when visit the https://vuejs.org/js/vue.min.js, i find it's loaded from ServiceWorker, how it works?
Screenshot of Chrome DevTool
A service worker is a piece of JavaScript code that your browser runs in the background. Service workers are given some special privileges over normal JavaScript code that runs in a browser, with a commonly used priveledge being their ability to intercept fetch events.
A fetch event is activated anytime the client requests a file. Many service workers will download all requested files into a cache. The second time the same file is requested, the service worker will step in and return the cached file without sending any http request.
In this particular case, a service worker is storing the contents of the vue file into a cache the first time it is downloaded, removing the need for an expensive network request on future page loads.
Related
I am serving my service worker from /worker.js and want it to intercept fetches to /localzip/*, but the fetch event is never fired.
I register it like this:
navigator.serviceWorker.register(
"worker.js",
{ scope: "/localzip/" }
);
And I claim all clients when it activates, so that I can start intercepting fetches from the current page immediately. I am sure that the service worker is activating and that clients.claim() is succeeding.
self.addEventListener("activate", (e) => {
// Important! Start processing fetches for all clients immediately.
//
// MDN: "When a service worker is initially registered, pages won't use it
// until they next load. The claim() method causes those pages to be
// controlled immediately."
e.waitUntil(clients.claim());
});
Chrome seems happy with it and the scope appears correct:
My fetch event handler is very simple:
self.addEventListener("fetch", (e) => {
console.log("Trying to make fetch happen!");
});
From my application, after the worker is active, I try to make a request, e.g.,
const response = await fetch("/localzip/lol.jpg");
The fetch does not appear to trigger the above event handler, and the browser instead tries to make a request directly to the server and logs GET http://localhost:3000/localzip/lol.jpg 404 (Not Found).
I have tried:
Making sure the latest version of my worker code is running.
Disabling / clearing caches to make sure the fetch isn't being handled by the browser's cache.
Hosting from an HTTPS server. (Chrome is supposed to support service workers on plaintext localhost for development.)
What more does it want from me?
Live demo: https://rgov.github.io/service-worker-zip-experiment/
Note that the scope is slightly different, and the fetch is performed by creating an <img> tag.
First, let's confirm you are not using hard-reload while testing your code. If you use hard-reload, all requests will not go through the service worker.
See https://web.dev/service-worker-lifecycle/#shift-reload
I also checked chrome://serviceworker-internals/ in Chrome, and your service worker has fetch handler.
Then, let's check the codes in detail.
After trying your demo page, I found a network request is handled by the service worker after clicking "Display image from zip archive" button since I can see this log:
Service Worker: Serving archive/maeby.jpg from zip archive
Then, the error is thrown:
Failed to load ‘https://rgov.github.io/localzip/maeby.jpg’. A ServiceWorker passed a promise to FetchEvent.respondWith() that rejected with ‘TypeError: db is undefined’.
This is caused by db object is not initialized properly. It would be worth confirming whether you see the DB related issue as I see in your demo. If not, my following statement might be incorrect.
I try to explain some service worker mechanism alongside my understanding of your code:
Timing of install handler
Your DB open code happens in the install handler only. This means DB object will be assigned only when the install handler is executed.
Please notice the install handler will be executed only when it's necessary. If a service worker exists already and does not need to update, the install handler won't be called. Hence, the db object in your code might not be always available.
Stop/Start Status
When the service worker does not handle events for a while (how long it would be is by browser's design), the service worker will go to stop/idle state.
When the service worker is stopped/idle (you can check the state in the devtools) and started again, the global db object will be undefined.
In my understanding, this is why I see the error TypeError: db is undefined’.
Whenever the service worker wakes up, the whole worker script will be executed. However, the execution of event handlers will depend on whether the events are coming.
How to prevent stop/idle for debugging?
Open your devtools for the page then the browser will keep it being alive.
Once you close the devtool, the service worker might go to "stop" soon.
Why does the service worker stops?
