I was lucky enough to look at a huge number of virtual appliances over the past few weeks. If you have not come across one yet, a virtual appliance is a virtual machine set up to handle a particular application. Like a traditional hardware appliance, a virtual one might come with a web-based management interface and be designed so customers don’t need to work directly with the virtual appliance’s operating system.

Virtual appliances have some distinct advantages over traditional appliances and servers. So many, in fact, that the age of the traditional hardware appliance could be coming to an end. And not before time, because although hardware appliances are usually much easier to deploy and manage than traditional full function servers, they are wasteful when it comes to power consumption, heat output and space requirements.

Each appliance also needs cabling to networks and power outlets, and before you can use them, most also need you to connect a serial interface to them to assign an IP address. This last point may seem trivial, but finding a PC with a serial port and a copy of HyperTerminal that’s within a cable’s length of a new appliance is not always easy.

Plus, several appliances we have seen in IT Week Labs over the last year rely on you buying two appliances if you want a reasonable degree of fault tolerance.

In contrast, virtual appliances can be run on whatever hardware you see fit, ranging from a cheap single-socket desktop for appliances that you just want to evaluate, through to extremely fault-tolerant multiprocessor server hardware and SAN storage for a virtual appliance that you need to be constantly available.

Like traditional appliances, virtual ones can automatically download updates and patches if they are needed. Some of the more interesting ones I have seen go one step further and integrate software running on a separate system with the virtual appliance. For example, the beta version of a desktop firewall I looked at uses a Windows .Net-based GUI to configure the firewall rules. The GUI is then used to generate a new read-only virtual appliance that actually runs the firewall software. The result is a very compact firewall – less than 5MB in size – that has no management interface. Configuration changes are entered into the Windows GUI, which then generates a new virtual machine based on that configuration. That’s one trick a hardware appliance can’t do.

There will be much more news about virtual appliances in the coming weeks, particularly following the announcement of a prize of $100,000 from virtualisation specialist VMware for the best virtual appliance. The winner will be announced today (August 14).