Lecture note 3 on software engineering and development

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Lecture 4
 Principles that Guide Practice

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Software Engineering Knowledge
 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.
Steve McConnell

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Principles that Guide Process - I
 Principle #1. Be agile. Whether the process model you choose
is prescriptive or agile, the basic tenets of agile development
should govern your approach.
 Principle #2. Focus on quality at every step. The exit condition
for every process activity, action, and task should focus on the
quality of the work product that has been produced.
 Principle #3. Be ready to adapt. Process is not a religious
experience and dogma has no place in it. When necessary,
adapt your approach to constraints imposed by the problem,
the people, and the project itself.
 Principle #4. Build an effective team. Software engineering
process and practice are important, but the bottom line is
people. Build a self-organizing team that has mutual trust and
respect.

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Principles that Guide Process - II
 Principle #5. Establish mechanisms for communication and
coordination. Projects fail because important information falls into
the cracks and/or stakeholders fail to coordinate their efforts to
create a successful end product.
 Principle #6. Manage change. The approach may be either formal or
informal, but mechanisms must be established to manage the way
changes are requested, assessed, approved and implemented.
 Principle #7. Assess risk. Lots of things can go wrong as software is
being developed. It’s essential that you establish contingency plans.
 Principle #8. Create work products that provide value for others.
Create only those work products that provide value for other
process activities, actions or tasks.

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Principles that Guide Practice
 Principle #1. Divide and conquer. Stated in a more technical
manner, analysis and design should always emphasize
separation of concerns (SoC).
 Principle #2. Understand the use of abstraction. At it core, an
abstraction is a simplification of some complex element of a
system used to communication meaning in a single phrase.
 Principle #3. Strive for consistency. A familiar context makes
software easier to use.
 Principle #4. Focus on the transfer of information. Pay special
attention to the analysis, design, construction, and testing of
interfaces.

These slides are designed to accompany Software Engineering: A Practitioner’s Approach, 7/e
(McGraw-Hill, 2009) Slides copyright 2009 by Roger Pressman. 6
Principles that Guide Practice
 Principle #5. Build software that exhibits effective
modularity. Separation of concerns (Principle #1)
establishes a philosophy for software. Modularity
provides a mechanism for realizing the philosophy.
 Principle #6. Look for patterns. Brad Appleton
[App00] suggests that: “The goal of patterns within the
software community is to create a body of literature to
help software developers resolve recurring problems
encountered throughout all of software development.
 Principle #7. When possible, represent the problem and
its solution from a number of different perspectives.
 Principle #8. Remember that someone will maintain the
software.

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Communication Principles
 Principle #1. Listen. Try to focus on the speaker’s words,
rather than formulating your response to those words.
 Principle # 2. Prepare before you communicate. Spend the
time to understand the problem before you meet with others.
 Principle # 3. Someone should facilitate the activity. Every
communication meeting should have a leader (a facilitator) to
keep the conversation moving in a productive direction; (2) to
mediate any conflict that does occur, and (3) to ensure than
other principles are followed.
 Principle #4. Face-to-face communication is best. But it
usually works better when some other representation of the
relevant information is present.

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Communication Principles
 Principle # 5. Take notes and document decisions. Someone participating in
the communication should serve as a “recorder” and write down all
important points and decisions.
 Principle # 6. Strive for collaboration. Collaboration and consensus occur
when the collective knowledge of members of the team is combined …
 Principle # 7. Stay focused, modularize your discussion. The more people
involved in any communication, the more likely that discussion will bounce
from one topic to the next.
 Principle # 8. If something is unclear, draw a picture.
 Principle # 9. (a) Once you agree to something, move on; (b) If you can’t
agree to something, move on; (c) If a feature or function is unclear and
cannot be clarified at the moment, move on.
 Principle # 10. Negotiation is not a contest or a game. It works best when
both parties win.

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Planning Principles
 Principle #1. Understand the scope of the project. It’s
impossible to use a roadmap if you don’t know where you’re
going. Scope provides the software team with a destination.
 Principle #2. Involve the customer in the planning activity.
The customer defines priorities and establishes project
constraints.
 Principle #3. Recognize that planning is iterative. A project
plan is never engraved in stone. As work begins, it very likely
that things will change.
 Principle #4. Estimate based on what you know. The intent
of estimation is to provide an indication of effort, cost, and
task duration, based on the team’s current understanding of
the work to be done.

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Planning Principles
 Principle #5. Consider risk as you define the plan. If you have identified
risks that have high impact and high probability, contingency planning is
necessary.
 Principle #6. Be realistic. People don’t work 100 percent of every day.
 Principle #7. Adjust granularity as you define the plan. Granularity refers
to the level of detail that is introduced as a project plan is developed.
 Principle #8. Define how you intend to ensure quality. The plan should
identify how the software team intends to ensure quality.
 Principle #9. Describe how you intend to accommodate change. Even the
best planning can be obviated by uncontrolled change.
 Principle #10. Track the plan frequently and make adjustments as
required. Software projects fall behind schedule one day at a time.