The service worker is designed for handling requests. If no request/event should be handled by a service worker, the service worker thread is not necessary to run for saving resources.
Please notice both fetch and message events (but not limited to) will awake the service worker.
See Prevent Service Worker from automatically stopping
Solution for the demo page
If the error is from the DB, this means the getFromZip function should open the DB when db is unavailable.
By the way, even without any change, your demo code works well in the following steps:
As a user, I open the demo page at the first time. (This is for ensuring that the install handler is called.)
I open the devtools ASAP once I see the page content. (This is for preventing the service worker goes to "stop" state)
I click "Download zip archive to IndexedDB" button.
I click "Display image from zip archive" button.
Then I can see the image is shown properly.
Jake Archibald pointed out that I was confused about the meaning of the service worker's scope, explained here:
We call pages, workers, and shared workers clients. Your service worker can only control clients that are in-scope. Once a client is "controlled", its fetches go through the in-scope service worker.
In other words:
The scope affects which pages (clients) get placed under the service worker's control and not which fetched URLs get intercepted.
I'm playing with the service worker API in my computer so I can grasp how can I benefit from it in my real world apps.
I came across a weird situation where I registered a service worker which intercepts fetch event so it can check its cache for requested content before sending a request to the origin.
The problem is that this code has an error which prevented the function from making the request, so my page is left blank; nothing happens.
As the service worker has been registered, the second time I load the page it intercepts the very first request (the one which loads the HTML). Because I have this bug, that fetch event fails, it never requests the HTML and all I see its a blank page.
In this situation, the only way I know to remove the bad service worker script is through chrome://serviceworker-internals/ console.
If this error gets to a live website, which is the best way to solve it?
Thanks!
I wanted to expand on some of the other answers here, and approach this from the point of view of "what strategies can I use when rolling out a service worker to production to ensure that I can make any needed changes"? Those changes might include fixing any minor bugs that you discover in production, or it might (but hopefully doesn't) include neutralizing the service worker due to an insurmountable bug—a so called "kill switch".
For the purposes of this answer, let's assume you call
navigator.serviceWorker.register('service-worker.js');
on your pages, meaning your service worker JavaScript resource is service-worker.js. (See below if you're not sure the exact service worker URL that was used—perhaps because you added a hash or versioning info to the service worker script.)
The question boils down to how you go about resolving the initial issue in your service-worker.js code. If it's a small bug fix, then you can obviously just make the change and redeploy your service-worker.js to your hosting environment. If there's no obvious bug fix, and you don't want to leave your users running the buggy service worker code while you take the time to work out a solution, it's a good idea to keep a simple, no-op service-worker.js handy, like the following:
// A simple, no-op service worker that takes immediate control.
self.addEventListener('install', () => {
// Skip over the "waiting" lifecycle state, to ensure that our
// new service worker is activated immediately, even if there's
// another tab open controlled by our older service worker code.
self.skipWaiting();
});
/*
self.addEventListener('activate', () => {
// Optional: Get a list of all the current open windows/tabs under
// our service worker's control, and force them to reload.
// This can "unbreak" any open windows/tabs as soon as the new
// service worker activates, rather than users having to manually reload.
self.clients.matchAll({type: 'window'}).then(windowClients => {
windowClients.forEach(windowClient => {
windowClient.navigate(windowClient.url);
});
});
});
*/
That should be all your no-op service-worker.js needs to contain. Because there's no fetch handler registered, all navigation and resource requests from controlled pages will end up going directly against the network, effectively giving you the same behavior you'd get without if there were no service worker at all.
Additional steps
It's possible to go further, and forcibly delete everything stored using the Cache Storage API, or to explicitly unregister the service worker entirely. For most common cases, that's probably going to be overkill, and following the above recommendations should be sufficient to get you in a state where your current users get the expected behavior, and you're ready to redeploy updates once you've fixed your bugs. There is some degree of overhead involved with starting up even a no-op service worker, so you can go the route of unregistering the service worker if you have no plans to redeploy meaningful service worker code.
