crowd-funder-for-time-pwa/web-push.md

Web Push notifications is a web browser messaging protocol defined by the W3C.

Discussions of this interesting technology are clouded because of a terminological morass.

To understand how Web Push operates, we need to observe that are three (and potentially four) parties involved. These are:

1. The user's web browser. Let's call that BROWSER
2. The Web Push Service Provider which is operated by the organization controlling the web browser's source code. Here named PROVIDER. An example of a PROVIDER is FCM (Firebase Cloud Messaging) which is owned by Google.
3. The Web Application that a user is visiting from their web browser. Let's call this the SERVICE (short for Web Push application service)
4. A Custom Web Push Intermediary Service, either third party or self-hosted. Called INTERMEDIARY here. FCM also may fit in this category if the SERVICE has an API key from FCM.]

The workflow works like this:

BROWSER visits a website which hosts a SERVICE.

The SERVICE asks BROWSER for its permission to subscribe to messages coming from the SERVICE.

The SERVICE will provide context and obtain explicit permission before prompting for notification permission:

In order to provide this context and explict permission a two-step opt-in process where the user is first presented with a pre-permission dialog box that explains what the notifications are for and why they are useful. This may help reduce the possibility of users clicking "don't allow".

Now, to explain what happens in Typescript, we can activate a browser's permission dialogue in this manner:

function askPermission(): Promise<NotificationPermission> {
  return new Promise(function(resolve, reject) {
    const permissionResult = Notification.requestPermission(function(result) {
      resolve(result);
    });

    if (permissionResult) {
      permissionResult.then(resolve, reject);
    }
  }).then(function(permissionResult) {
    if (permissionResult !== 'granted') {
      throw new Error("We weren't granted permission.");
    }
    return permissionResult;
  });
}

The Notification.permission property indicates the permission level for the current session and returns one of the following string values:

'granted': The user has granted permission for notifications.
'denied': The user has denied permission for notifications.
'default': The user has not made a choice yet.

Once the user has granted permission, the client application registers a service worker using the ServiceWorkerRegistration API.

The ServiceWorkerRegistration API is accessible via the browser's navigator object and the navigator.serviceWorker child object and ultimately directly accessible via the navigator.serviceWorker.register method which also creates the service worker or the navigator.serviceWorker.getRegistration method.

Once you have a ServiceWorkerRegistration object, that object will provide a child object named pushManager through which subscription and management of subscriptions may be done.

Let's go through the register method first:

navigator.serviceWorker.register('sw.js', { scope: '/' })
  .then(function(registration) {
    console.log('Service worker registered successfully:', registration);
  })
  .catch(function(error) {
    console.log('Service worker registration failed:', error);
  });

The sw.js file contains the logic for what a service worker should do. It executes in a separate thread of execution from the web page but provides a means of communicating between itself and the web page via messages.

Note that there is a scope can specify what network requests it may intercept.

The Vue project already has its own service worker but it is possible to create multiple service worker files by registering them on different scopes.

It is useful architecturally to specify a separate server worker file.

In the case of web push, the path of the scope only has reference to the domain of the service worker and no relationship to the pathing for the web push server. In order to specify more than one server workers each needs to be on different scope paths!

Here's a version which can be used for testing locally. Note there can be caching issues in your browser! Incognito is highly recommended.

sw-dev.ts

self.addEventListener('push', function(event: PushEvent) {
  console.log('Received a push message', event);

  const title = 'Push message';
  const body = 'The message body';
  const icon = '/images/icon-192x192.png';
  const tag = 'simple-push-demo-notification-tag';

  event.waitUntil(
    self.registration.showNotification(title, {
      body: body,
      icon: icon,
      tag: tag
    })
  );
});

vue.config.js

module.exports = {
  pwa: {
    workboxOptions: {
      importScripts: ['sw-dev.ts']
    }
  }
}

Once we have the service worker registered and the ServiceWorkerRegistration is returned, we then have access to a pushManager property object. This property allows us to continue with the web push work flow.

In the next step, BROWSER requests a data structure from SERVICE called a VAPID (Voluntary Application Server Identification) which is the public key from a key-pair.

The VAPID is a specification used to identify the application server (i.e. the SERVICE server) that is sending push messages through a push PROVIDER. It's an authentication mechanism that allows the server to demonstrate its identity to the push PROVIDER, by use of a public and private key pair. These keys are used by the SERVICE in encrypting messages being sent to the BROWSER, as well as being used by the BROWSER in decrypting the messages coming from the SERVICE.

The VAPID (Voluntary Application Server Identification) key provides more security and authenticity for web push notifications in the following ways:

Identifying the Application Server:

    The VAPID key is used to identify the application server that is sending
the push notifications. This ensures that the push notifications are
authentic and not sent by a malicious third party.

Encrypting the Messages:

    The VAPID key is used to sign the push notifications sent by the
application server, ensuring that they are not tampered with during
transmission. This provides an additional layer of security and
authenticity for the push notifications.

Adding Contact Information:

The VAPID key allows a web application to add contact information to
the push messages sent to the browser push service. This enables the
push service to contact the application server in case of need or
provide additional debug information about the push messages.