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Modeling Principles
 In software engineering work, two classes of models
can be created:
 Requirements models (also called analysis models)
represent the customer requirements by depicting
the software in three different domains: the
information domain, the functional domain, and
the behavioral domain.
 Design models represent characteristics of the
software that help practitioners to construct it
effectively: the architecture, the user interface, and
component-level detail.

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Requirements Modeling Principles
 Principle #1. The information domain of a problem must be
represented and understood.
 Principle #2. The functions that the software performs must be
defined.
 Principle #3. The behavior of the software (as a consequence of
external events) must be represented.
 Principle #4. The models that depict information, function,
and behavior must be partitioned in a manner that uncovers
detail in a layered (or hierarchical) fashion.
 Principle #5. The analysis task should move from essential
information toward implementation detail.

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Design Modeling Principles
 Principle #1. Design should be traceable to the requirements model.
 Principle #2. Always consider the architecture of the system to be built.
 Principle #3. Design of data is as important as design of processing
functions.
 Principle #5. User interface design should be tuned to the needs of the
end-user. However, in every case, it should stress ease of use.
 Principle #6. Component-level design should be functionally
independent.
 Principle #7. Components should be loosely coupled to one another and
to the external environment.
 Principle #8. Design representations (models) should be easily
understandable.
 Principle #9. The design should be developed iteratively. With each
iteration, the designer should strive for greater simplicity.

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Agile Modeling Principles
 Principle #1. The primary goal of the software team is to build software, not
create models.
 Principle #2. Travel light—don’t create more models than you need.
 Principle #3. Strive to produce the simplest model that will describe the
problem or the software.
 Principle #4. Build models in a way that makes them amenable to change.
 Principle #5. Be able to state an explicit purpose for each model that is created.
 Principle #6. Adapt the models you develop to the system at hand.
 Principle #7. Try to build useful models, but forget about building perfect
models.
 Principle #8. Don’t become dogmatic about the syntax of the model. If it
communicates content successfully, representation is secondary.
 Principle #9. If your instincts tell you a model isn’t right even though it seems
okay on paper, you probably have reason to be concerned.
 Principle #10. Get feedback as soon as you can.

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Construction Principles
 The construction activity encompasses a set of coding
and testing tasks that lead to operational software that
is ready for delivery to the customer or end-user.
 Coding principles and concepts are closely aligned
programming style, programming languages, and
programming methods.
 Testing principles and concepts lead to the design of
tests that systematically uncover different classes of
errors and to do so with a minimum amount of time
and effort.

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Preparation Principles
 Before you write one line of code, be sure you:
•Understand of the problem you’re trying to solve.
•Understand basic design principles and concepts.
•Pick a programming language that meets the needs of the
software to be built and the environment in which it will
operate.
•Select a programming environment that provides tools
that will make your work easier.
•Create a set of unit tests that will be applied once the
component you code is completed.

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Coding Principles
 As you begin writing code, be sure you:
• Constrain your algorithms by following structured programming
practice.
• Consider the use of pair programming
• Select data structures that will meet the needs of the design.
• Understand the software architecture and create interfaces that are
consistent with it.
• Keep conditional logic as simple as possible.
• Create nested loops in a way that makes them easily testable.
• Select meaningful variable names and follow other local coding
standards.
• Write code that is self-documenting.
• Create a visual layout (e.g., indentation and blank lines) that aids
understanding.

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Validation Principles
 After you’ve completed your first coding pass, be sure
you:
•Conduct a code walkthrough when appropriate.
•Perform unit tests and correct errors you’ve
uncovered.
•Refactor the code.

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Testing Principles
 Al Davis [Dav95] suggests the following:
 Principle #1. All tests should be traceable to customer
requirements.
 Principle #2. Tests should be planned long before testing
begins.
 Principle #3. The Pareto principle applies to software
testing.
 Principle #4. Testing should begin “in the small” and
progress toward testing “in the large.”
 Principle #5. Exhaustive testing is not possible.

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Deployment Principles
 Principle #1. Customer expectations for the software must be
managed. Too often, the customer expects more than the team
has promised to deliver, and disappointment occurs
immediately.
 Principle #2. A complete delivery package should be assembled
and tested.
 Principle #3. A support regime must be established before the
software is delivered. An end-user expects responsiveness and
accurate information when a question or problem arises.
 Principle #4. Appropriate instructional materials must be
provided to end-users.
 Principle #5. Buggy software should be fixed first, delivered
later.

Lecture note 3 on software engineering and development

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