Cover by David Flanagan

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Client-Side Databases

Web application architecture has traditionally featured HTML, CSS, and JavaScript on the client and a database on the server. Among the most surprising HTML5 APIs, therefore, are client-side databases. These are not just client-side APIs for accessing database servers across the network, but actual client-side databases stored on the user’s computer and directly accessed by JavaScript code in the browser.

The Web Storage API described in localStorage and sessionStorage can be thought of as a particularly simple kind of database that persists simple key/value pairs. But in addition, there are two client-side database APIs that are “real” databases. One, known as Web SQL Database, is a simple relational database that supports basic SQL queries. Chrome, Safari, and Opera implemented this API, but Firefox and IE have not, and likely never will. Work on the official specification of this API has stopped and this full-featured SQL database will probably never become an official standard nor an unofficial but interoperable feature of the web platform.

Standardization efforts are now focused on another database API, known as IndexedDB. It is too early to document this API in any detail (it is not covered in Part IV), but Firefox 4 and Chrome 11 include implementations, and this section includes a working example that demonstrates some of the most important features of the IndexedDB API.

IndexedDB is an object database, not a relational database, and it is much simpler ...

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