of FCP and LCP efficiency:

A bar chart comparing the RUM median FCP and LCP performance of no service worker, a "standard" service worker, and a streaming service worker. Both the "standard" and streaming service workers are faster than no service worker at all, but the streaming service worker beats out the "standard" service worker for both FCP and LCP.
Determine 6. A bar chart of ninety fifth percentile FCP and LCP RUM efficiency information throughout varied Service Employee varieties for the Weekly Timber web site.

The ninety fifth percentile of RUM information is a good place to evaluate the slowest experiences. On this case, we see that the streaming Service Employee confers a 40% and 51% enchancment in FCP and LCP, respectively, over no Service Employee in any respect. In comparison with the “commonplace” Service Employee, we see a discount in FCP and LCP by 19% and 43%, respectively. If these outcomes appear a bit squirrely in comparison with artificial metrics, keep in mind: that’s RUM information for you! You by no means know who’s going to go to your web site on which gadget on what community.

Whereas each FCP and LCP are boosted by the myriad advantages of streaming, navigation preload (in Chrome’s case), and sending much less markup by stitching collectively partials from each CacheStorage and the community, FCP is the clear winner. Perceptually talking, the profit is pronounced, as this video would counsel:

Now ask your self this: If that is the type of enchancment we will anticipate on such a small and easy web site, what would possibly we anticipate on a web site with bigger header and footer markup payloads?

Caveats and conclusions#section9

Are there trade-offs with this on the event aspect? Oh yeah.

As Philip Walton has famous, a cached header partial means the doc title should be up to date in JavaScript on every navigation by altering the worth of doc.title. It additionally means you’ll have to replace the navigation state in JavaScript to replicate the present web page if that’s one thing you do in your web site. Observe that this shouldn’t trigger indexing points, as Googlebot crawls pages with an unprimed cache.

There may be some challenges on websites with authentication. For instance, in case your website’s header shows the present authenticated consumer on log in, you’ll have to replace the header partial markup offered by CacheStorage in JavaScript on every navigation to replicate who’s authenticated. You might be able to do that by storing primary consumer information in localStorage and updating the UI from there.

There are actually different challenges, nevertheless it’ll be as much as you to weigh the user-facing advantages versus the event prices. In my view, this strategy has broad applicability in purposes comparable to blogs, advertising and marketing web sites, information web sites, ecommerce, and different typical use circumstances.

All in all, although, it’s akin to the efficiency enhancements and effectivity beneficial properties that you just’d get from an SPA. Solely the distinction is that you just’re not changing time-tested navigation mechanisms and grappling with all of the messiness that entails, however enhancing them. That’s the half I feel is de facto vital to contemplate in a world the place client-side routing is all the fashion.

“What about Workbox?,” you would possibly ask—and also you’d be proper to. Workbox simplifies quite a bit in the case of utilizing the Service Employee API, and also you’re not improper to achieve for it. Personally, I choose to work as near the steel as I can so I can achieve a greater understanding of what lies beneath abstractions like Workbox. Even so, Service Employee is tough. Use Workbox if it fits you. So far as frameworks go, its abstraction price may be very low.

No matter this strategy, I feel there’s unimaginable utility and energy in utilizing the Service Employee API to scale back the quantity of markup you ship. It advantages my shopper and all of the folks that use their web site. Due to Service Employee and the innovation round its use, my shopper’s web site is quicker within the far-flung elements of Wisconsin. That’s one thing I be ok with.

Particular due to Jake Archibald for his invaluable editorial recommendation, which, to place it mildly, significantly improved the standard of this text.

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