OpenStack "is the fastest-growing open source project in history. It's the Linux of cloud computing," Kemp said.

Mickos countered by saying many of OpenStack's supporters are hardware vendors, such as HP, IBM, Intel, AMD, Dell, NEC, Brocade, and Juniper. Enterprise cloud builders shouldn't expect them to produce the best cloud software. "Linus Torvalds worked on Linux for 10 years before he attracted a company to support him," and that company was "little Red Hat," not IBM or HP, he pointed out. Big-vendor support for OpenStack doesn't automatically translate into a successful software system, he continued.

Eucalyptus will build out API support for 80-90% of the cloud suppliers in the marketplace. "You can't get more open than that," Mickos asserted in turn. Amazon's APIs were "de facto standards in the marketplace" that Eucalyptus would continue to follow, he added.

"When you say 'open,' you don't have control of Amazon APIs. I don't think a de facto standard is a standard," countered Kemp.

The panel's moderator, Jo Maitland, a GigaOm Pro research director, challenged all three speakers to answer "which open source project has the most developers?"

Dholakia said CloudStack shouldn't be evaluated by its number of developers because it was a kernel of code surrounded by services. Kernel development needs to be lead by a tightly organized team of developers in its early stages, with open source contributions flowing in to augment that development after the kernel is in place. "It's extraordinarily early in the cloud game," he said. "The number of cloud developers today is paltry compared to what it will be in a short while ... It's foolhardy to pick a winner based on the number of developers at this stage," he said.

But numbers of developers is a factor that plays in OpenStack's favor; Kemp had a different answer. "Innovation is really the key here. It isn't just a code base that makes a community." OpenStack with its Essex release in May had 250 contributors, he added, throwing out a figure that neither CloudStack or Eucalyptus could match.

"There's something to be said for an incredibly tight team of developers leading a community, allowing an ecosystem to be created around it," countered Dholakia.

He then challenged Kemp to name the large implementers of OpenStack. Co-founder Rackspace is one and HP is another. But there have been few non-technical company implementations because the two-year-old project's code is still raw. In some cases, getting one code module to work in coordination with another is still difficult, but Kemp said the influx of new members to the project with integration expertise will solve the problem over the coming year.

As it begins to mature, Eucalyptus should include support for OpenStack among its supported APIs, Kemp asserted. It's not part of Eucalyptus today.

The wary Mickos countered: "Which APIs will we support? Fortunately, I don't have to make such decisions. We'll support any cloud APIs once they get to 30% of the market," he said.

"So it's just a matter of time," responded Kemp.

"It's just a matter of time," Mickos nodded, although the reserved nature of his assent suggested he foresaw a much longer time lapse than Kemp did.

The nature of the respective cloud packages then came under discussion. OpenStack is sometimes described as a loosely coupled platform of modules written in Python, in contrast to the more monolithic Eucalyptus and CloudStack, both written in Java.

"Monolithic versus modular," mused Dholakia. "If you're trying to serve customers, it very difficult to string 17 projects together and get them running," he said in his shorthand for "modular."

"You're not the Linux of cloud computing," added Mickos. "You're the Unix," he said, getting chuckles from a Silicon Valley crowd that recalled how Unix fractured into competing versions and user interfaces.

At this point, Mickos and Dholakia were frequently supporting each other in the face of Kemp's assertions. "Let's ask the customers" about which project's code will be adopted, Mickos said, urging Kemp to start with his former agency, NASA. NASA CIO Linda Cureton recently cited Amazon as a boon to the agency that had allowed it to save "almost a million dollars."