As discussed in our Google Code Blog post on HTML5 for Gmail Mobile, Google's new version of Gmail for iPhone and Android-powered devices uses the Web Storage Portability Layer (WSPL) to let the same database code run on browsers that provide either Gears or HTML5 structured storage facilities. The WSPL consists of a collection of classes that provide asynchronous transactional access to both Gears and HTML5 databases and can be found on Project Hosting on Google Code.
There are five basic classes:
google.wspl.Statement - A parametrizable SQL statement class
google.wspl.Transaction - Used to execute one or more Statements with ACID properties
google.wspl.Database - A connection to the backing database, also provides transaction support
google.wspl.DatabaseFactory - Creates the appropriate HTML5 or Gears database implementation
Also included in the distribution is a simple note-taking application with a persistent database cache built using the WSPL library. This application (along with Gmail mobile for iPhone and Android-powered devices) is an example of the cache pattern for building offline web applications. In the cache pattern, we insert a browser-local cache into the web application to break the synchronous link between user actions in the browser and server-generated responses. Instead, as shown below, we have two data flows. First, entirely local to the device, contents flow from the cache to the UI while changes made by the user update the cache. In the second flow, the cache asynchronously forwards user changes to the web server and receives updates in response.
By using this architectural pattern, a web application can made tolerant of a flaky (or even absent) network connection!
We'll be available at the Developer Sandbox at Google I/O to discuss the cache pattern, HTML5 development and the WSPL library. Check it out! If you have questions or comments, please visit our discussion list.