Interaction designers know that if you use a computer to control any other kind of device or process, the computer takes over. These computers can range in complexity from embedded chips on credit cards to globe-spanning distributed clusters. As long as it is possible to describe, and as long as economy favours them over a traditional mechanical or manual process, computer- and software-driven systems will prevail.

Because of the tendency for computers to usurp the behaviour of whatever they are attached to, it is useful to know how they themselves behave. The purpose of this series of articles is therefore to outfit interaction designers with an understanding of the basic properties and behaviours of computers, to help them make more informed designs and interact more productively with software engineers.

Promises Kept

Our concern as interaction designers, with regard to computational feasibility, is primarily around closing the gap between what we promise the people who use and who are affected by the software-driven product and what we deliver.

Some of these promises are explicit, but many are not. For instance, when we present a shutter button on a digital camera, we make a tacit pledge to capture what is in the frame the moment the button is pressed, as has been the case for a century. In practice, this is a difficult promise to keep. We commit in similar ways when we offer to search a computer's hard drive for a piece of data like a telephone number, or expose a hypertext link on a Web site.

As often as not, computer-driven artifacts and processes offer us immediacy and then leave us to wait. They offer completeness and produce glaring omissions. They offer us certitude and return fallacies. They offer us reliability and then summarily collapse. Whoever coined the maxim computers never lie has obviously never encountered a computer.

If we arm ourselves with an understanding of how these machines work, we can help keep the word of the systems we design, or at least not unduly inflate the expectations of the people who come in contact with them.