We recall from our previous discussion that software development is not simply coding – it is a multi-activity process. The process of software construction encompasses and includes answers to the following questions:
- What is the problem to be solved?
- What are the characteristics of the entity that is used to solve the problem?
- How will the entity be realized?
- How will the entity be constructed?
- What approach will be used to uncover errors that were made in the design and construction of the entity?
- How will the entity be supported over the long term when users of the entity request corrections, adaptations, and enhancements?
Software Requirements Definitions
Before talking about the requirement process in general and discussing different tools and techniques used for developing a good set of requirements, let us first look at a few definitions of software requirements.
Jones defines software requirements as a statement of needs by a user that triggers the development of a program or system.
Alan Davis defines software requirements as a user need or necessary feature, function, or attribute of a system that can be sensed from a position external to that system. According to Ian Summerville, requirements are a specification of what should be implemented. They are descriptions of how the system should behave, or of a system property or attribute. They may be a constraint on the development process of the system.
IEEE defines software requirements as:
- A condition or capability needed by user to solve a problem or achieve an objective.
- A condition or capability that must be met or possessed by a system or system component to satisfy a contract, standard, specification, or other formally imposed document.
- A documented representation of a condition or capability as in 1 or 2.
As can be seen, these definitions slightly differ from one another but essentially say the same thing: a software requirement is a document that describes all the services provided by the system along with the constraints under which it must operate.
Importance of Requirements
Many of the problems encountered in SW development are attributed to shortcoming in requirement gathering and documentation process. We cannot imagine start building a house without being fully satisfied after reviewing all the requirements and developing all kinds of maps and layouts but when it comes to software we really do not worry too much about paying attentions to this important phase. This problem has been studied in great detail and has been noted that 40-60% of all defects found in software projects can be traced back to poor requirements.
Role of Requirements
Software requirements document plays the central role in the entire software development process. To start with, it is needed in the project planning and feasibility phase. In this phase, a good understanding of the requirements is needed to determine the time and resources required to build the software. As a result of this analysis, the scope of the system may be reduced before embarking upon the software development.
Once these requirements have been finalized, the construction process starts. During this phase the software engineer starts designing and coding the software. Once again, the requirement document serves as the base reference document for these activities. It can be clearly seen that other activities such as user documentation and testing of the system would also need this document for their own deliverables.
On the other hand, the project manager would need this document to monitor and track the progress of the project and if needed, change the project scope by modifying this document through the change control process.
The following diagram depicts this central role of the software requirement document in the entire development process.