If you're already in a situation in which you're serving service-worker.js with HTTP caching directives giving it a lifetime that's longer than your users can wait for, keep in mind that a Shift + Reload on desktop browsers will force the page to reload outside of service worker control. Not every user will know how to do this, and it's not possible on mobile devices, though. So don't rely on Shift + Reload as a viable rollback plan.
What if you don't know the service worker URL?
The information above assumes that you know what the service worker URL is—service-worker.js, sw.js, or something else that's effectively constant. But what if you included some sort of versioning or hash information in your service worker script, like service-worker.abcd1234.js?
First of all, try to avoid this in the future—it's against best practices. But if you've already deployed a number of versioned service worker URLs already and you need to disable things for all users, regardless of which URL they might have registered, there is a way out.
Every time a browser makes a request for a service worker script, regardless of whether it's an initial registration or an update check, it will set an HTTP request header called Service-Worker:.
Assuming you have full control over your backend HTTP server, you can check incoming requests for the presence of this Service-Worker: header, and always respond with your no-op service worker script response, regardless of what the request URL is.
The specifics of configuring your web server to do this will vary from server to server.
The Clear-Site-Data: response header
A final note: some browsers will automatically clear out specific data and potentially unregister service workers when a special HTTP response header is returned as part of any response: Clear-Site-Data:.
Setting this header can be helpful when trying to recover from a bad service worker deployment, and kill-switch scenarios are included in the feature's specification as an example use case.
It's important to check the browser support story for Clear-Site-Data: before your rely solely on it as a kill-switch. As of July 2019, it's not supported in 100% of the browsers that support service workers, so at the moment, it's safest to use Clear-Site-Data: along with the techniques mentioned above if you're concerned about recovering from a faulty service worker in all browsers.
You can 'unregister' the service worker using javascript.
Here is an example:
if ('serviceWorker' in navigator) {
navigator.serviceWorker.getRegistrations().then(function (registrations) {
//returns installed service workers
if (registrations.length) {
for(let registration of registrations) {
registration.unregister();
}
}
});
}
That's a really nasty situation, that hopefully won't happen to you in production.
In that case, if you don't want to go through the developer tools of the different browsers, chrome://serviceworker-internals/ for blink based browsers, or about:serviceworkers (about:debugging#workers in the future) in Firefox, there are two things that come to my mind:
Use the serviceworker update mechanism. Your user agent will check if there is any change on the worker registered, will fetch it and will go through the activate phase again. So potentially you can change the serviceworker script, fix (purge caches, etc) any weird situation and continue working. The only downside is you will need to wait until the browser updates the worker that could be 1 day.
Add some kind of kill switch to your worker. Having a special url where you can point users to visit that can restore the status of your caches, etc.
I'm not sure if clearing your browser data will remove the worker, so that could be another option.
I haven't tested this, but there is an unregister() and an update() method on the ServiceWorkerRegistration object. you can get this from the navigator.serviceWorker.
navigator.serviceWorker.getRegistration('/').then(function(registration) {
registration.update();
});
update should then immediately check if there is a new serviceworker and if so install it. This bypasses the 24 hour waiting period and will download the serviceworker.js every time this javascript is encountered.
For live situations you need to alter the service worker at byte-level (put a comment on the first line, for instance) and it will be updated in the next 24 hours. You can emulate this with the chrome://serviceworker-internals/ in Chrome by clicking on Update button.
This should work even for situations when the service worker itself got cached as the step 9 of the update algorithm set a flag to bypass the service worker.
We had moved a site from godaddy.com to a regular WordPress install. Client (not us) had a serviceworker file (sw.js) cached into all their browsers which completely messed things up. Our site, a normal WordPress site, has no service workers.
It's like a virus, in that it's on every page, it does not come from our server and there is no way to get rid of it easily.
We made a new empty file called sw.js on the root of the server, then added the following to every page on the site.
