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Using HTML5 Application Cache to Create Offline Web Applications

Provides an easy way to prefetch some or all of your web app’s assets

HTML5 introduces Application Cache, a new feature that enables you to make web apps and sites available offline. The new specification also provides an easy way to prefetch some or all of your web app's assets (HTML files, images, CSS, JavaScript, and so on) while the client is still online. During this caching process, files are stored in an application cache, where they sit ready for future offline use.

Compare this to regular browser caching, in which pages that you visit are cached in the browser's cache based on server-side rules and client-side configuration. But-even if web pages are cached normally, this does not provide a reliable way for you to access pages while you're in offline mode (in an airplane, for example). In addition, an application cache can cache pages that have not been visited at all and are therefore typically unavailable in the regular browser cache. Prefetching files can even speed up your site's performance, though you are of course using bandwidth to download those files initially.

Regular caching can lead to undesired results while you're offline.
The mechanism that makes offline web applications available is simple: create a manifest (text) file that lists your app's assets and reference it in a manifest attribute in your web pages' html elements. Sounds simple? It is. Just remember that there are some common misconceptions about how offline web apps work, and because of all the caching the browser is now doing for you, you may get a few (tricky) surprises when you try to debug and test your web apps (online and offline). This article explains what to look out for, so you don't have to learn the hard way. This article also covers some lower level object and Application Cache event functionality that you can tap into. Sounds good? Great-let's get started!

Note: a set of starter files that make up an example offline web app are available at: http://tech.kaazing.com/training/offline/peter-lubbers-html5-offline-web-apps-presentation-code.zip

The Basics
Let's start with the basics. Consider a web app that has just a few web pages: an index page, a CSS file, images, and JavaScript files. This web site can be made available offline in less than five minutes (once you get the hang of it). Here's how:

  • Open your favorite text editor, create a new text file, and save it with the .manifest extension. For example, offline.manifest.
  • Add the following directive on the first line:
    CACHE MANIFEST
  • List the files you want to make available offline (one line per file), as shown in the following example:

CACHE MANIFEST
index.html
cache.html
resources/img/html5.png
resources/css/html5.css
resources/js/html5.js

Note
: files must be referenced relative to the manifest file. Full URLs are allowed as well.

  • Open the HTML files that you want to make available offline (index.html and cache.html in this case) and add the manifest attribute to the html elements as follows:
    <!DOCTYPE html>
    <html manifest="offline.manifest">
  • Configure your server to serve up .manifest files with the correct mime type (this is covered in more detail later).
  • Navigate to your site's index page. The page will load, and before storing the files locally, most browsers will ask if you want to make files available for offline use. The browser will then parse the manifest file and download all the assets specified in the file. Note: Unlike regular browser caching, you don't have to actually visit each page in order to load it into the application cache.
  • Go offline and continue to browse and enjoy your site.

Browser notification about offline storage

More Stories By Peter Lubbers

Peter Lubbers is the Director of Documentation and Training at Kaazing where he oversees all aspects of documentation and training. He is the co-author of the Apress book Pro HTML5 Programming and teaches HTML5 training courses. An HTML5 and WebSocket enthusiast, Peter frequently speaks at international events.

Prior to joining Kaazing, Peter worked as an information architect at Oracle, where he wrote many books. He also develops documentation automation solutions and two of his inventions are patented.

A native of the Netherlands, Peter served as a Special Forces commando in the Royal Dutch Green Berets. In his spare time (ha!) Peter likes to run ultra-marathons. He is the 2007 and 2009 ultrarunner.net series champion and three-time winner of the Tahoe Super Triple marathon. Peter lives on the edge of the Tahoe National Forest and loves to run in the Sierra Nevada foothills and around Lake Tahoe (preferably in one go!).

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