Also, while the virtual appliance can contain the necessary software components within its virtual disk image, there's no way to accompany the executable image with contextual information—things like how many virtual CPUs the application should have, or how the networking should be set up. That all needs to be left to the person doing the installation to configure—certainly not unachievable or perhaps even a very large burden compared to the usual slog of installing and configuring individual software components, but definitely not plug-and-play, once again.

Finally, virtual appliances are well-suited for an individual virtual machine that contain all necessary software within itself, but they fall short of the ability to deliver a more complex topology. For example, it's easy to envision an application that would be best suited to a multi-virtual machine architecture, an N-tier architecture, for example. Today, such a complex architecture could be delivered as multiple virtual appliances, but it would have to be accompanied with written instructions regarding configuration that would need to be executed manually.

The logic for virtual appliances is unassailable, however. From the perspective of IT shops, virtual appliances address software installation and configuration—the last frontier of operations automation. From the perspective of software vendors, virtual appliances hold the promise of reducing support costs, a large proportion of which are correlated with initial application install. From the perspective of virtualization providers, virtual appliances represent a further buttressing of the virtualization value proposition. And, from the perspective of the future global IT infrastructure, where the boundary between internal data centers and external cloud computing resources is erased, virtual appliances represent the easiest way to integrate those two worlds—one software component that can run anywhere.

This is a situation that cries out for a standard, which would offer a way for all participants to gain the benefits of virtual appliances while reducing the friction of multiple, competitive, incompatible implementations. And, fortunately, the Distributed Management Task Force has launched just such an initiative, the Open Virtual Machine (OVF) Format. OVF addresses the three key issues that have previously hindered virtual appliance adoption. In addition, OVF supports multiple virtual image formats, so that virtual machines for any hypervisor may be carried within an OVF payload. Furthermore, OVF payloads are digitally signed, ensuring tamper-free distribution and user peace of mind.

The rise of cloud computing is likely to make OVF even more important. Being able to put a complete application payload onto a remote cloud service will be much more convenient than implementing individual virtual machines and attempting to manage the licensing nightmare of remote systems.

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