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copied!<p>A little warning, my comments below are not based on running production apps on Windows Azure; so far I have just fooled around with it and read docs.</p> <blockquote> <p>Is it possible to programatically scale up and down the number of instances in Azure?</p> </blockquote> <p>Yes, but AFAIK Azure doesn't scale up and down by itself, you'll have to do so via web service calls (much the same as using the UI to do it by hand). See this <a href="http://msdn.microsoft.com/en-us/library/ee460807.aspx" rel="nofollow noreferrer">MSDN doc</a>, the section "Hosted Services", and read on from there.</p> <p>Something like the <a href="http://hudson-ci.org/" rel="nofollow noreferrer">Hudson CI server</a> can fairly easily be expanded to do basic work queue monitoring. Perhaps you need something more feature rich; there are quite many such systems from the whole "enterprise service bus" world.</p> <blockquote> <p>Are there any non-obvious issues of scaling down the number of instances?</p> </blockquote> <p>Hmn. You need to think about locking, i.e. how to ensure that each worker leaves the queue in a consistent state and completes its tasks before shutdown?</p> <p>Additionally, from the Azure price list: "Partial compute hours are billed as full hours.".</p> <blockquote> <p>How much time does Azure takes to "acknowledge" the scaling down of your application?</p> </blockquote> <p>Honestly, I don't know, but assuming that your application has cleanly exited its work, I would <em>assume</em> it to be a couple of minutes at most. They instantiate new servers fast enough, and I'm guessing that starting new servers takes more time than shutting them down.</p> <p>Try testing it. Make a cheap account, and start a few servers, shut them down, etc. Build a small prototype of your app (or use one of the Azure sample apps) and get a feel for it.</p> <p>Another aspect is: What are your guarantees of being able to <strong>scale up whenever you want to?</strong> Currently, there is no such guarantee with any cloud computing vendor AFAIK; but Amazon has had a pretty good track record on this. Azure is a new offering, and we don't know how good Microsoft's capacity estimation is, i.e. we don't know how well they'll perform on this over the coming months. I <strong>expect</strong> this to be a non-problem; that Microsoft will handle this well -- but I have no evidence of this yet.</p>

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