VMware has been stung into a response after weeks of attack on its cloud offerings from open source rivals.

VMware reminded customers on their website of the company's market-leading position in private cloud deployments, and seemingly takes some jabs at open source cloud platforms such as OpenStack, CloudStack and Eucalyptus, marking what could be an intensifying debate between open source and proprietary clouds.

As the open source players have been "jockeying for position," VMware's network of providers that offer public cloud compatibility with on-premise VMware-powered infrastructure has more than 100 public cloud vendors, it announced. The blog post plays on a theme of a fairy tale, equating the open source providers as "the three ugly sisters."

"While the ugly sisters were squabbling, customers were getting on with business and choosing their Cinderella as VMware," director of VMware's cloud platforms Mathew Lodge said, adding that the 100 partners and 350,000 customers using VMware products is "an order of magnitude greater than the ugly sisters' combined total."

There have been a variety of announcements in recent weeks related to open source cloud development platforms from those three open source players. Citrix announced that its plans to give CloudStack an Apache Software Foundation licence, in effect creating a competing open source platform to OpenStack, which itself released the fifth version of its software, codenamed Essex. Citrix officials later played down the competition between CloudStack and OpenStack, and instead attempted to frame the debate against VMware and other proprietary offerings. "This is about proprietary versus open source, and open source is going to win," Peder Ulander, vice president of product marketing for Citrix's Cloud Platforms Group, said recently.

Now, VMware is firing back. VMware writes that its family of public cloud partners, which include public cloud offerings in 24 countries, is "effectively forming the world's largest community of compatible public clouds." VCloud has grown from having five providers 18 months ago to doubling the number of providers it had in a one-year period, and now surpassing the 100-provider mark, it says.

Still though, competitors criticise VMware for the notion of locking the customers into that ecosystem. Lodge rejects those criticisms. "Openness to us means customers have a choice of where to run their clouds," he said. "Having 100 providers in the vCloud ecosystem gives customers more choices than really any other vendor, and we try to make it as easy as possible to move your workloads and data."

The common APIs, combined with using the vCloud Connector, make migrating data between VMware-powered private clouds and public clouds from VMware partners easy. Providers must prove an ability to upload and download data into the cloud using the vCloud Connector in order to be a partner in the ecosystem, Lodge says. "Openness is not about how you write software, it's about what you allow your customers to be able to do," Lodge says.

But open source backers say VMware's offerings do not provide true freedom of choice for customers. "They offer choice; it's choice as long as it's powered by vCloud," says Sameer Dholakia, group vice president and GM, Cloud Platforms Group, Citrix. "In our world, we believe open means open source," which he says is one of the reasons Citrix gave its CloudStack platform an Apache license. Dholakia says calling the 350,000 customers using VMware server virtualisation technology private clouds is "a gross abuse" of the term.

Another claim by some of the open source providers - particularly CloudStack and Eucalyptus - focus on their compatibility with Amazon Web Services. Lodge says despite AWS recently inking a partnership to advance API compatibility with Eucalyptus, that it's not as easy to migrate workloads out of the AWS system as the VMware ecosystem network is. "It's very challenging to have a hybrid cloud strategy when there's no easy way to easily move from the public to the private cloud," he says in relation to AWS.

Meanwhile as VMware is seemingly taking shots at open source IaaS providers, the company is taking a different approach related to its platform as a service offering, CloudFoundry, which the company celebrated the one-year anniversary of this week. The open source PaaS offering accepts outside code from a developer community, with one of its chief advantages, backers say, being its ability to run on a variety of infrastructure platforms.