We begin by describing the nature of engineering in general, including how it relates to:

• Crafts, and
• Sciences, including computer science

Next, we describe software engineering in particular, noting how the nature of software itself makes software engineering unique.

Engineering has its roots in two important human activities: crafts and sciences.

This section describes the nature of these activities and how engineering in general relates to them.

Understanding what it means to be a craft will require the following foundational concepts:

• Values
• Knowledge
• Quantification
• Creativity

A craft is concerned with the creation, and possibly the maintenance, of a type of product, that is produced for its value, which can be of two types:

• Aesthetic: The product is valued as an end in itself, or
• Utilitarian: The product is a means to another end

In either case, practicing a craft requires a knowledge of values and the means for achieving them.

Practicing a craft requires two kinds of knowledge:

• Knowledge of the behavior of the products and their constituents — knowing that, or declarative knowledge.

  However, declarative knowledge by itself does not ensure achievement of value.

• A craft also requires the development of tools and techniques involved in production, as well as the skills involved in their use: knowing how, or procedural knowledge.

Practice is needed to develop habits in the use of the tools and techniques.

These habits partially automate the achievement of value, freeing the mind to deal with more complex issues that may arise.

The knowledge of values together with the practiced habits make up the work ethics of a craft.

Quantification — the use of numeric or symbolic abstractions — may be essential in the construction of a finished product:

• Quantification of physical properties (e.g. temperature) of raw materials
• Quantification of design models (e.g. musical notation)

A full-blown mathematical theory may or may not be needed.

Practicing a craft is a creative effort, and it takes imagination to:

• Put raw materials together in new ways to create a product of value.
• Use tools and techniques in new ways to enhance the value of the product.

Creativity is an important quality that distingushes expert practitioners from average practitioners.

In the early stages of development of a craft, understanding how things behave is of primary importance; understanding why is of secondary importance.

Scientific understanding also begins with an understanding of how things behave, but a science ultimately focuses more on why they behave as they do.

With more complex products and production processes, just knowing how things behave is not sufficient; There is also an increased need for understanding causal relationships — the answers to "why" questions.

The answers to these questions often have predictive power that is essential for developing new techniques and methodologies of product design and production.

When the causal knowledge about the behavior of the product and its constituents is well-developed in a craft it becomes a science.

The human mind does not deal well with a large, unorganized collection of facts.

In order to serve human needs, related facts must be organized into a theory, with a single theory encompassing and explaining a large number of facts.

A science is a body of causal knowledge about a subject matter, organized into theories.

Theories provide explanations of relationships between characteristics of objects covered by the subject matter and the behavior of those objects.

In some areas of science, this understanding can be used to predict behavior, leading to a utilitarian value for science — applied science.

However, a well developed science has a core which is concerned with understanding for its own sake — pure science.

Most sciences attempt a quantification of the relevent concepts if it is practical.

In some well-developed sciences, such as physics and chemistry, the quantification plays a central role, resulting in a mathematical theory or model at the heart of the science.

In some areas of science, such as evolutionary theory in biology, measurement is not always practical and the mathematical theory is not very well developed.

Practicing a science is a creative effort. It takes imagination to:

• Put facts together in a way that makes sense
• Develop concepts that have explanatory power

Creativity is an important quality that distinguishes expert scientists from average scientists.

Pure scientific values have to do with the quality of, not products, but scientific thinking itself:

• Correctness of scientific inference
• Explanatory power
• Simplicity
• Testability

Applied science may involve producing products with greater functionality or higher quality, requiring knowledge of the causal factors that affect its value.

Such causal knowledge involves:

• Factors involved in the quality of a product
• Means for quantifying those factors
• Characteristics that control those factors
• Principles that focus attention on important value-related product design issues

When a science is concerned with this kind of causal knowledge, it is an engineering science.

