Dive into html5 7

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Ersistent local storage is one of the areas where native client applications have held an advantage over web applications. For native applications, the operating system typically provides an abstraction layer for storing and retrieving application-specific data like preferences or runtime state. These values may be stored in the registry, INI files, XML files, or some other place according to platform convention. If your native client application needs local storage beyond key/value pairs, you can embed your own database, invent your own file format, or any number of other solutions.

Historically, web applications have had none of these luxuries. Cookies were invented early in the web’s history, and indeed they can be used for persistent local storage of small amounts of data. But they have three potentially dealbreaking downsides:

Cookies are included with every HTTP request, thereby slowing down your web application by needlessly transmitting the same data over and over
Cookies are included with every HTTP request, thereby sending data unencrypted over the internet (unless your entire web application is served over SSL)
Cookies are limited to about 4 KB of data – enough to slow down your application (see above), but not enough to be terribly useful
What we really want is

A lot of storage space
On the client
That persists beyond a page refresh
And isn’t transmitted to the server
Before HTML5, all attempts to achieve this were ultimately unsatisfactory in different ways.


In the beginning, there was only Internet Explorer. Or at least, that’s what Microsoft wanted the world to think. To that end, as part of the First Great Browser Wars, Microsoft

invented a great many things and included them in their browser-to-end-all-browser-wars, Internet Explorer. One of these things was called DHTML Behaviors, and one of these behaviors was called userData.

UserData allows web pages to store up to 64 KB of data per domain, in a hierarchical XML-based structure. (Trusted domains, such as intranet sites, can store 10 times that amount. And hey, 640 KB ought to be enough for anybody.) IE does not present any form of permissions dialog, and there is no allowance for increasing the amount of storage available.

In 2002, Adobe introduced a feature in Flash 6 that gained the unfortunate and misleading name of “Flash cookies.” Within the Flash environment, the feature is properly known as Local Shared Objects. Briefly, it allows Flash objects to store up to 100 KB of data per domain. Brad Neuberg developed an early prototype of a Flash-to-JavaScript bridge called AMASS (AJAX Massive Storage System), but it was limited by some of Flash’s design quirks. By 2006, with the advent of ExternalInterface in Flash 8, accessing LSOs from JavaScript became an order of magnitude easier and faster. Brad rewrote AMASS and integrated it into the popular Dojo Toolkit under the moniker dojox. storage. Flash gives each domain 100 KB of storage “for free.” Beyond that, it prompts the user for each order of magnitude increase in data storage (1 Mb, 10 Mb, and so on).

In 2007, Google launched Gears, an open source browser plugin aimed at providing additional capabilities in browsers. (We’ve previously discussed Gears in the context of providing a geolocation API in Internet Explorer.

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Dive into html5 7