Improving Delivery Rates:

Using the VAPID key can help improve the overall performance of web push
notifications, specifically improving delivery rates. By streamlining the
delivery process, the chance of delivery errors along the way is lessened.

If the BROWSER accepts and grants permission to subscribe to receiving from the SERVICE Web Push messages, then the BROWSER makes a subscription request to PROVIDER which creates and stores a special URL for that BROWSER.

Here's a bit of code describing the above process:

// b64 is the VAPID
b64 = 'BEl62iUYgUivxIkv69yViEuiBIa-Ib9-SkvMeAtA3LFgDzkrxZJjSgSnfckjBJuBkr3qBUYIHBQFLXYp5Nksh8U';
const applicationServerKey = urlBase64ToUint8Array(b64);
const options: PushSubscriptionOptions = {
  userVisibleOnly: true,
  applicationServerKey: applicationServerKey
};

registration.pushManager.subscribe(options)
  .then(function(subscription) {
    console.log('Push subscription successful:', subscription);
  })
  .catch(function(error) {
    console.error('Push subscription failed:', error);
  });

In this example, the applicationServerKey variable contains the VAPID public key, which is converted to a Uint8Array using a function such as this:

export function toUint8Array(base64String: string, atobFn: typeof atob): Uint8Array {
	const padding = "=".repeat((4 - (base64String.length % 4)) % 4);
	const base64 = (base64String + padding).replace(/-/g, "+").replace(/_/g, "/");

	const rawData = atobFn(base64);
	const outputArray = new Uint8Array(rawData.length);

	for (let i = 0; i < rawData.length; ++i) {
		outputArray[i] = rawData.charCodeAt(i);
	}
	return outputArray;
}

The options object is of type PushSubscriptionOptions, which includes the userVisibleOnly and applicationServerKey (ie VAPID public key) properties.

options: An object that contains the options used for creating the
subscription. This object itself has the following sub-properties:

  applicationServerKey: A public key your push service uses for application
  server identification. This is normally a Uint8Array.

  userVisibleOnly: A boolean value indicating that the push messages that
  are sent should be made visible to the user through a notification.
  This is often set to true.

The subscribe() method returns a Promise that resolves to a PushSubscription object containing details of the subscription, such as the endpoint URL and the public key. The returned data would have a form like this:

{
  "endpoint": "https://some.pushservice.com/some/unique/identifier",
  "expirationTime": null,
  "keys": {
    "p256dh": "some_base64_encoded_string",
    "auth": "some_other_base64_encoded_string"
  }
}

endpoint: A string representing the endpoint URL for the push service. This
URL is essentially the push service address to which the push message would
be sent for this particular subscription.

expirationTime: A DOMHighResTimeStamp (which is basically a number or null)
representing the subscription's expiration time in milliseconds since
01 January, 1970 UTC. This can be null if the subscription never expires.

The BROWSER will, internally, then use that URL to check for incoming messages by way of the service worker we described earlier. The BROWSER also sends this URL back to SERVICE which will use that URL to send messages to the BROWSER via the PROVIDER.

Ultimately, the actual internal process of receiving messages varies from BROWSER to BROWSER. Approaches vary from long-polling HTTP connections to WebSockets. A lot of handwaving and voodoo magic. The bottom line is that the BROWSER itself manages the connection to the PROVIDER whilst the SERVICE must send messages via the PROVIDER so that they reach the BROWSER service worker.

Just to remind us that in our service worker our code for receiving messages will look something like this:

self.addEventListener('push', function(event: PushEvent) {
  console.log('Received a push message', event);

  const title = 'Push message';
  const body = 'The message body';
  const icon = '/images/icon-192x192.png';
  const tag = 'simple-push-demo-notification-tag';

  event.waitUntil(
    self.registration.showNotification(title, {
      body: body,
      icon: icon,
      tag: tag
    })
  );
});

Now to address the issue of receiving notification messages on mobile devices. It should be noted that Web Push messages are only received when BROWSER is open, except in the cases of Chrome and Firefox mobile BROWSERS. In iOS, the mobile application (in our case a PWA) must be added to the Home Screen and permissions must be explicitly granted that allow the application to receive push notifications. Further, with an iOS device the user must enable wake on notification to have their device light-up when it receives a notification (https://support.apple.com/enus/HT208081).

So what about #4? - The INTERMEDIARY. Well, It is possible under very special circumstances to create your own Web Push PROVIDER. The only case I've found so far relates to making an Android Custom ROM. (An Android Custom ROM is a customized version of the Android Operating System.) There are open source IMTERMEDIARY products such as UnifiedPush (https://unifiedpush.org/) which can fulfill this role. If you are using iOS you are not permitted to make or use your own custom Web Push PROVIDER. Apple will never allow anyone to do that. Apple has none of its own.

It is, however, possible to have a sort of proxy working between your SERVICE and FCM (or iOS). Services that mash up various Push notification services (like OneSignal) can perform in the role of such proxies.

#4 -The INTERMEDIARY- doesn't appear to be anything we should be spending our time on.