From the Editor
IEEE Software, Vol. 16, No. 2, March/April 1999
Software Engineering Principles
practitioners think of software engineering knowledge almost exclusively as
knowledge of specific technologies: Java, Perl, html, C++, Linux, Windows
NT, and so on. Knowledge of specific technology details is necessary to
perform computer programming. If someone assigns you to write a program in
C++, you have to know something about C++ to get your program to work.
You often hear people say that software development
knowledge has a 3 year half-life: half of what you need to know today will
be obsolete within 3 years. In the domain of technology-related knowledge,
that’s probably about right. But there is another kind of software
development knowledge—a kind that I think of as "software engineering
principles"—that does not have a three year half-life. These software
engineering principles are likely to serve a professional programmer
throughout his or her career.
Essence and Accident
In 1987, Fred Brooks published an influential paper, "No
Silver Bullets—Essence and Accident in Software Engineering" (Computer,
April 1987). The main contention of Brooks’ paper was that no single tool or
methodology—no "silver bullet"— portended a 10-to-1 improvement in
productivity over the next decade. The reasoning behind that claim is highly
relevant to the distinction between technology knowledge and software
In using the words "essence" and "accident," Brooks drew on
an ancient philosophical tradition of distinguishing between "essential" and
"accidental" properties. Essential properties are those properties that a
thing must have to be that thing: A car must have an engine, wheels, and a
transmission in order to be a car. These are essential properties. A car
might or might not have a V8 or a V6, studded snow tires or racing slicks,
an automatic or a manual transmission. These are "accidental" properties,
the properties that arise by happenstance and do not affect the basic
car-ness of the car.
According to Brooks, the most difficult work of software
development is not representing the concepts faithfully in a specific
computer programming language (coding) or checking the fidelity of that
representation (testing)—that is the accidental part of software
Brooks argued that the essence of software engineering
consists of working out the specification, design, and verification of a
highly precise and richly detailed set of interlocking concepts. What makes
software development difficult is its essential complexity, conformity,
changeability, and invisibility. Computer programs are complex by their
nature. Even if you could invent a programming language that operated at the
level of the problem domain, programming would still be a complicated
activity because you would still need to define relationships between
real-world entities precisely, identify exception cases, anticipate all
possible state transitions, and so on. Strip away the accidental work
involved in representing these factors in some specific programming language
within some specific operating system, and you still have the essential
difficulty of developing and debugging the underlying real-world concepts.
Adding to its inherent complexity, additional complexity
arises from the fact that software cannot be created in isolation, but must
conform to real-world constraints—pre-existing hardware, third party
components, government regulations, legacy data formats, and so on. The
software designer often faces inflexible, external factors that limit the
extent to which complexity can be reduced.
Another source of complexity is software’s changeability.
The more successful a program is, the more uses people will find for it, and
the more it will be adapted beyond the domain for which it was originally
A final source of software complexity arises from
software’s inherent invisibility. Software doesn’t exist in a way
that can be represented using geometric models. Attempts to represent even
simple, static logic flow geometrically quickly become absurdly complicated,
as anyone who has ever tried to draw a full flow chart for even a relatively
simple program will attest. Adding to the problem, software doesn’t exist
meaningfully in static form; it only exists meaningfully when it’s executed.
So even the absurdly complicated geometrical representations turn out to
present software’s structure as simpler as it really is because they ignore
the time dimension.
Brooks argued that the major gains to be realized from
addressing the accidental elements of software engineering have already been
made: the invention of high level languages, movement to interactive
computing from batch processing, and development of powerful integrated
environments. Any further order-of-magnitude improvements can be made only
by addressing software’s essential difficulties—the complexity, conformity,
changeability, and invisibility inherent to software development.
Software Engineering Principles
Knowledge that addresses what Brooks calls the essential
difficulty of software engineering is what I think of as "software
engineering principles." During the past 30 years, since the first NATO
conference on software engineering in 1968, the software industry has come a
long ways in identifying the essential knowledge that a software engineer
needs in order to develop software effectively.
One way to look at the state of this body of knowledge is
to think of it in terms of the total body of knowledge needed to create a
complex software system and the proportion of that body of knowledge that is
relatively stable—the part that is not going to be obsolete within three
years. As the figure below illustrates, at the time of the 1968 NATO
conference, that stable core was relatively small.
How small was this body of knowledge at that time? Consider
that the first fully correct binary search algorithm was published just six
years before the NATO conference. Böhm and Jacopini published the paper that
made elimination of the goto and structured programming possible only 2
years before the conference. Edsger Dijkstra wrote his famous "GoTo
Statement Considered Harmful" letter to the editor the same year as the
conference. Larry Constantine, Glenford Myers, and Wayne Stevens didn’t
publish the first paper on structured design until six years after the
conference. Tom Gilb published the first book on software metrics nine years
after the conference. Tom DeMarco’s landmark book on structured analysis
didn’t appear until 1979. Anyone who tried to identify a stable core of
knowledge in 1968 would have had their work cut out for them. It’s hard to
find much knowledge that would have been considered part of a "stable core"
of software engineering knowledge in 1968 that is still in use today—perhaps
only about 10 percent.
In contrast, I think that the same diagram drawn today
would look more like this:
We still do not have perfect knowledge of what is needed to
develop software effectively, so the stable core now occupies only perhaps
75 percent of the knowledge needed to develop a software system. But we
probably don’t ever want a core that makes up 100 percent of the body of
knowledge—that would indicate that software engineering had reached a steady
state and was immune to future improvements.
Considered from the point of view of the body of knowledge
of software engineering, I think we’ve made significant progress in the
areas Brooks refers to as the "essential difficulties" of software
development. We now have reference books that are adequate or good on the
topics of requirements development, design, construction, testing, reviews,
quality assurance, software project management, algorithms, and user
interface design, just to name a few topics, and new and better books that
codify the knowledge needed to be a software engineer are appearing
regularly. An investment in learning software engineering principles is a
particular good investment for a software professional to make because that
knowledge will last a whole career—not be half obsolete within three years.
As a software development professional, you need knowledge
of specific technologies to do your job. But you need knowledge of software
engineering principles to do your job well. A continuing pursuit of
such knowledge is one mark of a true professional.
Editor: Steve McConnell, Construx Software, 11820 Northup
Way #E200, Bellevue, WA 98005.