Over the last five years, since the release of the first open-source version of OpenNebula in March 2008, we have been involved in many presentations, discussions and meetings where people wanted to know how OpenNebula compares with the rest of open-source Cloud Management Platforms (CMPs), mostly with Eucalyptus and OpenStack. The most common understanding is that all CMPs are competing in the same market, trying to fill the same gap. Consequently, people jump to the wrong conclusion that after years of a fierce competition, there will only be one winner, a single open-source CMP in the market. However, as discussed by Joe Brockmeier in his post "It's Not Highlander, There Can Be More Than One Open Source Cloud", there is room in the market for several open-source CMPs that, addressing different cloud niches, will fit together into a broad open cloud ecosystem.

We have prepared this article to briefly describe our experience about the different types of cloud models, and our view about how the main open-source CMPs are targeting their needs. Do not expect a table comparing side-by-side the size of the communities, technical features of the different tools, or the management structure of the projects. We have tried to focus only on their general approaches, on their overall position in the cloud market, and, of course we have tried to be as neutral as possible.

Two Different Cloud Models
Although there are as many ways to understand cloud computing as there are organizations planning to build a cloud, they mostly fall between two extreme cloud models:

• Datacenter Virtualization: On one side, there are businesses that understand cloud as an extension of virtualization in the datacenter; hence looking for a vCloud-like infrastructure automation tool to orchestrate and simplify the management of the virtualized resources.
• Infrastructure Provision: On the other side, there are businesses that understand cloud as an AWS-like cloud on-premise; hence looking for a provisioning tool to supply virtualized resources on-demand.

Yes, we know, we said that we wanted to focus only on open-source CMPs. However we have intentionally used two of the principal cloud "products", VMware vCloud and AWS, a proprietary CMP and a cloud service, because they are the most well known implementations of both models. We will go even a step ahead and claim that most of the users in the first cloud model explicitly express their willingness to find an open alternative to vCloud because it is too expensive, because they want to avoid vendor lock-in, or because it cannot be adapted to meet their needs. Equally, users in the second cloud model explicitly mention Amazon as the type of cloud they want to build internally.

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