Computer science is the study of computational processes:

• How to specify them:
  • Formal languages
  • Data abstraction
  • Procedural abstraction
• How to reason about them:
  • Algorithmic efficiency
  • Computability
  • Complexity
• How to build machines that bring them about:
  • Architecture
  • Operating Systems

Computer science is both similar to and different than mathematics and pure science in important ways.

Theoretical computer science, the formal study of computational structures and computation itself, is mathematical in nature.

Non-theoretical computer science, concerned with the creation of software and the machines that run it, can be empirical and experimental.

In either case, computer science differs from natural science in that the computational objects it studies are objects of its own making.

The computational objects CS both creates and studies, as far as software is concerned, are abstractions.

Mathematics also creates its own subject matter in the form of abstractions, but there is a crucial difference:

• Mathematical abstractions are inference structures
• Computer science abstractions are interaction patterns

Values in mathematics, like the values of pure science, concern the quality of mathematical thinking itself:

• Rigor and consistency
• Correctness of inference
• Simplicity and elegance

Values in computer science concern the best ways to create and control the computational objects they create, whether they are:

• Data structures
• Algorithms
• Programs that implement them

Since creating and controlling computational objects requires causal knowledge, computer science is best understood as an engineering science.

Conceptually, engineering disciplines arise from crafts as their knowledge bases evolve into sciences. If a craft is producing a utilitarian product then there is a demand for:

• Greater functionality of the product
• Higher quality in the product
• Greater quantities of the product

Meeting these demands results in more complexity in the product, or in the production process, or both.

For a craft to become an engineering discipline it must develop a better understanding of problems involved in planning and managing the product development process and the people involved.

Production of complex products, large quantities of products, and more functional products all require a significant amount of planning:

• When the product is complex or the product has greater functionality, planning focuses on designing the product.
• When large quantities of a product are produced, the planning focuses on designing a production process.
• The magnitude of this effort may require that the people involved develop specialized skills:
  • Some continue with the actual production
  • Others are involved in the design of products and production processes.

Planning is part of any engineering discipline.

An engineering discipline builds on the same knowledge bases as a craft, including knowledge of tools and techniques. However,

• These tools and techniques are developed within the engineering discipline more as aids to design rather than production.
• The partial automation of the achievement of values is of even greater importance because an engineer has more complex issues to deal with.

As in a craft, practice is needed to develop habits in their use.

The knowledge of values together with the practiced habits make up the work ethics of an engineering discipline.

Production of complex products requires the use of teams of engineers to do the design. This requires:

• A well-defined design process.
• A management structure to:
  • Guide people through the process
  • Identify risks involved
  • Allocate necessary resources
• Engineering managers must themselves have an understanding of the engineering discipline

To prepare engineers to work on and lead teams, management is an important part of engineering education.

Because software engineering is engineering, it inherits the features already mentioned about engineering in general.

Software engineering also inherits from computer science as its mother discipline.

However, software engineering has unique characteristics and problems owing to the uniqueness of software as a product.

These differences arise primarily in three areas: the nature, functionality, and maintenance of software.

Software engineering is characterized by its primary product, which is software — programs that bring about a computational process. Many of the subject areas of computer science deal with software products:

• Programming languages
• Implementations of algorithms and data structures
• Systems software

Because of this, the boundary between computer science and software engineering is sometimes vague; in particular, the values of software engineering are an important element of computer science.

Whatever the context, the production of large quantities of software characterizes software engineering.

Software has an inherently dual nature:

• It is physical:
  • Can be handled on physical media such as hard drives, optical disks, memory sticks
  • Runs as electronic processes on a physical processor
• It is abstract:
  • Written in formal language
  • Describes abstractions such as objects

Due to the dual nature of software, creation of physical products and creation of software differs:

• The production of large quantities of some physical products (say automobiles) introduces a great deal of complexity into the production process.
• The complexity of large pieces of software is in the design process:
  • Deals with abstractions
  • Requires large intellectual effort

Reproducing software, even large quantities of copies, is relatively easy once it has been designed.

Software engineering is concerned almost exclusively with the design of the product and not the production process.

