There is the Cloud that provides value by requiring as few changes as possible. And there is the Cloud that provides value by raising the abstraction and operation level. The backward-compatible Cloud versus the forward-compatible Cloud.
The main selling point of the backward-compatible Cloud is that you can take your existing applications, tools, configurations, customizations, processes etc and transition them more or less as they are. It’s what allowed hypervisors to spread so quickly in the enterprise.
The main selling point of the forward-compatible Cloud is that you are more productive and focused. Fewer configuration items to worry about, fewer stack components to install/monitor/update, you can focus on your application and your business goals. You develop and manage at the level of application concepts, not systems. Bottom line, you write and deploy applications more quickly, cheaply and reliably.
To a large extent this maps to the distinction between IaaS and PaaS, but it’s not that simple. For example, a PaaS that endeavors to be a complete JEE environment is mainly aiming for the backward-compatible value proposition. On the other hand, EC2 spot instances, while part of the IaaS layer, are of the forward-compatible kind: not meant to run your current applications unchanged, but rather to give you ways to create applications that better align with your business goals.
Part of the confusion is that it’s sometimes unclear whether a given environment is aiming for forward-compatibility (and voluntary simplification) or whether its goal is backward-compatibility but it hasn’t yet achieved it. Take EC2 for example. At first it didn’t look much like a traditional datacenter, beyond the ability to create hosts. Then we got fixed IP, EBS, boot from EBS, etc and it got more and more realistic to run applications unchanged. But not quite, as this recent complaint by Hoff illustrates. He wants a lot more control on the network setup so he can deploy existing n-tier applications that have specific network topology/config requirements without re-engineering them. It’s a perfectly reasonable request, in the context of the backward-compatible Cloud value proposition. But one that will never be granted by a Cloud that aims for forward-compatibility.
Similarly, the forward-compatible Cloud doesn’t always successfully abstract away lower-level concerns. It’s one thing to say you don’t have to worry about backup and security but it means that you now have to make sure that your Cloud provider handles them at an acceptable level. And even on technical grounds, abstractions still leak. Take Google App Engine, for example. In theory you only deal with requests and not even think about the servers that process them (you have no idea how many servers are used). That’s nice, but once a while your Java application gets a DeadlineExceededException. That’s because the GAE platform had to start using a new JVM to serve this request (for example, your traffic is growing or the JVM previously used went down) and it took too long for the application to load in the new JVM, resulting in this loading request being killed. So you, as the developer, have to take special steps to mitigate a problem that originates at a lower level of the stack than you’re supposed to be concerned about.
All in all, the distinction between backward-compatible and forward-compatible Clouds is not a classification (most Cloud environments are a mix). Rather, it’s another mental axis on which to project your Cloud plans. It’s another way to think about the benefits that you expect from your use of the Cloud. Both providers and consumers should understand what they are aiming for on that axis. Hopefully this can help prevent shout matches of the “it’s a bug, no it’s a feature” variety.
[UPDATED 2010/3/4: Apparently, Steve Ballmer thinks along the same lines. Though the way he sees it, Azure is forward-compatible, while Amazon is backward-compatible: "I think Amazon has done a nice job of helping you take the server-based programming model - the programming model of yesterday, that is not scale-agnostic - and then bringing it into the cloud. On the other hand, what we're trying to do with Azure is let you write a different kind of application."]
[UPDATED 2010/3/5: I now have the quasi-proof that indeed Steve Ballmer stole the idea from my blog. Look at this entry in my HTTP log. This visitor came the evening before Steve’s “Cloud” talk at the University of Washington. I guess I am not the only one to procrastinate until the 11th hour when I have a deadline. Every piece of information in this log entry points at Steve Ballmer. How can it be anyone other than him?
184.108.40.206 - - [03/Mar/2010:23:51:52 -0800] "GET /archives/1198 HTTP/1.0" 200 4820 "http://www.bing.com/search?q=Brilliant+Cloud+Insight" "Mozilla/1.22 (compatible; MSIE 2.0; Microsoft Bob)"
(in case you are not fluent in the syntax and semantics of HTTP log files, this is a joke)]