<script>
if (navigator && navigator.serviceWorker && navigator.serviceWorker.getRegistration) {
navigator.serviceWorker.getRegistration('/').then(function(registration) {
if (registration) {
registration.update();
registration.unregister();
}
});
}
</script>
In case it helps someone else, I was trying to kill off service workers that were running in browsers that had hit a production site that used to register them.
I solved it by publishing a service-worker.js that contained just this:
self.globalThis.registration.unregister();
In my react application have a service worker working in the browser as expected. Unfortunately when the application is built and packaged with electron the service worker does not work. It seems to register but the service worker throws a error - uncaught in promise typeerror failed to fetch
The only real difference I can see is the origin of the worker. In electron it is stating a file path where as the one in the browser is stating the http://localhost path. This is using electron 11. The application is written in react from create-react app which has not been ejected.
This error originates from CORS(Cross Origin Resource Sharing). The thing is if you delete your service worker content and leave maybe a simple console.log statement the app would work. That is because in your fetch handler in service worker you are manually sending requests with Fetch API.
Unfortunately, CORS rules which are set inside your application are lost inside your service worker if you use the Fetch API.
If you really want to have that fetch handler in your service worker you have two choices:
You can customize the fetch handler so it doesn't do anything for index.html(which is the most important file for you). In that case if you don't do anything and you don't use event.respondWith, the request will not be stopped by service worker and your CORS headers will not be lost.
This is the tougher option. You need to set your CORS headers manually in the service worker fetch handler.
I'm using Electron, which is based on Chromium, to create an offline desktop application.
The application uses a remote site, and we are using a service worker to offline parts of the site. Everything is working great, except for a certain situation that I call the "airplane wifi situation".
Using Charles, I have restricted download bandwidth to 100bytes/s. The connection is sent through webview.loadURL which eventually calls LoadURLWithParams in Chromium. The problem is that it does not fail and then activate the service worker, like no connection at all would. Once the request is sent, it waits forever for the response.
My question is, how do I timeout the request after a certain amount of time and load everything from the service worker as if the user was truly offline?
An alternative to writing this yourself is to use the sw-toolbox library, which provides routing and runtime caching strategies for service workers, along with some built in options for helping with these sorts of advanced use cases. In particular, you'd want to use the networkTimeoutSeconds parameter to configure the amount of time to wait for a response from the network before you fall back to a previously cached response.
You can use it like the following:
toolbox.router.get(
new RegExp('my-api\\.com'),
toolbox.networkFirst, {
networkTimeoutSeconds: 10
}
);
That would configure a route that matched GET requests with URLs containing my-api.com, and applied a network-first strategy that will automatically fall back to the previously cached response after 10 seconds.
I am using service worker to implement web push notifications. Whenever I change some code of service-worker, that change is not reflected in service-worker on browser unless I delete cookie/cache.
Is this normal behaviour or I have to add some function to update service-worker?
Service worker files are cached for a Max of 24 hours if the cache header is sent with the service worker file.
First step is to set the cache headers to 0 to not cache.
When a browser finds a new service worker it will download and install it. It won't take affect until all pages that are currently controlled by the service worker are closed. For a normal user this isn't a problem. During development in chrome you can use Ctrl+ shift + R to do a hard refresh which forces a page not to be controlled by service worker, allowing your be service worker take control on the next refresh.
Final option is to use skip waiting in install step and claim in the activate step to force a new service worker to instantly activate and control any pages. If earn against this as it's easy to get into weird scenarios.
Update: Browsers are changing this default behavior - Firefox will now ignore the cache header and other browsers are likely to implement the same behaviour
To answer your specific question: yes, the behaviour is intentional and yes, yo can call an update function. Use update() method on the service worker registration. From MDN:
The update method of the ServiceWorkerRegistration interface attempts to update the service worker. It fetches the worker's script URL, and if the new worker is not byte-by-byte identical to the current worker, it installs the new worker. The fetch of the worker bypasses any browser caches if the previous fetch occurred over 24 hours ago.
Notice it says the SW fetch will bypass any browser cache if the previous fetch is older than 24h so you should disable caches while developing service workers.