Running a Service in the Azure Cloud
How to build an Azure calendar aggregator - Part 1
By: Jon Udell
Dec. 29, 2008 03:10 PM
For over two weeks now, I’ve been running a service in the Azure cloud that aggregates calendar events from Eventful.com and from a diverse set of iCalendar feeds. My aim is to recreate and then extend my experimental elmcity.info community information hub, while exploring and documenting the evolution of Azure and the layered services emerging on top of it.
I haven’t written a whole lot about programming for a while, because I’ve trying to to explain the whys and wherefores of syndication-oriented communication to a wider audience. But as I build out this service I’m learning a lot about cloud-based software development in general, and about Azure in particular, and I want to narrate this work. I’ll try to do it in a way that will inform developers who currently use Microsoft tools and technologies, as well as those who don’t. But I’ll also try to be accessible to folks who don’t write software, yet would like to learn something about the opportunities that cloud computing is creating as well as the challenges it poses.
The service, as it currently exists, is running as an Azure worker role. That means it does input, processing, and output, but presents no user interface. The inputs are Eventful.com, accessed by way of its API, and a growing set of public iCalendar feeds. The processing involves reading calendar events and normalizing them to a common intermediate format. The output is currently XML to the Azure blob store, one file for Eventful and another for the iCalendar feeds.
I’m only allocating one instance of this worker process, and that’s probably enough horsepower for any single community’s events. But I’d like to be able to scale out the aggregator to serve other communities as well, potentially many others. Turning up the dial to do that would be a nice illustration — and test — of the cloud computing fabric.
The existing aggregator at elmcity.info is written in Python, and my original plan was to port it with minimal change to IronPython on Azure. That didn’t work out because, although bare-bones IronPython code runs on Azure as I show here, you quickly run into restrictions imposed by Azure’s security sandbox. The trust policy, defined here, is based on a feature of the .NET platform known as code access security (CAS).
When you upload code to the Azure cloud, or run it in the local development fabric, the hosting environment only partly trusts your code, and also only partly trusts any components used by your code. This is part of a layered, defense-in-depth security strategy, prudent for the same reason that it’s prudent to run your own computer as a partly-trusted user instead of an all-powerful administrator. It is also problematic for the same reason. A lot of Windows applications used to require administrative privilege in order to run properly, and some — though fewer month by month — still do. Similarly, a lot of .NET components that run happily in the fully-trusted environment of your local computer won’t run in Azure’s medium-trust environment, or (what’s nearly equivalent) in Internet Information Server 7 (IIS 7) when its security mode is set to medium trust.
I am no expert on the subject of code access security, but here’s what I think:
I think that IronPython is likely to become such a component, once the dust settles from the recent 2.0 release. (If you care about this issue, you can vote up its priority.) Meanwhile I’ve been working in C#, which has been a fascinating experience. On the one hand, I believe that dynamic languages like Python are excellent choice for agile development everywhere, and especially in the fluid environment of the cloud. On the other hand, I’m not a language bigot and have always appreciated the virtues of statically-typed languages.
My basic philosophy has always been to use a mix of best-of-breed tools in order to gain maximum leverage. The combination of IronPython and C#, on the .NET platform, is a really powerful one, for the same reason that the Jython/Java combo is. On this project, even though I am not yet deploying any code written in IronPython, I often use IronPython to test C# components that I’ve written or acquired.
Along the way, I’ve been recalling something IronPython’s creator, Jim Hugunin, said at the Professional Developers Conference back in October. Jim’s talk followed one by Anders Hejlsberg, the creator of C#. Anders showed an experimental future version of C# that makes use of the Dynamic Language Runtime which supports IronPython and IronRuby on .NET. The effect was to create an island of dynamic typing within C#’s otherwise statically-typed world. We all appreciated the delicious irony of a static type called ‘dynamic’.
Jim might have sounded a bit wistful when he said: “I’m not sure what a dynamic language is any more.” But I think this blurring of boundaries is a wonderful thing. Many smart people I deeply respect value the static typing of C#. Some of the same smart people, and many different ones, value the dynamic typing in languages like Ruby and Python. If I can leverage the union of what all of those smart people find valuable, I’ll happily do so.
I’ll have more to say about this project, and of course code to share, as things evolve. Meanwhile, though, I want to acknowledge Doug Day at DDay Software. When I switched from Python to C#, the key component I needed was an iCalendar module equivalent to MaxM’s excellent Python iCalendar module, which I’m using at elmcity.info. Doug’s DDay.iCal met the need. It’s a solid, cleanly-built, open source .NET component that enables code written in any of the .NET family of languages to parse, and generate, iCalendar (RFC 2445) files.
And now back to the project, which reminds me of the era at BYTE during which I got to build stuff while writing about what I was building. It’s great fun. And as John Leeke so eloquently says, it engages the mind, the hands, and the heart.
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