Service workers and offline web apps

A service worker is a script that a browser keeps, separate from the page, which can answer the page's requests from a saved copy. This is how a web app can open on a train with no signal.

You will learn what it does, how it saves files, and how to choose what to do with each request.

Normally a page asks the network for a file and waits. With a service worker, the request goes to the worker first. The worker can answer from the cache or pass the request on to the network.

Because it can answer without the network, a page can open when the phone has no signal.

A service worker can answer any request for its site, so a fake one would be dangerous. A browser therefore allows it only on a secure address, which means https. A page on your own computer, called localhost, is also allowed for testing.

A browser may refuse a worker on a plain preview address. When that happens its developer console says so. On a secure address you would expect the worker to be listed as activated and running.

docker exec jupyter python3 steps/serve.py

The script asks the lab's preview server for the page, the manifest and the worker file. Each answers with 200, which means it was found, and a content type.

The worker file is an ordinary file the server hands out. Whether the browser then runs it as a worker is the browser's decision, and it depends on the address.

const CACHE = "habits-v1";
const FILES = ["/", "/style.css", "/app.js"];

self.addEventListener("install", (event) => {
  event.waitUntil(caches.open(CACHE).then((c) => c.addAll(FILES)));
});

// install runs once. It opens a cache and saves every listed file.
// If one file cannot be saved, the whole install fails.

docker exec jupyter python3 steps/offline.py

The script finds the cache name, habits-v1, and the list of eight files the worker saves. It checks that each file exists on disk.

Seven exist. One does not, /offline.html. That is a real fault, because a worker saves the whole list in one step. If one file is missing, the install fails and nothing works offline.