Brittle (software)

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The term brittle, when applied to software, means software that may appear reliable, but will fail badly when presented with unusual data.

Brittleness in software can be caused by algorithms that do not work well for the full range of input data. A good example is an algorithm that allows a divide by zero to occur, or a curve fitting equation that is used to extrapolate beyond the data that it was fitted to. Another cause of brittleness is the use of data structures that restrict values. This was commonly seen in the late 1990s as people realized that their software only had room for a 2 digit year entry; this lead to the sudden updating of tremendous quantities of brittle software before the year 2000. Another more commonly encountered form of brittleness is in graphical user interfaces (GUIs) that make invalid assumptions. For example, a user may be running on a low resolution display, and the software will open a window too large to fit the display. Another common problem is expressed when a user uses a color scheme other than the default, causing text to be rendered in the same color as the background, or a user uses a font other than the default, which won't fit in the allowed space and cuts off instructions and labels.

More subtle forms of brittleness commonly occur in artificial intelligence systems. These systems often rely on significant assumptions about the input data. When these assumptions aren't met (and as they may not be stated, this could be an easy case), then the system will respond in completely unpredicable ways. This is not surprising when you consider that many forms of artificial intelligence are based on the same principles as curve fitting.

See also: Ariane 5 Flight 501, where an overflow in a velocity calculation shut down a rocket's navigational system. Corner case, a common area where systems are brittle.