Computer Appliance - Internal Structure

Internal Structure

There are several design patterns adopted by computer appliance vendors, a few of which are shown below. Since the whole concept of an appliance rests on keeping such implementation details away from the end user, it is difficult to match these patterns to specific appliances, particularly since they can and do change without affecting external capabilities or performance.

1. Special chip - the vendor builds an ASIC, without any separate "software" or operating system. The appliance has a limited interface, usually terminal console or web-based, to allow some basic configuration by the IT staff. The manufacturer often provides some way of accessing deeper configuration mechanisms. Azul Systems' Vega 3 Java Compute Appliance is an example; special hardware modifications to the chip enable Java application scaling.
2. Special software kernel - the vendor uses or creates a general-purpose computer, and designs a new operating system that integrates the application into the operating system. Cisco's IOS is an example; the Unix-like operating system has firewall functions and network/firewall configuration commands built into it. Sometimes, the device is also sealed, so the consumer has no access to reinstall the operating system or replace it with another operating system. The consumer may also be restricted to a small group of configuration commands, while the more detailed and lower level functions of the operating system are only available to the vendor. The more this "locked down" approach is carried out, the closer this type of device comes to appearing like an ASIC device.
3. Specialized application - off-the-shelf computers and operating systems are used, but the user interface and "box" are designed so the user cannot access anything on the computer, except for the application interface that the vendor has created. Since the underlying computing architecture is locked down and essentially invisible, it becomes difficult to discern that the device really functions on top of general purpose hardware and operating systems. Linux, and BSD to a lesser degree, has become the operating system of choice for this type of appliance. Recently the term software appliance has also been used to describe such a pre-packaged, black-box combination.
4. Virtual appliance - here the hardware itself has disappeared entirely and become a so-called virtual appliance (also known as a virtual software appliance) using any one of a number of virtual machine technologies. Within this virtual machine is essentially the same stack of software plus the operating system as in the "specialized application" alternative.

Sometimes, these techniques are mixed. For example, a VPN appliance might contain a limited access software firewall running on Linux, with an encryption ASIC to speed up VPN access.

Some computer appliances use solid state storage, while others use a hard drive to load an operating system. Again, the two methods might be mixed—an ASIC print server might allow an optional hard drive for job queueing, or a Linux-based device may encode Linux in firmware, so that a hard drive is not needed to load the operating system.