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Web site Developer I Advertising and marketing I Social Media Advertising and marketing I Content material Creators I Branding Creators I Administration I System SolutionReliably Ship an HTTP Request as a Person Leaves a Web page | CSS-Methods

Web site Developer I Advertising and marketing I Social Media Advertising and marketing I Content material Creators I Branding Creators I Administration I System SolutionReliably Ship an HTTP Request as a Person Leaves a Web page | CSS-Methods

Web site Developer I Advertising and marketing I Social Media Advertising and marketing I Content material Creators I Branding Creators I Administration I System Resolution

On a number of events, I’ve wanted to ship off an HTTP request with some knowledge to log when a consumer does one thing like navigate to a unique web page or submit a kind. Think about this contrived instance of sending some info to an exterior service when a hyperlink is clicked:

<a href="https://css-tricks.com/some-other-page" id="hyperlink">Go to Web page</a>

<script>
doc.getElementById('hyperlink').addEventListener('click on', (e) => {
  fetch("/log", {
    technique: "POST",
    headers: {
      "Content material-Kind": "software/json"
    }, 
    physique: JSON.stringify({
      some: "knowledge"
    })
  });
});
</script>

There’s nothing terribly difficult happening right here. The hyperlink is permitted to behave because it usually would (I’m not utilizing e.preventDefault()), however earlier than that habits happens, a POST request is triggered on click on. There’s no want to attend for any kind of response. I simply need it to be despatched to no matter service I’m hitting.

On first look, you would possibly count on the dispatch of that request to be synchronous, after which we’d proceed navigating away from the web page whereas another server efficiently handles that request. However because it seems, that’s not what all the time occurs.

Browsers don’t assure to protect open HTTP requests

When one thing happens to terminate a web page within the browser, there’s no assure that an in-process HTTP request shall be profitable (see extra concerning the “terminated” and different states of a web page’s lifecycle). The reliability of these requests might rely on a number of issues — community connection, software efficiency, and even the configuration of the exterior service itself.

In consequence, sending knowledge at these moments could be something however dependable, which presents a probably important drawback should you’re counting on these logs to make data-sensitive enterprise choices.

To assist illustrate this unreliability, I arrange a small Categorical software with a web page utilizing the code included above. When the hyperlink is clicked, the browser navigates to /different, however earlier than that occurs, a POST request is fired off.

Whereas all the pieces occurs, I’ve the browser’s Community tab open, and I’m utilizing a “Sluggish 3G” connection pace. As soon as the web page masses and I’ve cleared the log off, issues look fairly quiet:

Viewing HTTP request in the network tab

However as quickly because the hyperlink is clicked, issues go awry. When navigation happens, the request is cancelled.

Viewing HTTP request fail in the network tab

And that leaves us with little confidence that the exterior service was truly ready course of the request. Simply to confirm this habits, it additionally happens once we navigate programmatically with window.location:

doc.getElementById('hyperlink').addEventListener('click on', (e) => {
+ e.preventDefault();

  // Request is queued, however cancelled as quickly as navigation happens. 
  fetch("/log", {
    technique: "POST",
    headers: {
      "Content material-Kind": "software/json"
    }, 
    physique: JSON.stringify({
      some: 'knowledge'
    }),
  });

+ window.location = e.goal.href;
});

No matter how or when navigation happens and the energetic web page is terminated, these unfinished requests are in danger for being deserted.

However why are they cancelled?

The basis of the problem is that, by default, XHR requests (by way of fetch or XMLHttpRequest) are asynchronous and non-blocking. As quickly because the request is queued, the precise work of the request is handed off to a browser-level API behind the scenes.

Because it pertains to efficiency, that is good — you don’t need requests hogging the primary thread. Nevertheless it additionally means there’s a danger of them being abandoned when a web page enters into that “terminated” state, leaving no assure that any of that behind-the-scenes work reaches completion. Right here’s how Google summarizes that particular lifecycle state:

A web page is within the terminated state as soon as it has began being unloaded and cleared from reminiscence by the browser. No new duties can begin on this state, and in-progress duties could also be killed in the event that they run too lengthy.

