1. Service Workers
The Practical Bits
@patmeenan
Patrick Meenan

2. Slides
Slideshare: http://www.slideshare.net/patrickmeenan
Twitter: @patmeenan (and to #velocityconf)
Velocity Site, attached to session information
Video: https://www.youtube.com/user/patmeenan
Office Hours: Friday, May 29 from 12:30pm-1:00pm
@patmeenan

3. Chrome Team @Google (performance)
WebPageTest
pmeenan@webpagetest.org
@patmeenan

4. The Real Experts
Alex Russell
@slightlylate
Jake Archibald
@jaffathecake

5. Service Workers

6. DOM
Resource Fetching in Chrome

7. DOM
Resource Fetching in Chrome
In-memory
Resource
Cache

8. DOM
Resource Fetching in Chrome
In-memory
Resource
Cache
Resource
Fetcher

9. DOM
Resource Fetching in Chrome
Resource
Fetcher
Net Stack

10. DOM
Resource Fetching in Chrome
In-memory
Resource
Cache
Resource
Fetcher
Net Stack
Disk
Cache

11. DOM
Resource Fetching in Chrome
In-memory
Resource
Cache
Resource
Fetcher
Net Stack Net
Disk
Cache

12. DOM
Resource Fetching in Chrome
In-memory
Resource
Cache
Resource
Fetcher
Net Stack Net
Disk
Cache
Service
Worker
SW Added and
Removed Here!

13. Capabilities
Sees every request for your document
- Including cross-origin
- And headers
Can synthesize responses
Supports fetch
Has a programmable cache and Indexed DB

14. Limitations
HTTPS documents only
Not active for first view
iFrames are separate documents
Non-CORS Cross-origin responses are opaque
No global state
- or concept of a “page”
No Sync API’s (localstorage, XHR, etc)

15. http://memegenerator.net/instance/62336209

16. Registering
if ('serviceWorker' in navigator) {
navigator.serviceWorker.register('/my-app/sw.js', {
scope: '/my-app/'
});
}
https://github.com/slightlyoff/ServiceWorker/blob/master/explainer.md

17. Registering
if ('serviceWorker' in navigator) {
navigator.serviceWorker.register('/my-app/sw.js', {
scope: '/my-app/'
});
}
https://github.com/slightlyoff/ServiceWorker/blob/master/explainer.md

18. Registering
if ('serviceWorker' in navigator) {
navigator.serviceWorker.register('/my-app/sw.js', {
scope: '/my-app/'
});
}
https://github.com/slightlyoff/ServiceWorker/blob/master/explainer.md

19. Registering
if ('serviceWorker' in navigator) {
navigator.serviceWorker.register('/my-app/sw.js', {
scope: '/my-app/'
});
}
https://github.com/slightlyoff/ServiceWorker/blob/master/explainer.md

20. Install / activate
self.addEventListener('install', function(event) {
event.waitUntil(
fetchStuffAndInitDatabases()
);
});
self.addEventListener('activate', function(event) {
// You're good to go!
});
https://github.com/slightlyoff/ServiceWorker/blob/master/explainer.md

21. Network Intercepting
self.addEventListener('fetch', function(event) {
event.respondWith(new Response("Hello world!"));
});
https://github.com/slightlyoff/ServiceWorker/blob/master/explainer.md

22. Enough Theory – Let’s Use It

23. Offline Content
 Most benefit for Single-Page Apps where app/data are separated
 Service Workers become progressive enhancement for offline support

24. Offline Content
 Pre-cache offline content
 Pass live requests through when online
- Caching the responses
 Serve cached responses when offline (if available)
 Serve offline-specific versions otherwise

25. Offline Content – Pre-caching
self.addEventListener( ‘install’, function(event) {
event.waitUntil(
caches.open(‘my-offline-cache-v1’).then( function(cache) {
return cache.addAll([
‘/site.js’,
‘/images/offline.png’,
‘/offline.html’]);
}));});

26. Offline Content – Pre-caching
self.addEventListener( ‘install’, function(event) {
event.waitUntil(
caches.open(‘my-offline-cache-v1’).then( function(cache) {
return cache.addAll([
‘/site.js’,
‘/images/offline.png’,
‘/offline.html’]);
}));});

