Peter Lubbers - Pro HTML 5 Programming

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CHAPTER 10 ■ CREATING HTML5 OFFLINE WEB APPLICATIONS
The HTML5 offline application cache makes it possible to augment an application to run without a
network connection. You do not need a connection to the Internet just to draft an e-mail. HTML5
introduces the offline application cache that allows a Web application to run without network
connectivity.
An application developer can specify specific additional resources comprising an HTML5
application (HTML, CSS, JavaScript, and images) to make an application available for offline use. There
are many use cases for this, for example:
• Read and compose e-mail
• Edit documents
• Edit and display presentations
• Create to-do lists
Using offline storage can avoid the normal network requests needed to load an application. If the
cache manifest is up to date, the browser knows it does not need to check if the other resources are also
up to date, and most of the application can load very quickly out of the local application cache.
Additionally, loading resources out of a cache (instead of making multiple HTTP requests to see if
resources have been updated) saves bandwidth, which can be especially important for mobile web
applications. Currently, slower loading is one way that web applications suffer in comparison with
desktop applications. Caching can offset that.
The application cache gives developers explicit control over caching. The cache manifest file allows
you to group related resources into a logical application. This is a powerful concept that can give web
applications some of the characteristics of desktop applications. You can use this additional power in
new, creative ways.
Resources identified in the cache manifest file create what is known as an application cache, which
is the place where browsers store the resources persistently, typically on disk. Some browsers give users
a way to view the data in the application cache. For example, the about:cache page in the latest version
of Firefox shows you details about the application cache and a way to view individual files in the cache,
as shown in Figure 10-1.