In brief, the browser is designed with the belief that when a web page is dismissed, there’s no must proceed to course of any background processes queued by it.

So, what are our choices?

Maybe the obvious method to keep away from this drawback is, as a lot as potential, to delay the consumer motion till the request returns a response. Up to now, this has been achieved the flawed means by use of the synchronous flag supported inside XMLHttpRequest. However utilizing it fully blocks the primary thread, inflicting a number of efficiency points — I’ve written about a few of this previously — so the thought shouldn’t even be entertained. In truth, it’s on its means out of the platform (Chrome v80+ has already eliminated it).

As an alternative, should you’re going to take this sort of method, it’s higher to attend for a Promise to resolve as a response is returned. After it’s again, you’ll be able to safely carry out the habits. Utilizing our snippet from earlier, that may look one thing like this:

doc.getElementById('hyperlink').addEventListener('click on', async (e) => {
  e.preventDefault();

  // Watch for response to return again...
  await fetch("/log", {
    technique: "POST",
    headers: {
      "Content material-Kind": "software/json"
    }, 
    physique: JSON.stringify({
      some: 'knowledge'
    }),
  });

  // ...and THEN navigate away.
   window.location = e.goal.href;
});

That will get the job achieved, however there are some non-trivial drawbacks.

First, it compromises the consumer’s expertise by delaying the specified habits from occurring. Gathering analytics knowledge definitely advantages the enterprise (and hopefully future customers), however it’s lower than ultimate to make your current customers to pay the price to comprehend these advantages. To not point out, as an exterior dependency, any latency or different efficiency points inside the service itself shall be surfaced to the consumer. If timeouts out of your analytics service trigger a buyer from finishing a high-value motion, everybody loses.

Second, this method isn’t as dependable because it initially sounds, since some termination behaviors can’t be programmatically delayed. For instance, e.preventDefault() is ineffective in delaying somebody from closing a browser tab. So, at greatest, it’ll cowl gathering knowledge for some consumer actions, however not sufficient to have the ability to belief it comprehensively.

Instructing the browser to protect excellent requests

Fortunately, there are alternatives to protect excellent HTTP requests which might be constructed into the overwhelming majority of browsers, and that don’t require consumer expertise to be compromised.

Utilizing Fetch’s keepalive flag

If the keepalive flag is about to true when utilizing fetch(), the corresponding request will stay open, even when the web page that initiated that request is terminated. Utilizing our preliminary instance, that’d make for an implementation that appears like this:

<a href="https://css-tricks.com/some-other-page" id="hyperlink">Go to Web page</a>

<script>
  doc.getElementById('hyperlink').addEventListener('click on', (e) => {
    fetch("/log", {
      technique: "POST",
      headers: {
        "Content material-Kind": "software/json"
      }, 
      physique: JSON.stringify({
        some: "knowledge"
      }), 
      keepalive: true
    });
  });
</script>

When that hyperlink is clicked and web page navigation happens, no request cancellation happens:

Viewing HTTP request succeed in the network tab

As an alternative, we’re left with an (unknown) standing, just because the energetic web page by no means waited round to obtain any kind of response.

A one-liner like this a straightforward repair, particularly when it’s a part of a generally used browser API. However should you’re in search of a extra targeted choice with a less complicated interface, there’s one other means with just about the identical browser assist.

