Xen is open source code distributed under the GPL Version 2, which requires a vendor to make its code changes freely available to customers. That means competitors only need buy one copy of, say, Citrix XenServer to examine the source code. In effect, all Xen changes in Citrix's proprietary product will be shared with the community. "We have no fear of making all the bits available," says Simon Crosby, CTO of the XenSource unit inside Citrix.
The Xen vendor community has "stumbled on a model that endorses differentiation by all vendors," he says, while collaboration continues on the core Xen hypervisor. The Xen project is devoted to "producing the best engine, not the entire car." Vendors will surround the Xen hypervisor with management and development tools, and applications for niches such as disaster recovery or energy conservation.
When Citrix acquired XenSource, control of the Xen project went to a new advisory board of the five top contributors plus representatives of Citrix, Hewlett-Packard, IBM, Intel, Novell, Red Hat, and Sun. Since then, Xen has moved rapidly to achieve parity in many respects with market leader VMware's hypervisor, ESX Server. As Xen becomes the basis for more proprietary product sets, it's likely to evolve faster than any open source effort could on its own.
A New Model: Open Source Software After It's Acquired