Thursday, October 30, 2008

2 Days with Google Gears

I had a chance to play with Google Gears on a mockup project recently. I was surprised to learn that my understanding of Gears was not the same as the reality of Gears. I had expected it to be a JS library to facilitate rich HTML applications to handle spotty network connections. It turned out that Gears is a browser plugin that adds useful tools that any rich web app developer would find useful, though they are all aligned to handle an app losing its network connection. In the two days that I spent playing with Gears, I was pretty much blown away by the power and simplicity of it.

Besides understanding what Gears is, it's important to also understand what Gears is not. Gears doesn't try to be a UI library, or a general "glue" library (see JQuery, Prototype, MochiKit, MooTools, or any other Javascript framework). It doesn't really force you to code in any particular way (this is a bit of a lie, but more on that later). Just as a sampling, Gears includes:

  • A cross-browser compatible XMLHttpRequest
  • Local database storage
  • A web resource cache, so that items can still be fetched even if the network connection goes down. This is nearly completely transparent to the developer, and works for resources loaded by Gears or by the browser itself.
  • A mechanism to run scripts in the background (for real).

In our little demo, we have a set of checkboxes that can be checked. Initially, these would perform an asynchronous post to the server, where they would update some server-side state. However, if the app goes offline, nothing will ever reach the server. We modified this to enqueue changes into a local database, with a background script pulling items out of the database and sending them to the server. If the server ever goes down, that thread simply stops pulling items out of the database. In addition, we set our app up so that its resources (HTML, Javascript, CSS, and images) were initially cached when the app is first loaded. A neat feature of Gears seems to be that it will monitor the apps that it knows about and will automatically update its cache if the cache ever gets stale. Unfortunately, it's not perfect. It depends on the developer updating a version string, which causes Gears to update its cache from the source.

One problem that we had is that the HTML that we served up would include information about what items were checked. That is to say, when you would load the page, we would serve up some <input type="checkbox" /> and <input type="checkbox" checked="checked" /> elements. This makes total sense in a traditional web app. The client requests the page, and you serve it, and everybody is happy. Every time the page is served, it is reconstructed with the current state of the data. As you might imagine, this caused all kinds of problems for us. Concretely, we noticed that every time the page was reloaded (whether the network connection is up or down), the browser would display the state of the page as it was when the cache first acquired it. In a real application, that could mean that you are seeing data that is several months out of date. Now you see how I lied earlier. Gears does influence the way you code your application, but its requirements are about the same as those of any Javascript-heavy web app. As long as you separate your application presentation from your data, you should be fine.

Another thing that surprised and greatly pleased me was Gears' WorkerPool implementation. As everybody knows, it is impossible to run Javascript in the background in a normal web browser. I think that's because multi-threaded programming is hard, and Javascript can be pretty hairy as it is. I think that the browser designers have held off on implementing a threading solution out of fear that multithreaded Javascript would cause the apocalypse. As it turns out, though, Gears' implementation is both simple and powerful. Gears uses a message-passing mechanism for communication, with absolutely no shared state. This is great news. As far as I can tell, just as your main JS code has an event loop, each worker also has an event loop. Whenever a message is sent from the main JS code to a worker, that message is copied and onMessage is invoked on by that worker's event loop. Likewise, when a worker sends a message back to the main JS, the message is copied and onMessage is invoked on the main event loop. This has some interesting implications. For one, none of the workers have access to the DOM, or to global variables defined on the page, and cannot participate in closures with mainline Javascript code. By placing a concrete wall between your page and your workers, Gears forces you to think about the interactions that the page and the worker will have, and that's a Good Thing. I'm sure that it's still possible for threading to ruin you, it's just a lot harder with a scheme like this.

And that's it. There's more to Gears that what I described (though not much more). It also includes some geolocation bits (presumably for Android, and maybe Safari Mobile, integration), desktop integration stuff, a standards-compliant timer, a file multi-chooser (yay!), and a binary data type (as opposed to String, which is for textual content). It's a shame that Gears is still in beta. I would really like to see some sites that use it. Of course, since I just recently installed Gears, there might be some sites that do and I never realized it.

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