Utilizing Navigator.sendBeacon()

The Navigator.sendBeacon()operate is particularly supposed for sending one-way requests (beacons). A fundamental implementation seems to be like this, sending a POST with stringified JSON and a “textual content/plain” Content material-Kind:

navigator.sendBeacon('/log', JSON.stringify({
  some: "knowledge"
}));

However this API doesn’t allow you to ship customized headers. So, to ensure that us to ship our knowledge as “software/json”, we’ll must make a small tweak and use a Blob:

<a href="https://css-tricks.com/some-other-page" id="hyperlink">Go to Web page</a>

<script>
  doc.getElementById('hyperlink').addEventListener('click on', (e) => {
    const blob = new Blob([JSON.stringify({ some: "data" })], { sort: 'software/json; charset=UTF-8' });
    navigator.sendBeacon('/log', blob));
  });
</script>

In the long run, we get the identical consequence — a request that’s allowed to finish even after web page navigation. However there’s one thing extra happening which will give it an edge over fetch(): beacons are despatched with a low precedence.

To show, right here’s what’s proven within the Community tab when each fetch() with keepalive and sendBeacon() are used on the identical time:

Viewing HTTP request in the network tab

By default, fetch() will get a “Excessive” precedence, whereas the beacon (famous because the “ping” sort above) have the “Lowest” precedence. For requests that aren’t essential to the performance of the web page, it is a good factor. Taken straight from the Beacon specification:

This specification defines an interface that […] minimizes useful resource competition with different time-critical operations, whereas making certain that such requests are nonetheless processed and delivered to vacation spot.

Put one other means, sendBeacon() ensures its requests keep out of the way in which of people who actually matter to your software and your consumer’s expertise.

An honorable point out for the ping attribute

It’s price mentioning {that a} rising variety of browsers assist the ping attribute. When hooked up to hyperlinks, it’ll hearth off a small POST request:

<a href="http://localhost:3000/other" ping="http://localhost:3000/log">
  Go to Different Web page
</a>

And people requests headers will comprise the web page on which the hyperlink was clicked (ping-from), in addition to the href worth of that hyperlink (ping-to):

headers: {
  'ping-from': 'http://localhost:3000/',
  'ping-to': 'http://localhost:3000/other'
  'content-type': 'textual content/ping'
  // ...different headers
},

It’s technically just like sending a beacon, however has a number of notable limitations:

  1. It’s strictly restricted to be used on hyperlinks, which makes it a non-starter if it’s essential to observe knowledge related to different interactions, like button clicks or kind submissions.
  2. Browser assist is sweet, however not nice. On the time of this writing, Firefox particularly doesn’t have it enabled by default.
  3. You’re unable to ship any customized knowledge together with the request. As talked about, essentially the most you’ll get is a few ping-* headers, together with no matter different headers are alongside for the journey.

All issues thought-about, ping is an effective device should you’re superb with sending easy requests and don’t need to write any customized JavaScript. However should you’re needing to ship something of extra substance, it may not be one of the best factor to achieve for.

So, which one ought to I attain for?

There are undoubtedly tradeoffs to utilizing both fetch with keepalive or sendBeacon() to ship your last-second requests. To assist discern which is essentially the most acceptable for various circumstances, listed below are some issues to contemplate:

You would possibly go together with fetch() + keepalive if:

  • It is advisable simply cross customized headers with the request.
  • You need to make a GET request to a service, relatively than a POST.
  • You’re supporting older browsers (like IE) and have already got a fetch polyfill being loaded.

However sendBeacon() could be a more sensible choice if:

  • You’re making easy service requests that don’t want a lot customization.
  • You like the cleaner, extra elegant API.
  • You need to assure that your requests don’t compete with different high-priority requests being despatched within the software.

Keep away from repeating my errors

There’s a motive I selected to do a deep dive into the character of how browsers deal with in-process requests as a web page is terminated. Some time again, my group noticed a sudden change within the frequency of a specific sort of analytics log after we started firing the request simply as a kind was being submitted. The change was abrupt and important — a ~30% drop from what we had been seeing traditionally.

Digging into the explanations this drawback arose, in addition to the instruments which might be out there to keep away from it once more, saved the day. So, if something, I’m hoping that understanding the nuances of those challenges assist somebody keep away from among the ache we bumped into. Pleased logging!

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