Besides having a dual nature, software is also unique in the scope of its application.

The magnitude of this scope has an impact on the software development process.

Non-software product categories are targeted toward specific uses:

• Transportation
• Shelter
• Entertainment

Software, however, can be applied to the automation of any human task.

In particular, any engineering process can benefit from the support of software.

Therefore, any engineering process can become, in part, a software engineering project.

There is no single domain of knowledge that covers the behavior of all software products.

• There is no single body of theory that tells software engineering how to specify and measure the behavior of its products.
• Software engineering has computer science, but:
  • Computer science does deal with the components of software and some of the tools and techniques involved in its construction
  • Computer science does not and cannot deal with all of the kinds of behavior exhibited by software.

The values of software engineering are driven by customers who have an unlimited range of needs. As a result,

• Classification of the wide range of values is a virtually impossible task.
• When values can be identified, they vary considerably in importance; a value that is unimportant in one software product may be crucial in another.

For these reasons, the ethical and value-oriented side of software engineering takes on a more prominent role.

The extensive variety of software functionality and values can be dealt with by software engineers specializing in a particular domain:

• Software development firms can specialize, for example, in business and accounting software
• Firms whose main business is not software, for example banks, can create a department dedicated to software development

Demand for a software product often does not warrant specialized software development firms or departments. A second option is that people that are trained in the application area develop programming skills:

• A workable option for relatively simple software
• For more complex software, the lack of software understanding may outweigh the advantage of familiarity with the application domain.

A more general solution to the problem is to build domain analysis into the software development process.

In this approach, a significant amount of time early in development is devoted to:

• Communicating with experts in the application area, noting important concepts and their relationships.
• Building a domain model that can be readily converted into software.

Another important part of the process involves building the product in stages and getting feedback from the customer about the preliminary versions.

In other engineering disciplines, product maintenance is concerned with the deterioration of a product over time.

Software, as an abstraction, does not deteriorate, and can be readily and cheaply reproduced.

In software engineering, the maintenance objective is improving the product.

Software maintenance is perhaps better described as software evolution.

There are three general types of software maintenance, defined by the types of improvements.

• Corrective: modifications that correct defects.
• Perfective: modifications to:
  • Add new behavior
  • Modify old behavior to make it more suitable to users
  • Modify structure to simplify future maintenance (refactoring).
• Adaptive: modifications that aim towards adaptation for new or modified platforms.

The low cost of software reproduction and installation makes it practical to develop software that is continuously upgraded while in use.

It has been estimated that 90 percent of the software development effort is involved with maintenance.

This figure reflects overall effort, including unsuccessful software. For successful software with a long history, the figure is nearer to 100 percent.

Since so much of successful development effort is involved in maintenance, the interests of developers weigh heavily in software values that ease maintenance.

These values include designing for change, documenting design, and effective and rigorous testing.

The burden of software maintenance is eased for the developer if software design is structured so that future changes can be made more easily:

• Minimize the complexity and nature of interactions between components through decoupling and cohesion.
• Use architectural and design patterns that reduce the impact of changes.

The burden of software maintenance is eased for the developer through design documentation:

• With a poorly documented design, software maintainers will spend a lot more time studying the existing software before making changes.
• Working with undocumented aspects of design are also likely to introduce new errors.

The burden of software maintenance is eased for the developer if extra effort is spent in developing test software and procedures that can be reused:

• Software engineers routinely engage in a technique called refactoring: making design changes to software that do not change its functionality.
• The changes are made for the purpose of simplifying future maintenance.
• After a refactoring, the original test software can be reused without modification.

Software engineering can be defined as:

The process of solving customers' problems through the systematic development and maintenance (evolution) of large, high-quality software systems within cost, time, and other constraints.

This requires:

• Satisfying the software stakeholders, one of which is the customer
• Implementing the best practices for systematic development and maintenance of software
• Knowing what it means for software to be of high-quality
• How to trade off cost, time, and other constraints

Each of these in turn requires knowledge of software engineering values.