Virtual appliances are provided to the user or customer as files, via either electronic downloads or physical distribution.
[3] With the rise of virtualization as a platform for hosted services provision, virtual appliances have come to provide a direct route for traditional on-premises applications to be rapidly redeployed in a software as a service (SaaS) mode – without requiring major application re-architecture for multi-tenancy.
Traditional approaches to SaaS, such as that touted by Salesforce.com, leverage shared infrastructure by forcing massive change and increased complexity on the software stack.
Prior to EC2, single-tenant hosted models were too expensive, leading to the failure of many early ASP offerings.
Furthermore, in contrast to the multi-tenancy approaches to SaaS, a virtual appliance can also be deployed on-premises for customers that need local network access to the running application, or have security requirements that a third-party hosting model does not meet.