Doing IT the App Engine Way

Google first announced its App Engine technology in 2008. The Internet's search superstar will host your Web application (webapp) on its infrastructure, initially at no charge. Only when your webapp gets “busy” will Google start charging you. By “busy”, Google means something in the range of five million “page views” per month. Hit that threshold, and Google will come looking for your credit-card details.

Apps for Google's App Engine are written in Python (with Java recently added to the mix). As most of you know, Python's creator (Guido van Rossum) works for Google and spends a reported 50% of his time working on Python's ongoing improvement and the developer ecosystem that surrounds this increasingly popular general-purpose programming and scripting technology.

Unlike most other webapp development technologies and frameworks that require you to host your webapp yourself (or find a friendly cost-effective ISP to host your webapp and its dependent technologies for you), Google's App Engine abstracts away the hosting part. Simply build your webapp to the App Engine standard, upload it to Google, and it's then deployed in the Google “cloud”. Google handles backups, load balancing, spikes in access, deployment, caches and the like. All you have to worry about is your code, as there's no more deployment distractions. And, when it comes to App Engine, it's all about the code, which is just how we programmers like things, isn't it?

In this article, I explain how to build a very simple App Engine project using the Python API. By the time you've worked through to the end, you should know enough to be in a better position to decide if Google's App Engine is something you want to spend more time learning.

Most modern webapp technologies are organized around the Model-View-Controller pattern (MVC). In essence, an MVC-compliant webapp is divided into three parts: the model handles interactions with your webapp's data, the view looks after the user interface, and the controller provides the application or business logic that glues everything together. In theory, changes to how your webapp looks (the view) should not significantly affect how your webapp stores its data (the model), or at least any effects should be minimal and localized. Of course, the MVC pattern can be applied to any application domain, but it is particularly appropriate within the Web context, as each component is (typically) physically separate from the others. For instance, your view code runs within a browser on your client PC. Your controller code runs on your Web server, and your model may be deployed on some sort of datastore, which may or may not run on its own hardware.

App Engine conforms to the MVC pattern, and what's nice about App Engine is that each of the MVC components is realized in code files that are easy to keep separate from one another. You can put your code into one big source code file if you like, but to be honest, all but the most trivial webapps benefit from splitting out each MVC component into its own source file.

Of course, App Engine doesn't really care how you structure your code (only that it's correct), nor does App Engine care how your webapp operates. If you want to write a basic CGI webapp, you can. You can use Python's WSGI standard too, which is the standard we'll use here. So, let's learn what's involved by building a simple MVC-patterned webapp with Google App Engine.

To build an App Engine webapp, you need two things: Google's App Engine SDK and release 2.5 of Python.

Although you'll deploy your App Engine webapp to Google's cloud, during development, it is possible to test your webapp on your local machine. All you need is a copy of the App Engine SDK, which is easy to get. Go to the App Engine download page (see Resources) and click on the link for the Python SDK. From the page displayed, select the Linux download (which is a ZIP file at the time of this writing). The current App Engine documentation as well as a Google Plugin for Eclipse also are available for download (should such things interest you).

With the download complete, installation is a breeze. Simply unzip the downloaded file into a directory of your choosing. I unzipped into my HOME directory, and a new directory called google_appengine was created.

Since the end of 2008, Python comes in three distinct flavors: the standard 2.5 release (which maintains backward-compatibility with the installed base), the 2.6 release (which is designed to bridge the move from the 2.5 release to future releases) and release 3 (which is the new, backward-incompatible Python). App Engine targets a customized and optimized version of the 2.5 release of Python that runs in Google's cloud. You can try to run your webapp under 2.6, but my experience has been less than satisfactory, although it can sometimes work. The 3.1 release of Python currently is not supported, so don't waste your time trying to use it with App Engine.