Introduction to Software Engineering/Architecture/Design

Software Design
The result of the software requirements analysis (SRA) usually is a specification. The design helps us turn this specification into a working system. As we have seen there are different kinds of software designs, the IEEE Std 610.12-1990 Standard Glossary of Software Engineering Terminology defines the following distinctions:
 * Architectural Design: the process of defining a collection of hardware and software components and their interfaces to establish the framework for the development of a computer system.
 * Detailed Design: the process of refining and expanding the preliminary design of a system or component to the extent that the design is sufficiently complete to begin implementation.
 * Functional Design: the process of defining the working relationships among the components of a system.
 * Preliminary Design: the process of analyzing design alternatives and defining the architecture, components, interfaces, and timing/sizing estimates for a system or components.

Hence software design includes architectural views, but also low-level component and algorithm implementation issues. Depending on the type, a software design may be platform-independent or platform-specific.

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:


 * 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.
 * Extensibility - New capabilities can be added to the software without major changes to the underlying architecture.
 * Fault-tolerance - The software is resistant to and able to recover from component failure.
 * 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.
 * Mod - modification with slight or no modification.
 * 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.
 * Security - The software is able to withstand hostile acts and influences.
 * Usability - The software user interface must be usable for its target user/audience. Default values for the parameters must be chosen so that they are a good choice for the majority of the users.

Modeling Language
Designers are assisted be the existence of modeling languages. They can be used to express information, knowledge or systems in a structure that is defined by a consistent set of rules. A modeling language can be graphical or textual. Examples of graphical modelling languages for software design are:
 * Unified Modeling Language (UML) is a general modeling language to describe software both structurally and behaviorally. It has a graphical notation and allows for extension with a Profile (UML).
 * Flowchart is a schematic representation of an algorithm or a stepwise process,
 * Business Process Modeling Notation (BPMN) is an example of a Process Modeling language.
 * Systems Modeling Language (SysML) is a new general-purpose modeling language for systems engineering.

There is quite a few more, but we will concentrate mostly on the UML as we will see in the next chapter.