Software design

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Software design is a process of problem-solving and planning for a software solution. After the purpose and specifications of software is determined, software developers will design or employ designers to develop a plan for a solution. It includes low-level component and algorithm implementation issues as well as the architectural view.

The software requirements analysis (SRA) step of a software development process yields specifications that are used in software engineering. If the software is "semiautomated" or user centered, software design may involve user experience design yielding a story board to help determine those specifications. If the software is completely automated (meaning no user or user interface), a software design may be as simple as a flow chart or text describing a planned sequence of events. There are also semi-formal methods like Unified Modeling Language and Fundamental modeling concepts. In either case some documentation of the plan is usually the product of the design.

A software design may be platform-independent or platform-specific, depending on the availability of the technology called for by the design.

Design Considerations

There are many aspects to consider in the design of a piece of software. The importance of each should reflect the goals the software is trying to achieve. Some of these aspects are:

• Extensibility - New capabilities can be added to the software without major changes to the underlying architecture.
• Robustness - The software is able to operate under stress or tolerate unpredictable or invalid input. For example, it can be designed with a resilience to low memory conditions.
• Reliability - The software is able to perform a required function under stated conditions for a specified period of time.
• Fault-tolerance - The software is resistant to and able to recover from component failure.
• Security - The software is able to withstand hostile acts and influences.
• Maintainability - The software can be restored to a specified condition within a specified period of time. For example, antivirus software may include the ability to periodically receive virus definition updates in order to maintain the software's effectiveness.
• Compatibility - The software is able to operate with other products that are designed for interoperability with another product. For example, a piece of software may be backward-compatible with an older version of itself.
• Modularity - the resulting software comprises of well defined, independent components. That leads to better maintainability. The components could be then implemented and tested in isolation before being integrated to form a desired software system. This allows division of work in a software development project.
• Reuse - the modular components designed should capture the essence of the functionality expected out of them and no more or less. This single-minded purpose render the components reusable wherever there are similar needs in other designs.

Design Patterns

A software designer or architect may identify a design problem which has been solved by others before. A template or pattern describing a solution to a common problem is known as a design pattern. (Design pattern (computer science)). The reuse of such patterns can speed up the software development process, having been tested and proved in the past.

Design methodologies

Design methodologies aim to provide a template process or a framework for the actual design of a system. They aim to simplify the actual process of designing a system and aim to enforce some standard design principles which improve the quality of a design. One of the earlier design methodologies is the Responsibility Driven Design (RDD) pioneered by Rebecca Wirth et al. It forms the basis of the URDAD, the Use Case, Responsibility-Driven Analysis and Design method which aims to generate a technology neutral design which is then mapped onto one's choice of implementation architecture and technologies.

Usage

Software design documentation may be reviewed or presented to allow constraints, specifications and even requirements to be adjusted prior to programming. Redesign may occur after review of a programmed simulation or prototype. It is possible to design software in the process of programming, without a plan or requirement analysis, but for more complex projects this would not be considered a professional approach. A separate design prior to programming allows for multidisciplinary designers and Subject Matter Experts (SMEs) to collaborate with highly-skilled programmers for software that is both useful and technically sound.