Category Archives: OVM

“If you have a stove, a saucepan and a bottle of cold water, how can you make boiling water?”

If you ask this question to a mathematician, they’ll think about it a while, and finally tell you to pour the water in the saucepan, light up the stove and put the saucepan on it until the water boils. Makes sense. Then ask them a slightly different question: “if you have a stove and a saucepan filled with cold water, how can you make boiling water?”. They’ll look at you and ask “can I also have a bottle”? If you agree to that request they’ll triumphantly announce: “pour the water from the saucepan into the bottle and we are back to the previous problem, which is already solved.”

In addition to making fun of mathematicians, this is a good illustration of the “fake machine” approach to utility computing embodied by Amazon’s EC2. There is plenty of practical value in emulating physical machines (either in your data center, using VMWare/Xen/OVM or at a utility provider’s site, e.g. EC2). They are all rooted in the fact that there is a huge amount of code written with the assumption that it is running on an identified physical machine (or set of machines), and you want to keep using that code. This will remain true for many many years to come, but is it the future of utility computing?

Google’s App Engine is a clear break from this set of assumptions. From this perspective, the App Engine is more interesting for what it doesn’t provide than for what it provides. As the description of the Sandbox explains:

“An App Engine application runs on many web servers simultaneously. Any web request can go to any web server, and multiple requests from the same user may be handled by different web servers. Distribution across multiple web servers is how App Engine ensures your application stays available while serving many simultaneous users [not to mention that this is also how they keep their costs low — William]. To allow App Engine to distribute your application in this way, the application runs in a restricted ‘sandbox’ environment.”

The page then goes on to succinctly list the limitations of the sandbox (no filesystem, limited networking, no threads, no long-lived requests, no low-level OS functions). The limitations are better described and commented upon here but even that article misses one major limitation, mentioned here: the lack of scheduler/cron.

Rather than a feature-by-feature comparison between the App Engine and EC2 (which Amazon would won handily at this point), what is interesting is to compare the underlying philosophies. Even with Amazon EC2, you don’t get every single feature your local hardware can deliver. For example, in its initial release EC2 didn’t offer a filesystem, only a storage-as-a-service interface (S3 and then SimpleDB). But Amazon worked hard to fix this as quickly as possible in order to be appear as similar to a physical infrastructure as possible. In this entry, announcing persistent storage for EC2, Amazon’s CTO takes pain to highlight this achievement:

“Persistent storage for Amazon EC2 will be offered in the form of storage volumes which you can mount into your EC2 instance as a raw block storage device. It basically looks like an unformatted hard disk. Once you have the volume mounted for the first time you can format it with any file system you want or if you have advanced applications such as high-end database engines, you could use it directly.”

and

“And the great thing is it that it is all done with using standard technologies such that you can use this with any kind of application, middleware or any infrastructure software, whether it is legacy or brand new.”

Amazon works hard to hide (from the application code) the fact that the infrastructure is a huge, shared, distributed system. The beauty (and business value) of their offering is that while the legacy code thinks it is running in a good old data center, the paying customer derives benefits from the fact that this is not the case (e.g. fast/easy/cheap provisioning and reduced management responsibilities).

Google, on the other hand, embraces the change in underlying infrastructure and requires your code to use new abstractions that are optimized for that infrastructure.

To use an automotive analogy, Amazon is offering car drivers to switch to a gas/electric hybrid that refuels in today’s gas stations while Google is pushing for a direct jump to hydrogen fuel cells.

History is rarely kind to promoters of radical departures. The software industry is especially fond of layering the new on top of the old (a practice that has been enabled by the constant increase in underlying computing capacity). If you are wondering why your command prompt, shell terminal or text editor opens with a default width of 80 characters, take a trip back to 1928, when IBM defined its 80-columns punch card format. Will Google beat the odds or be forced to be more accommodating of existing code?

It’s not the idea of moving to a more abstracted development framework that worries me about Google’s offering (JEE, Spring and Ruby on Rails show that developers want this move anyway, for productivity reasons, even if there is no change in the underlying infrastructure to further motivate it). It’s the fact that by defining their offering at the level of this framework (as opposed to one level below, like Amazon), Google puts itself in the position of having to select the right framework. Sure, they can support more than one. But the speed of evolution in that area of the software industry shows that it’s not mature enough (yet?) for any party to guess where application frameworks are going. Community experimentation has been driving application frameworks, and Google App Engine can’t support this. It can only select and freeze a few framework.

Time will tell which approach works best, whether they should exist side by side or whether they slowly merge into a “best of both worlds” offering (Amazon already offers many features, like snapshots, that aim for this “best of both worlds”). Unmanaged code (e.g. C/C++ compiled programs) and managed code (JVM or CLR) have been coexisting for a while now. Traditional applications and utility-enabled applications may do so in the future. For all I know, Google may decide that it makes business sense for them too to offer a Xen-based solution like EC2 and Amazon may decide to offer a more abstracted utility computing environment along the lines of the App Engine. But at this point, I am glad that the leaders in utility computing have taken different paths as this will allow the whole industry to experiment and progress more quickly.

The comparison is somewhat blurred by the fact that the Google offering has not reached the same maturity level as Amazon’s. It has restrictions that are not directly related to the requirements of the underlying infrastructure. For example, I don’t see how the distributed infrastructure prevents the existence of a scheduling service for background jobs. I expect this to be fixed soon. Also, Amazon has a full commercial offering, with a price list and an ecosystem of tools, why Google only offers a very limited beta environment for which you can’t buy extra capacity (but this too is changing).