Software Engineering-Specification Principles


There is no doubt that the mode of specification has much to do with the quality of solution. Software engineers who have been forced to work with incomplete, inconsistent, or misleading specifications have experienced the frustration and confusion that invariably results. The quality, timeliness, and completeness of the software suffers as a consequence.

Specification Principles

Specification, regardless of the mode through which we accomplish it, may be viewed as a representation process. Requirements are represented in a manner that ultimately leads to successful software implementation. A number of specification principles, adapted from the work of Balzer and Goodman, can be proposed:

1. Separate functionality from implementation.

2. Develop a model of the desired behavior of a system that encompasses data and the functional responses of a system to various stimuli from the environment.

3. Establish the context in which software operates by specifying the manner in which other system components interact with software.

4. Define the environment in which the system operates and indicate how “a highly intertwined collection of agents react to stimuli in the environment (changes to objects) produced by those agents".

5. Create a cognitive model rather than a design or implementation model. The cognitive model describes a system as perceived by its user community.

6. Recognize that “the specifications must be tolerant of incompleteness and augmentable.” A specification is always a model—an abstraction—of some real (or envisioned) situation that is normally quite complex. Hence, it will be incomplete and will exist at many levels of detail.

7. Establish the content and structure of a specification in a way that will enable it to be amenable to change.

This list of basic specification principles provides a basis for representing software requirements. However, principles must be translated into realization.

Representation

We have already seen that software requirements may be specified in a variety of ways. However, if requirements are committed to paper or an electronic presentation medium (and they almost always should be!) a simple set of guidelines is well worth following:

Representation format and content should be relevant to the problem. A general outline for the contents of a Software Requirements Specification can be developed. However, the representation forms contained within the specification are likely to vary with the application area. For example, a specification for a manufacturing automation system might use different symbology, diagrams and language than the specification for a programming language compiler.

Information contained within the specification should be nested. Representations should reveal layers of information so that a reader can move to the level of detail required. Paragraph and diagram numbering schemes should indicate the level of detail that is being presented. It is sometimes worthwhile to present the same information at different levels of abstraction to aid in understanding.

Diagrams and other notational forms should be restricted in number and consistent in use. Confusing or inconsistent notation, whether graphical or symbolic, degrades understanding and fosters errors.

Representations should be revisable. The content of a specification will change. Ideally, CASE tools should be available to update all representations that are affected by each change.

Investigators have conducted numerous studies  on human factors associated with specification. There appears to be little doubt that symbology and arrangement affect understanding. However, software engineers appear to have individual preferences for specific symbolic and diagrammatic forms. Familiarity often lies at the root of a person's preference, but other more tangible factors such as spatial arrangement, easily recognizable patterns, and degree of formality often dictate an individual's choice.

The Software Requirements Specification

The Software Requirements Specification is produced at the culmination of the analysis task. The function and performance allocated to software as part of system engineering are refined by establishing a complete information description, a detailed functional description, a representation of system behavior, an indication of performance requirements and design constraints, appropriate validation criteria, and other information pertinent to requirements. The National Bureau of Standards, IEEE (Standard No. 830-1984), and the U.S. Department of Defense have all proposed candidate formats for software requirements specifications (as well as other software engineering documentation).

The Introduction of the software requirements specification states the goals and objectives of the software, describing it in the context of the computer-based system. Actually, the Introduction may be nothing more than the software scope of the planning document.

The Information Description provides a detailed description of the problem that the software must solve. Information content, flow, and structure are documented. Hardware, software, and human interfaces are described for external system elements and internal software functions.

A description of each function required to solve the problem is presented in the Functional Description. A processing narrative is provided for each function, design constraints are stated and justified, performance characteristics are stated, and one or more diagrams are included to graphically represent the overall structure of the software and interplay among software functions and other system elements. The Behavioral Description section of the specification examines the operation of the software as a consequence of external events and internally generated control characteristics.

Validation Criteria is probably the most important and, ironically, the most often neglected section of the Software Requirements Specification. How do we recognize a successful implementation? What classes of tests must be conducted to validate function, performance, and constraints? We neglect this section because completing it demands a thorough understanding of software requirements—something that we often do not have at this stage. Yet, specification of validation criteria acts as an implicit review of all other requirements. It is essential that time and attention be given to this section.

Finally, the specification includes a Bibliography and Appendix. The bibliography contains references to all documents that relate to the software. These include other software engineering documentation, technical references, vendor literature, and standards. The appendix contains information that supplements the specifications. Tabular data, detailed description of algorithms, charts, graphs, and other material are presented as appendixes.
In many cases the Software Requirements Specification may be accompanied by an executable prototype (which in some cases may replace the specification), a paper prototype or a Preliminary User's Manual. The Preliminary User's Manual presents the software as a black box. That is, heavy emphasis is placed on user input and the resultant output. The manual can serve as a valuable tool for uncovering problems at the human/machine interface.
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