Google Gears (officially called just Gears) is Google’s open source software that enables offline access to normally web-based services. When you incorporate Gears into your application or gadget code, that application can function when the user is not connected to the Internet—effectively turning your web-based application into traditional desktop software.

Why is that important?

Imagine, for example, that you’re working on a big report using Google Docs, Google’s web-based word processor. All is well and good until you head off to your in-laws’ house for the weekend. The in-laws, unfortunately, don’t have an Internet connection in the house, and you can’t sneak away to use the WiFi service in the local Starbucks. So, while you could be working on your report during your downtime, you’re instead stuck with a computer that can’t access either your application or your document because it isn’t connected to the Internet.

In this instance, if you could run your application offline and if your document was stored on your own computer, you could keep working and get your report done on schedule. Well, that’s what Google Gears does—lets you run web-based applications locally, with no Internet connection necessary.

As you might recall from Chapter 19, “Using Google Docs,” Google does offer offline workability for its web-based applications. This offline mode is facilitated by—you guessed it—Google Gears.

Now any application developer can utilize Gears to enable their own web-based applications to run locally without an Internet connection. If your application can benefit from this offline functionality, you need to incorporate Gears into your development plans.

A traditional web-based application works as shown in Figure 46.1. The application’s user interface, displayed within the user’s web browser, constantly accesses the Internet, where the application itself is stored. Separately, the application’s data—via the browser-based data layer—also has to access the Internet to store the data online.

Figure 46.1. The flowchart for a traditional web-based application. (Illustration courtesy Google.)

Google Gears works in a slightly different manner. Key to its operation is the installation of a database engine on the user’s computer. This database locally caches the user’s data, which was previously stored exclusively on the Web.

This is accomplished via means of a data switch, as shown in Figure 46.2. This switch directs the data information to the local database when the user is offline, or to the web-based storage when the user is online. Gears also adds a Sync Engine that synchronizes the offline data with the online data.

Figure 46.2. The flowchart for a Gears-enabled application. (Illustration courtesy Google.)

When the user is offline, then, Gears-enabled applications use data from the local cache instead of from the web-based service. Thus, a user can run your application in offline mode, using the locally cached data; when he goes back online, the local data is synchronized with the version stored on the web.

To make this all work, Gears utilizes three core modules, each of which has its own API:

Like all of Google’s developers tools, Google Gears is open source and free for all developers to use.

To use Google Gears with your application, you have to add explicit Gears code into your application’s source code. In addition, all of your application’s users must have Gears installed on their computers.