Introducing Service Workers

Weaving the Web

For as long as I can remember, the World Wide Web has had an inferiority complex. Back in the ’90s, it was outshone by CD-ROMs (ask your parents). They had video, audio, and a richness that the web couldn’t match. But they lacked links — you couldn’t link from something in one CD-ROM to something in another CD-ROM. They faded away. The web grew.

Service Workers

The technology that makes this bewitching offline sorcery possible is a browser feature called service workers. You might have heard of them. You might have heard that they’re something to do with JavaScript, and technically they are…but conceptually they’re very different from other kinds of scripts.

Browsers and Servers

Let’s take a step back and think about how the World Wide Web works. It’s a beautiful ballet of client and server. The client is usually a web browser — or, to use the parlance of web standards, a user agent: a piece of software that acts on behalf of the user.

Diagram of the request/response cycle between a user and a server
Fig 1.1: Browsers send URL requests to servers, and servers respond by sending files.
Diagram of the request/response cycle between a user and a server with a service worker being the first thing the response hits
Fig 1.2: Service workers tell the web browser to do something before they send the request to queue up a URL.

Getting Your Head Around Service Workers

A service worker is like a cookie. Cookies are downloaded from a web server and installed in a browser. You can go to your browser’s preferences and see all the cookies that have been installed by sites you’ve visited. Cookies are very small and very simple little text files. A website can set a cookie, read a cookie, and update a cookie. A service worker script is much more powerful. It contains a set of instructions that the browser will consult before making any requests to the site that originally installed the service worker.

Safety First

Service workers are powerful. Once a service worker has been installed on your machine, it lies in wait, like a patient spider waiting to feel the vibrations of a particular thread.

  1. HTTPS only.

Securing Your Site

Enabling HTTPS on your site opens up a whole series of secure-only browser features — like the JavaScript APIs for geolocation, payments, notifications, and service workers. Even if you never plan to add a service worker to your site, it’s still a good idea to switch to HTTPS. A secure connection makes it trickier for snoopers to see who’s visiting which websites. Your website might not contain particularly sensitive information, but when someone visits your site, that’s between you and your visitor. Enabling HTTPS won’t stop unethical surveillance by the NSA, but it makes the surveillance slightly more difficult.

Screenshot of
Fig 1.3: The website of EFF’s Certbot.



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Jeremy Keith

Jeremy Keith


A web developer and author living and working in Brighton, England. Everything I post on Medium is a copy — the originals are on my own website,