What steps can you take to guarantee that a service worker consistently caches the same set of files?

Question

What steps can you take to guarantee that a service worker consistently caches the same set of files?

My progressive web app (PWA) is made up of various files like index.html, manifest.json, bundle.js, and serviceWorker.js. Updating the app involves uploading all these files to my host, which in my case is Firebase. I use firebase deploy for this purpose.

Usually, the update process works smoothly: Existing users see the old version initially, but the new service worker automatically installs any updated files in the background. Then, when the user next opens the app, the new version activates and becomes visible.

However, there is a issue when a user accesses the app shortly after an update. It appears that the host delivers the new serviceWorker.js but serves the old bundle.js. Consequently, the service worker ends up caching the old bundle.js, leading to users experiencing outdated functionality or even encountering a mix of new and old files.

I realize it could be argued that the hosting platform should update atomically, but since I have no control over Firebase, this seems unlikely. Moreover, given that browsers send independent fetch requests, there's no guarantee that all files returned will be consistent.

For reference, here is my serviceWorker.js file. The cacheName strings such as "app1-a0f43550e414" are generated by my build pipeline. The hash value represents the latest version of bundle.js, ensuring that the cache is only updated if the content of bundle.js has changed.

"use strict";
const appName = "app1";
...

I've considered consolidating all my HTML, CSS, and JavaScript into a single large file to prevent inconsistencies. However, PWAs require separate supporting files such as the service worker, manifest, and icons that can't be bundled. Even if I bundle what I can, users may still end up with an outdated version of the bundle along with inconsistent supporting files. Additionally, I aim to reduce bundling and have more individual files in future updates for better granularity.

Another idea was to upload bundle.js and other files under different filenames for each version. The service worker's fetch function could hide the name change so that files like index.html can still refer to it as bundle.js. But this approach raises concerns about the initial loading of the app in a browser, as well as potential limitations on renaming certain key files like index.html or manifest.json.

javascript typescript firebase progressive-web-apps service-worker

Answer 1

Answer №1

If the request for bundle.js in your install handler seems to be served from the HTTP cache rather than the network, you may need to make some adjustments.

Consider updating the code snippet below:

cache.add(cacheLookup[cacheName])

You can modify it to explicitly create a new Request object with the cache mode set to

reload</code to ensure that the response is fetched from the network instead of the cache:</p>
<pre class="lang-js"><code>cache.add(new Request(cacheLookup[cacheName], {cache: 'reload'})

Alternatively, if you are worried about non-atomic global deployments and have the capability to generate hashes like sha256 during the build process, consider implementing subresource integrity to verify the response data received by your service worker.

To integrate this into your code, you will need to calculate the correct sha256 hashes for each file during build time:

const cacheLookup = {
    "sha256-[hash]": "/",
    "sha256-[hash]": "bundle.js",
    "sha256-[hash]": "manifest.json"
};

// Later...
cache.add(new Request(cacheLookup[cacheName], {
  cache: 'reload',
  integrity: cacheName,
}));

If there is an SRI mismatch, the request for a resource will fail, leading to rejection of the cache.add() operation and ultimately the failure of the service worker installation. The installation will be retried during the next update check, hopefully resolving the issue once the deployment is complete and the SRI matches correctly.

Answer 2

If the request for bundle.js in your install handler seems to be served from the HTTP cache rather than the network, you may need to make some adjustments.

Consider updating the code snippet below:

cache.add(cacheLookup[cacheName])

You can modify it to explicitly create a new Request object with the cache mode set to

reload</code to ensure that the response is fetched from the network instead of the cache:</p>
<pre class="lang-js"><code>cache.add(new Request(cacheLookup[cacheName], {cache: 'reload'})

Alternatively, if you are worried about non-atomic global deployments and have the capability to generate hashes like sha256 during the build process, consider implementing subresource integrity to verify the response data received by your service worker.

To integrate this into your code, you will need to calculate the correct sha256 hashes for each file during build time:

const cacheLookup = {
    "sha256-[hash]": "/",
    "sha256-[hash]": "bundle.js",
    "sha256-[hash]": "manifest.json"
};

// Later...
cache.add(new Request(cacheLookup[cacheName], {
  cache: 'reload',
  integrity: cacheName,
}));

If there is an SRI mismatch, the request for a resource will fail, leading to rejection of the cache.add() operation and ultimately the failure of the service worker installation. The installation will be retried during the next update check, hopefully resolving the issue once the deployment is complete and the SRI matches correctly.

What steps can you take to guarantee that a service worker consistently caches the same set of files?

Answer №1

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