A concrete example of the virtual appliances approach to delivering SaaS is the Amazon Elastic Compute Cloud (EC2)—a grid of Xen hypervisor nodes coupled with the availability of pre-packaged virtual appliances in the Amazon Machine Image format. Amazon EC2 reduces the cost-barrier to the point where it becomes feasible to have each customer of the hosted service provisioned with their own virtual appliance instance(s) rather than forcing them to share common instances. Prior to EC2, single-tenant hosted models were too expensive, leading to the failure of many early ASP offerings.