Q. OK, but why is OpenStack any different?

A. There are several reasons why OpenStack is different and worthy of your attention. The first is that it really is open source. Each component has been released under the terms of the Apache licence, which is slightly more permissive than the GPL.

It means you can release any changes you make under a different licence, for instance, and this licence issue has been turned into a feature by the OpenStack team.

"We strongly believe that an open development model is the only way to foster badly-needed cloud standards, remove the fear of proprietary lock-in for cloud customers and create a large ecosystem that spans cloud providers," is part of their five-minute overview of the project.

With an open API, open formats and completely modifiable source code, you can see why, when so much money is being spent on moving to the cloud, OpenStack makes so much sense.

It's also relatively straightforward for companies to play with private clouds on their own networks, and then farm them out to OpenStack providers such as Rackspace when it begins to make sense.

Another reason is that OpenStack has become something of a phenomenon. You need only to look at the official list of the 180 companies that confess to using it. You'll find the likes of Dell, AMD, Cisco, HP, AT&T, Broadcom and Yahoo alongside Rackspace.

Sadly, NASA is currently moving away from OpenStack in favour of Amazon.

You'll also find a list of Linux heavyweights getting closer to OpenStack, including Red Hat, SUSE and Canonical. Red Hat previously had its own cloud solutions, including Aeolus, which is a neat software suite for deploying your own virtual machines internally and across multiple incompatible clouds.

It was finally tempted by OpenStack in April, after it moved to a new foundation governance model.

Q. But we thought Canonical had made a big deal over using Eucalyptus for its cloud?

A. It did, originally. But about a year ago, the company behind Ubuntu announced it was replacing Eucalyptus with OpenStack.

The decision seems to have been made because Eucalyptus isn't as open as OpenStack. It's open source, but there's six months between each release, for example. OpenStack, on the other hand, is developed in the open and doesn't have a proprietary component.

Q. So there's more than one component to OpenStack?

A. Yes, there are actually three. The first is called OpenStack Compute. This is the 'Nova' alluded to in that initial blog post, and it's the infrastructure part. It's where the virtual machines actually live.

Compute provides API access to their configuration and management. It's where the developers can not only get involved with the virtual hardware, they can also build for scaling and concurrency.

Despite the low-level nature of these kinds of operations, Compute is written in Python, and many developers choose to use the Python bindings. There is wide-ranging support for the different virtual machine backends - a part known as the hypervisor.

This is in contrast to Eucalyptus, which supports only Xen and KVM, and that was after considerable help from Canonical.

OpenStack will work with KVM and Xen, but also VMWare, LXC (Linux Containers - look out for a tutorial next month), User Mode Linux and even Qemu, although the documentation admits the last two are best used for development purposes rather than live.

Q. And what are the other components?

A. With the infrastructure being handled by Nova, the other major requirement for a cloud system is storage. Unlike the computer you're reading this on, storage in the cloud needs to cater for distributed systems, processes and dynamic capacity.

This component in OpenStack is called Object Storage, or Swift, and it was initially developed by Rackspace. Like Nova, it's also written in Python.

It caters for both file (object) and block storage, which can be used for persistent storage by the hypervisor.

There are many advanced features, such as the ability to easily add extra capacity, or heal failures automatically, and it can scale to sizes still unfamiliar to most users - multiple petabytes and billions of objects.

Finally, Swift is tied to Nova using the OpenStack Image Service, which handles the discovery, registration and delivery for virtual disk images.