1.
JavaScript Service Worker
Design Patterns
for Better User Experience
Doug Reeder
reeder.29@gmail.com
@reeder29 on Twitter
https://hominidsoftware.com
DougReeder on GitHub

2.
Classical Request Path
browser
engine
server
proxy
server
cache
Indexed
DB
cache
classic
proxy

3.
What they are
• Programmable network proxy
• A separate thread, with separate context (like Web
Workers)
• Killed between events (don’t use globals)
• Each origin may have multiple scopes
• All tabs for pages in a scope share a single s.w.
• Each s.w. may control multiple caches

4.
What they can do
• Intercept network requests, serve them from multiple sources &
transform them
• Access new Cache API
• Construct responses using new Stream API
• Can’t access DOM
• Communicate via Requests, postMessage (structured clone of
objects), IndexedDB & Cache API
• Receive Web Push messages
• [Chrome only] Background Sync: event when connectivity restored

5.
New Request Path
browser
engine
server
proxy
server
cache
service
worker
cache
cache
server
Indexed
DB
Web Push

6.
Why this matters 1:
Network is the Enhancement
• Latency not going away & bandwidth physics-limited
• Lie-fi, erratic & costly cell data
• Underprovisioned servers, activity surges, misconfigured
routers
• Resources & data can be immediately served from cache
or IDB, then updated via network
• Controlled caching: per-article, recent, predictive
• Load-balance between continents

7.
Why this is important 2:
Threads & Streaming
• More convenient than Web Workers for I/O transforms
• Fetch JSON, xform to HTML
• Crypto, other xforms
• Move persistence layer to another thread
• Unobtrusive analytics
• UI stays snappy
• Progressive rendering keeps users engaged

8.
Why this is important 3:
Sync with tab closed
• Web Push: server pushes msg, OS+browser
activates s.w. (& usually raises notification)
• Store push payload in Indexed DB (typically)
• Web page not loaded until user taps notification
• [Chrome only] Background Sync when back
online
• Msg: store outbox in IDB, register for B.S.

9.
Why this is important 4:
Installability
• PWA “Install to Homescreen” prompt
• HTTPS
• App Manifest
• S.w. is most difficult prerequisite
• User prompted to install only after he’s shown
evidence of repeated use

10.
Coding & Debugging
• requires HTTPS - use surge.sh (or other) to test
• Use ES 2016
• Register s.w. using requestIdleCallback()
• Use service-worker-mock (on npm) to run unit
tests in Node.js
• close tab completely for new version; not reload

11.
Caching Strategies
• Cache only (e.g. static assets) like AppCache
• Cache, fall back to network
• Network, fall back to cache
• Cache, then update (e.g. user avatars)
• Cache, network update, refresh (requires
postMessage)

12.
Design Patterns
• Does not have to be app nor SPA!
• Different HTML when offline: https://
chris.bolin.co/offline

13.
browser
engine
server
proxy
server
cache
service
worker
cache
cache
Offline/Nework-tolerant
• S.w. pre-caches static assets on 1st load
• On subsequent load, browser checks for updated s.w.
If changed, deletes old cache, pre-caches new assets

14.
• 2nd & later loads avoid network delay, flakiness;
your app is never broken by a bad network
• Basic data caching:
• paths outside scope handled by classic proxy
+cache
• “network, fall back to cache” doesn’t require
app logic to change

15.
Offline Articles
browser
engine
server
proxy
server
cache
service
worker
article
cacheasset
cache
• Asset cache + 1/article; cache name is article title
• In modern browsers, show “save for offline” button:
add to cache in foreground
• Default offline page lists caches, thus offline articles

16.
• Like a podcatcher for the web; articles could
include interactive demos, VR, AR, tools
• Wifi-only tablet can hold conference schedule,
guide to landmark or games for your kid

17.
Load-Balancing
Between Data Centers
• Heuristics & hysteresis for server selection
• Response time accounts for network
congestion, server load
browser
engine
serverservice
worker
server
server

18.
Revenant Multipage Site
• DOM must be rebuilt & JS parsed on every
navigation.
• Page navigation requests full page HTML. For
old browser, server returns previously
concatenated <head> + header + footer +
navigation + content.

19.
browser
engine
server
proxy
server
cache
service
worker
cache
cache
• In new browser, s.w. intercepts, streams <head> +
header + footer+ navigation from cache immediately,
then content from cache or network.

20.
• Browser displays content as it streams in
• Network efficiency of Turbolinks, without dirty
environment
• Only 1 copy of common HTML
• Can retrofit old site, if markup can be divided
between fixed & variable parts

21.
Greenfield Pattern:
SPA w/ Search Repeater
• UI like https://serenenotes.hominidsoftware.com/
• SPA requests search results HTML. For old
browser, server converts DB result to HTML

22.
browser
engine
server
proxy
server
cache
service
worker
Indexed
DB
• In modern browser, s.w. intercepts,
queries Indexed DB & requests
JSON from server
• IDB results xformed to HTML
• Server results de-duped, xformed,
then update IDB

23.
• Need code to xform objects to HTML in two
places
• Store 10,000 records locally, 10,000,000 on
server, do full-text search & get recently
accessed results right away

24.
Indexed DB Centered
• All data fits in Indexed DB
• Foreground accesses Indexed DB
• S.w. feeds Indexed DB
browser
engine
server
proxy
serverservice
worker
Indexed
DB
Web Push

25.
Where does this lead us?
• Web apps w/ less friction & less session-oriented:
• Snappy response at all times
• UI & some content available immediately
• Fire & forget editing
• New content pushed; users don’t need to check in
• Poor connections mean you’re (somewhat) out-of-
date, not out-of-luck.

26.
• When you’re offline, you can return to where
you’ve been (or prepared to go), but can’t go
someplace totally new
• It’s easier to grab some content & unplug - to
relax, or make a considered reply.
• Whether people will choose to engage
deliberately is unknown.

27.
Questions?

28.
Sites to try
• offline app:
https://serenenotes.hominidsoftware.com
• offline-only: https://chris.bolin.co/offline
• service-worker-mock: https://github.com/
pinterest/service-workers/
• Is Service Worker Ready: https://
jakearchibald.github.io/isserviceworkerready/