1. Software is developed or engineered, it is not manufactured in the classical sense.
Although some similarities exist between software development and hardware manufacture, the two activities are fundamentally different. In both activities, high quality is achieved through good design, but the manufacturing phase for hardware can introduce quality problems that are nonexistent (or easily corrected) for software. Software costs are concentrated in engineering. This means that software projects cannot be managed as if they were manufacturing projects.
Figure 1: Failure Curve for hardware
2. Software doesn’t “wear out.”
Figure 1 depicts failure rate as a function of time for hardware. The relationship, often called the “bathtub curve,” indicates that hardware exhibits relatively high failure rates early in its life (these failures are often attributable to design or manufacturing defects); defects are corrected and the failure rate drops to a steady-state level (ideally, quite low) for some period of time. As time passes, however, the failure rate rises again as hardware components suffer from the cumulative affects of dust, vibration, abuse, temperature extremes, and many other environmental maladies. Stated simply, the hardware begins to wear out.
Software is not susceptible to the environmental maladies that cause hardware to wear out. In theory, therefore, the failure rate curve for software should take the form of the “idealized curve” shown in Figure 2. Undiscovered defects will cause high failure rates early in the life of a program. However, these are corrected (ideally, without introducing other errors) and the curve flattens as shown.The idealized curve is a gross oversimplification of actual failure models (see
This seeming contradiction can best be explained by considering the “actual curve” shown in Figure 2. During its life, software will undergo change (maintenance).
Figure 2: Idealized and Actual failure curves for software
When a hardware component wears out, it is replaced by a spare part. There are no software spare parts. Every software failure indicates an error in design or in the process through which design was translated into machine executable code. Therefore, software maintenance involves considerably more complexity than hardware maintenance.
3. Although the industry is moving toward component-based assembly, most software continues to be custom built.
As an engineering discipline evolves, a collection of standard design components is created. The reusable components have been created so that the engineer can concentrate on the truly innovative elements of a design, that is, the parts of the design that represent something new. In the hardware world, component reuse is a natural part of the engineering process. In the software world, it is something that has only begun to be achieved on a broad scale.
Software Engineering 