27. Offline Content – Pre-caching
self.addEventListener( ‘install’, function(event) {
event.waitUntil(
caches.open(‘my-offline-cache-v1’).then( function(cache) {
return cache.addAll([
‘/site.js’,
‘/images/offline.png’,
‘/offline.html’]);
}));});

28. Offline Content – Pre-caching
self.addEventListener( ‘install’, function(event) {
event.waitUntil(
caches.open(‘my-offline-cache-v1’).then( function(cache) {
return cache.addAll([
‘/site.js’,
‘/images/offline.png’,
‘/offline.html’]);
}));});

29. Offline Content – Pre-caching
self.addEventListener( ‘install’, function(event) {
event.waitUntil(
caches.open(‘my-offline-cache-v1’).then( function(cache) {
return cache.addAll([
‘/site.js’,
‘/images/offline.png’,
‘/offline.html’]);
}));});

30. Offline Content – Fetch Processing
self.addEventListener( 'fetch', function(event) {
if (navigator.online) {
event.respondWith( onlineRequest(event.request) );
} else {
event.respondWith( offlineRequest(event.request) );
}
});

31. Offline Content – Fetch Processing
self.addEventListener( 'fetch', function(event) {
if (navigator.online) {
event.respondWith( onlineRequest(event.request) );
} else {
event.respondWith( offlineRequest(event.request) );
}
});

32. Offline Content – Fetch Processing
self.addEventListener( 'fetch', function(event) {
if (navigator.online) {
event.respondWith( onlineRequest(event.request) );
} else {
event.respondWith( offlineRequest(event.request) );
}
});

33. Offline Content – Fetch Processing
self.addEventListener( 'fetch', function(event) {
if (navigator.online) {
event.respondWith( onlineRequest(event.request) );
} else {
event.respondWith( offlineRequest(event.request) );
}
});

34. Offline Content – Fetch Processing
self.addEventListener( 'fetch', function(event) {
if (navigator.online) {
event.respondWith( onlineRequest(event.request) );
} else {
event.respondWith( offlineRequest(event.request) );
}
});

35. Offline Content – Online Response
function onlineRequest(request) {
return caches.match(request)
.then(function(response) {
…
}
);
}

36. Offline Content – Online Response
function onlineRequest(request) {
return caches.match(request).then(function(response) {
if (response) {
return response;
} else {
return fetch(request);
}
});
}

37. Offline Content – Online Response
function onlineRequest(request) {
return caches.match(request).then(function(response) {
if (response) {
return response;
} else {
return fetch(request);
}
});
}

38. Offline Content – Online Response
…
var fetchRequest = request.clone();
return fetch(fetchRequest).then( function(response) {
var responseToCache = response.clone();
caches.open(CACHE_NAME).then( function(cache) {
cache.put(request, responseToCache);
});
return response;
});

39. Offline Content – Online Response
…
var fetchRequest = request.clone();
return fetch(fetchRequest).then( function(response) {
var responseToCache = response.clone();
caches.open(CACHE_NAME).then( function(cache) {
cache.put(request, responseToCache);
});
return response;
});

40. Offline Content – Online Response
…
var fetchRequest = request.clone();
return fetch(fetchRequest).then( function(response) {
var responseToCache = response.clone();
caches.open(CACHE_NAME).then( function(cache) {
cache.put(request, responseToCache);
});
return response;
});

41. Offline Content – Online Response
…
var fetchRequest = request.clone();
return fetch(fetchRequest).then( function(response) {
var responseToCache = response.clone();
caches.open(CACHE_NAME).then( function(cache) {
cache.put(request, responseToCache);
});
return response;
});

42. Offline Content – Online Response
…
var fetchRequest = request.clone();
return fetch(fetchRequest).then( function(response) {
var responseToCache = response.clone();
caches.open(CACHE_NAME).then( function(cache) {
cache.put(request, responseToCache);
});
return response;
});

