Building an Offline Page for theguardian.com — JSConf Budapest, May 2016

You’re on a train to work and you open up the Guardian app on your phone. A tunnel surrounds you, but the app still works in very much the same way as it usually would—despite your lack of internet connection, you still get the full experience, only the content shown will be stale. If you tried the same for the Guardian website, however, it wouldn’t load at all. Native apps have long had the tools to deal with these situations, in order to deliver rich user experiences whatever the user’s situation may be. With service workers, the web is catching up. This talk will explain how Oliver used service workers to build an offline page for theguardian.com.

Transcript

1. Building an Offline Page for theguardian.com Oliver Joseph Ash – JSConf Budapest, May 2016
 @OliverJAsh
2. @OliverJAsh
5. web vs native
6. • Content is cached • Experience: • offline: stale content remains • server down: stale content remains • poor connection: stale while revalidate • good connection: stale while revalidate Native app
7. Experience: • offline: nothing • server down: nothing • poor connection: white screen of death • good connection: new content Website
9. We can do better!
10. /developer-blog
11. How it works
12. Prototype built in < 1 day
13. 1. What is a service worker?
16. What is a service worker? • Script that runs in the background • Useful for features with no user interaction, e.g.: • listen to push events, useful for pushing notifications • intercept and handle network requests • future: • background sync • alarms (e.g. reminders) • geofencing • A progressive enhancement • Trusted origins only (HTTPS, localhost) • Chrome, Opera, and Firefox stable
17. www.theguardian.com https://
18. https://…/info …/science …/technology …/business
19. <script> if (navigator.serviceWorker) { navigator.serviceWorker.register('/service-worker.js'); } </script> 1. Create and register the service worker
20. 1 2
23. 2. Prime the cache
24. var version = 1; var staticCacheName = 'static' + version; var updateCache = function () { return caches.open(staticCacheName) .then(function (cache) { return cache.addAll([ '/offline-page', '/assets/css/main.css', '/assets/js/main.js' ]); }); }; self.addEventListener('install', function (event) { event.waitUntil(updateCache()); }); • install event: get ready! • Cache the assets needed later • Version the cache
25. 1 2
26. 3. Handle requests with fetch
27. const staticCacheName = 'static'; const version = 1; const updateCache = () => ( caches.open(staticCacheName + version) .then(cache => cache.addAll([ '/offline-page.html', '/assets/css/main.css', '/assets/js/main.js' ]); ); ); self.addEventListener('install', function (event) { event.waitUntil(updateCache()); }); self.addEventListener('fetch', function (event) { event.respondWith(fetch(event.request)); }); • Default: just fetch • Override default • Intercept network requests to: • fetch from the network • read from the cache • construct your own response fetch events
30. • e.g., use templating to construct a HTML response from JSON Service worker: custom responses self.addEventListener('fetch', function (event) { var responseBody = '<h1>Hello, World!</h1>'; var responseOptions = { headers: { 'Content-Type': 'text/html' } }; var response = new Response( responseBody, responseOptions ); event.respondWith(response); });
31. Mutable (HTML) Immutable (assets: CSS, JS)
32. HTML
33. Network first, then cache
34. var doesRequestAcceptHtml = function (request) { return request.headers.get('Accept') .split(',') .some(function (type) { return type === ‘text/html’; }); }; self.addEventListener('fetch', function (event) { var request = event.request; if (doesRequestAcceptHtml(request)) { event.respondWith(fetch(request)); } });
35. self.addEventListener('fetch', function (event) { var request = event.request; if (doesRequestAcceptHtml(request)) { event.respondWith( fetch(request).catch(function () { return caches.match('/offline-page'); }) ); } });
37. Assets
39. self.addEventListener('fetch', function (event) { var request = event.request; if (doesRequestAcceptHtml(request)) { event.respondWith( fetch(request).catch(function () { return caches.match('/offline-page'); }) ); } else { event.respondWith( caches.match(request) ); } });
41. Cache first, then network
42. self.addEventListener('fetch', function (event) { var request = event.request; if (doesRequestAcceptHtml(request)) { event.respondWith( fetch(request).catch(function () { return caches.match('/offline-page'); }) ); } else { event.respondWith( caches.match(request).then(function (response) { return response || fetch(request); }) ); } });
43. 4. Updating the crossword
44. var version = 1; var cacheNameSuffix = 'static' + '-' + version; var getISODate = function () { return new Date().toISOString().split('T')[0]; }; var getCacheName = function () { return getISODate() + '-' + cacheNameSuffix; }; var isLatestCacheName = function (key) { return key === getCacheName(); }; var deleteOldCaches = function () { var getOldCacheKeys = function () { return caches.keys().then(function (keys) { return keys.filter(function (key) { return !isLatestCacheName(key); }); }); }; return getOldCacheKeys().then(function (oldCacheKeys) { return Promise.all(oldCacheKeys.map(function (key) { return caches.delete(key); })); }); }; var isCacheUpdated = function () { return caches.keys().then(function (keys) { return keys.some(isLatestCacheName); }); }; isCacheUpdated().then(function (isUpdated) { if (!isUpdated) { updateCache().then(deleteOldCaches); } }); // <normal fetch handling code>
46. Future usages of service worker on theguardian.com • Offline page will increase in significance • Offline page can be extended • show stale content, like native apps • show personalised content, downloaded ahead of time • show content that has been “saved for later” • Go fully offline-first
47. Offline-first • Instantly respond to navigations with a “shell” • Improves the experience for users with poor connections • No more white screen of death • Show stale content whilst fetching new content
51. Problems and caveats • Browser bugs in both Chrome and Firefox • Interleaving of versions in CDN cache
52. service-worker.js v1.css offline-page.html caches caches depends on
53. 0 60 120 Seconds /v1.css TTL: 1 year /v2.css TTL: 1 year /offline-page.html TTL: 60 seconds /service-worker.js TTL: 60 seconds v1 v2 v1 v2 Deploy
54. service-worker.js v2.css offline-page.html v2 v1 v2 v1.css v1 caches caches depends on
55. // /offline-page.json { "html": "<html><!-- v1 --></html>", "assets": ["/v1.css"] } Solution: cache manifest
56. Why? Is this valuable? • Fun • Insignificant usage due to HTTPS/browser support • … but good to plant the seed • Iron out browser bugs, pushes the web forward
57. “If we only use features that work in IE8, we're condemning ourselves to live in an IE8 world.” — Nolan Lawson
58. Conclusion • Service workers allow us to progressively enhance the experience for: • offline users • users with poor connections • It’s easy to build an offline page • A simple offline page is a good place to start
59. Further reading • https://www.theguardian.com/info/developer-blog/2015/nov/04/ building-an-offline-page-for-theguardiancom • https://github.com/slightlyoff/ServiceWorker/blob/master/explainer.md • https://www.theguardian.com/service-worker.js
60. Thank you Like what we’re doing? developers.theguardian.com @OliverJAsh