Someone asked me what actually was cloud computing, and I found it rather difficult to answer concisely and coherently.  Much arm waving was involved, and comments like “you know, like Amazon”, and “you get billed for your usage”.  I thought it was worth looking at a clear, simple definition!

What is cloud computing?

Cloud computing is

“a computing capability that provides an abstraction between the computing resource and its underlying technical architecture (e.g., servers, storage, networks), enabling convenient, on-demand network access to a shared pool of configurable computing resources that can be rapidly provisioned and released with minimal management effort or service provider interaction.”

according to the National Institute of Standards and technology.  Thats fine, and the more technical among us could probably leave it at that.  How would you try describing cloud computing to your CEO, though?

Cloud computing can be thought of as a way of pooling the resources of a number of computers together, and then pouring those resources into whatever applications you decide to run on the top of them in a very simple way, allowing you to set up new applications very quickly and easily.  Instead of having to build a new server for a new application, you can pour some more resource from the cloud for it.  If the resources in the cloud are running low, just add some more servers to the pool.

In the past, there have been a variety of mechanisms that do parts of this, but haven’t applied it all together. 

Older, similar technologies

Clustering computers has been done for some time.  This allows computers running a application to work together, providing redundancy for the application, and generally some performance improvement.  How does this differ from a cloud?  Well, a cluster was generally quite complex to set up, and only really focussed on a single application.  You couldn’t pour resources from all the machines into a new application quickly at all.

Web farms can support many web applications, running over lots of web servers.  How does this differ from a cloud?  Well, web farms again tended to be hard to configure, and it was very difficult to apply specific levels of resources to specific web sites – if one web application used lots of processing power, the others could start to run more slowly.  Administration and maintenance to balance these problems, and to roll out new applications were often complex.

Virtual server providers are almost like mini clouds, where running virtual machines are provisioned simply and easily across the available hardware.  How do they differ from cloud computing?  Well, cloud computing separates off storage and processing power, allowing you to apply more storage or more processing power as needed to any application running on the infrastructure.  It adds another level of abstraction, which allows much more effective (and quick and easy) application of the necessary storage space and processing power from all the resources pooled in the cloud, rather than limiting it to a single virtual server, which can then be quiite complex to change.

All of these technologies really combine to provide a simple infrastructure on top of a pool of servers – you have the performance and resilence of a cluster, without the headaches of setting it up, and without the limitations of the focus on a single application.  You have the distibution of a web farm, allowing multiple applications to share resources, but with much easier tools to support rolling out new applications or providing additional resources to underperforming ones.  You have the benefits of virtual servers, but with better applications of storage space and processing power to your actual applications.

Confusing technologies and terms

There are two points that many people don’t quite understand about cloud computing, even if they grasp how it can be applied.  These are:

Does a cloud have to be on the Internet?

No!  A cloud can be run inside a private corporation, or even at home if you have the resources.   Ubuntu provide cloud deployment and management tools as part of their open source enterprise servers.   The reason cloud computing is synonomous with the internet in people’s minds is that the real benefits of cloud computing require massive, massive deployments of hardware to see the real benefits and cost savings.  Having said that, even a small cloud might be better for supporting a slew of internal web applications than 15 individual servers.

Because of the costs of a large cloud, only companies like Amazon have successfully deployed them as yet.  They use clouds to sell processing power and storage space to any one who wants to buy them.

Will I be charged by processing use or storage space?

This is actually a separate concept, called Utility computing, where you are charged for your system use like you are charged for gas, or electricity, hence the name.  This is by no means integral to the concept of cloud computing – though Amazon EC2, the biggest cloud deployment, does use this model to charge for access.  It doesn’t mean every has to charge this way, and an internal corporate cloud would just cost electrical consumption, hardware and licensing.

Don’t confuse the two – utility computing is a model of charging for computer use and storage space, while cloud computing is an infrastructure for pooling computer resources to be poured simply into applications on demand.  They may both be used at the same time, but aren’t the same thing.