43. Offline Content – Online Response
…
var fetchRequest = request.clone();
return fetch(fetchRequest).then( function(response) {
var responseToCache = response.clone();
caches.open(CACHE_NAME).then( function(cache) {
cache.put(request, responseToCache);
});
return response;
});

44. Offline Content – Offline Response
function offlineRequest(request) {
if (request.url.match(/.png$/)) {
return caches.match(‘/images/offline.png’);
} else if (request.url.match(/.html$/)) {
return caches.match(‘/offline.html’);
}
…
}

45. Offline Content – Offline Response
function offlineRequest(request) {
if (request.url.match(/.png$/)) {
return caches.match(‘/images/offline.png’);
} else if (request.url.match(/.html$/)) {
return caches.match(‘/offline.html’);
}
…
}

46. Other Uses
Not limited to applications designed for offline/SPA
Legacy apps + fetch intercept = Awesome possibilities

47. Custom Error Pages
Fetch Request Normally
For non-200 responses serve a local error page
Not just server errors:
- DNS failures
- CDN/Proxy Errors
- Intermediaries

48. SPOF Prevention
Identify requests of interest
- 3rd
-party javascript
- Fonts
Set a timer
Pass fetch requests through
If timer expires before fetch completes generate error response

49. CDN/Origin Failover
Identify CDN requests by URL
Set a timer
Pass fetch request through
If timer expires, create fetch request to origin
Respond with first fetch request to complete

50. Multi-Origin/CDN
Identify CDN requests by URL
Replace CDN domain with alternate CDN (or origin)
Keep track of performance by origin
Prefer faster origin (keep measuring)
Could also race the CDNs in parallel
- Be careful about increased data

51. Stale-While-Revalidate
Respond with local cached resource
- Ignoring Expires, max-age, etc.
Pass fetch request through in parallel
Update cache with new response
Works best for known resources (analytics/ads JS, etc)

52. Prefetch
Custom response headers with prefetch resources
When idle, prefetch suggested resources

53. Resource Prioritization/Scheduling
Track in-flight requests
Make scheduling decisions as new requests come in
Custom application-aware scheduling logic
- Delay JS if it is known to be at the end
- Delay footer images
- etc

54. Delta Compression
Delta compression for build->build updates
- Include version in URL scheme
- Get latest version number from cache
- Request delta from server
- Apply patch
- Cache new version
New compression algorithms

55. Custom Compression
New compression algorithms
- Brotli
- Fractal image compression
- JSON-specific dictionaries
- Application-specific
Prove-out new algorithms before standardizing
…as long as it can be implemented in JS and code size is
reasonable

56. Incremental Progressive Images
Identify JPEG image requests from known origin
Synthesize response
Range request (or smarter) for first few scans
Stream initial range into synthesized response
Range request for remaining image (some point later)
Append remaining data into synthesized response

57. Drawing Images Locally
 930 x 11,362 px WebPageTest waterfall
 351KB compressed PNG
 42 MB in-memory (server) to generate
 < 20KB compressed JSON data to describe
 Prefer actual images for WPT waterfalls for easier embedding
- Otherwise SVG, Canvas or DOM would work

58. Drawing Images Locally*
Identify appropriate requests (i.e. waterfall.png?...)
Fetch data necessary to draw image
Draw to canvas
Extract as PNG
Synthesize PNG response
* Work in progress, coming soon

59. Metrics/Debugging
Pass all requests through
Passively observe timings and success
Report metrics back
Gives visibility into failures and requests that are in flight
- Unlike resource timing

60. And there’s more…

61. Required for…
Push Notifications
- https://gauntface.com/blog/2014/12/15/push-notifications-service-worker
Background Sync
- https://github.com/slightlyoff/BackgroundSync/blob/master/explainer.md
GeoFencing
- https://github.com/slightlyoff/Geofencing

62. Availability
Chrome Stable (40+)
Opera (27+)
Firefox (Nightly)

63. Composable
Service workers can “importScripts”
Begging for a framework to handle pluggable pipeline
- With compatible plugins for processing
- Delta compression + scheduling + Stale While Revalidate…
- Just a matter of importing the libs

64. Thank You!
@patmeenan
pmeenan@webpagetest.org