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HomeSoftware DevelopmentNow THAT’S What I Name Service Employee! – A Checklist Aside

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

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

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

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

Getting out of the woods#section2

Weekly Timber is a consumer of mine that gives logging companies in central Wisconsin. For them, a quick web site is significant. Their enterprise is positioned in Waushara County, and like many rural stretches in the US, 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 sooner.

Wisconsin has farmland for days, but it surely additionally has loads of forests. While 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 trying elsewhere in case you’re left ready too lengthy on a crappy community connection.

I initially didn’t imagine a Service Employee was vital for Weekly Timber’s web site. In spite of everything, if issues have been a lot quick to start out with, why complicate issues? Alternatively, understanding that my consumer companies not simply Waushara County, however a lot of central Wisconsin, even a barebones Service Employee may very well be the sort of progressive enhancement that provides resilience within the locations it could be wanted most.

The primary Service Employee I wrote for my consumer’s web site—which I’ll discuss with henceforth because the “normal” Service Employee—used three well-documented caching methods:

  1. Precache CSS and JavaScript property for all pages when the Service Employee is put in when the window’s load occasion fires.
  2. Serve static property out of CacheStorage if accessible. If a static asset isn’t in CacheStorage, retrieve it from the community, then cache it for future visits.
  3. For HTML property, 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 property.

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 Consumer Monitoring (RUM). Meaning these beneficial properties aren’t simply theoretical, however an actual enchancment for actual individuals.

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 must entry the community, it takes about 23 milliseconds to “obtain” the asset from CacheStorage.

This efficiency increase is from bypassing the community fully for static property already in CacheStorage—notably render-blocking stylesheets. The same profit is realized after 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.

If you happen to’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 instances—should 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. An extended max-age worth works, too, however the mixture of Service Employee API and CacheStorage offers you much more flexibility.

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

A greater, sooner 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 after 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 should respect. Customers above all else, all the time.

The Service Employee API’s innovation area is appreciable. How you’re employed inside that area can have a giant 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:

  • Cut 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 yet one more trick: precache header and footer partials, then mix them with content material partials from the community. This not solely reduces how a lot knowledge we obtain from the community, but it surely 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 concept of mixing precached header and footer partials with community content material on the fly looks like a Single Web page Software (SPA), you’re not far off. Like an SPA, you’ll want to use the “app shell” mannequin to your web site. Solely as an alternative of a client-side router plowing content material into one piece of minimal markup, you must consider your web site as three separate components:

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

For my consumer’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 totally 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 should be legitimate markup within the sense that every one tags should be closed inside every partial. The one factor that counts within the closing sense is that the mixture of those partials have to be legitimate markup.

To begin, you’ll must precache separate header and footer partials when the Service Employee is put in. For my consumer’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
    // Other assets worth precaching

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

Each web page have to 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 web site is static, this implies producing an entire 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 consumer—you should utilize an HTTP request header to instruct the server to ship full pages or content material partials.

The laborious 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 with a view to keep away from the Service Employee startup penalty, we’ll must 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 have to 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.enable() : true

As a result of navigation preload isn’t supported in every single place, we now have to do the same old function 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 consists of pruning these previous caches, in addition to ready for each purchasers.declare() (which tells the Service Employee to claim management instantly fairly 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 move 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 return...

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

  1. The preloaded response is offered in occasion.preloadResponse. Nonetheless, as Jake Archibald notes, the worth of occasion.preloadResponse shall be undefined in browsers that don’t help navigation preload. Subsequently, we should move 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 by way of 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 essentially the most 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 difficult. 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 fairly than the total web page markup.

Nonetheless, as a result of navigation preload isn’t accessible in all browsers, we ship a special header in these eventualities. In Weekly Timber’s case, we fall again to a customized X-Content material-Mode header. In my consumer’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) {

require_once("consists of/house.php");

if (USE_PARTIAL === false) {


The factor to be hip to right here is that it is best to specify a Differ 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 shall be accessible nearly 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 shortly as attainable. 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 laborious. After I wrote my very own operate to merge streamed responses and tousled a important step—which ended up not enhancing web page efficiency, thoughts you—I modified Jake Archibald’s mergeResponses() operate to swimsuit my wants:

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

  let doneResolve,

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

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

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

        if (completed) {

          if (!readers[0]) {


          return this.pull(controller);

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

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

  return {
    response: new Response(readable, {

As common, there’s rather a lot 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 offered, it will all the time 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 completed is created. In it, we assign the promise’s resolve() and reject() capabilities to the exterior variables doneResolve and doneReject, respectively. These shall 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 shall 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 totally streamed. This repeats till there are not any extra readers to course of.
  6. When all is completed, the merged stream of responses is returned as a single response, and we return it with a Content material-Sort header worth of textual content/html.

That is a lot less complicated in case 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 { completed, response } = await mergeResponses([


On the finish of the fetch() occasion handler, we move the header and footer partials from CacheStorage to the mergeResponses() operate, and move 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 problem?#section8

It is a lot of stuff to do, and it’s difficult! You may mess one thing up, or possibly your web site’s structure isn’t well-suited to this precise strategy. So it’s necessary to ask: are the efficiency advantages well worth the work? In my opinion? Sure! The artificial efficiency beneficial properties aren’t dangerous 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 knowledge throughout numerous 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 consumer’s web site with a low-end Nokia 2 Android telephone on a throttled “Quick 3G” connection in Chrome’s developer instruments. Every class was examined ten instances on the homepage. The takeaways listed here are:

  • No Service Employee in any respect is barely sooner than the “normal” Service Employee with less complicated caching patterns than the streaming variant. Like, ever so barely sooner. This can be as a result of delay launched by Service Employee startup, nevertheless, the RUM knowledge I’ll go over reveals a special case.
  • Each LCP and FCP are tightly coupled in eventualities the place there’s no Service Employee or when the “normal” 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.
  • Nonetheless, 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 instances.
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 knowledge throughout numerous 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 “normal” 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 unbelievable! Nonetheless, in comparison with the “normal” Service Employee, LCP is a contact slower.

As a result of the lengthy tail of efficiency is necessary, 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 knowledge throughout numerous Service Employee varieties for the Weekly Timber web site.

The ninety fifth percentile of RUM knowledge is a superb 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 “normal” 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 knowledge 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 sort of enchancment we will anticipate on such a small and easy web site, what may we anticipate on an internet 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 have to be up to date in JavaScript on every navigation by altering the worth of doc.title. It additionally means you’ll must 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 web site’s header shows the present authenticated consumer on log in, you will have to replace the header partial markup offered by CacheStorage in JavaScript on every navigation to replicate who’s authenticated. You could possibly do that by storing primary consumer knowledge in localStorage and updating the UI from there.

There are actually different challenges, but it surely’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 reminiscent of blogs, advertising and marketing web sites, information web sites, ecommerce, and different typical use instances.

All in all, although, it’s akin to the efficiency enhancements and effectivity beneficial properties that you simply’d get from an SPA. Solely the distinction is that you simply’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 admittedly necessary to think about in a world the place client-side routing is all the fashion.

“What about Workbox?,” you may ask—and also you’d be proper to. Workbox simplifies rather a lot in relation to utilizing the Service Employee API, and also you’re not flawed to achieve for it. Personally, I desire to work as near the metallic as I can so I can acquire 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 value may be very low.

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

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



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