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A constraint is a restriction on
the degree of freedom you have in providing a
solution. Constraints are effectively global
requirements, such as limited development resources or a
decision by senior management that restricts the way you
develop a system. Constraints can be economic,
political, technical, or environmental and pertain to
your project resources, schedule, target environment, or
to the system itself. Figure 1
presents several potential constraints for the
university system. Constraints are documented in a
similar manner to
business rules and
technical requirements.
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Figure 1. Constraints
(summary form).
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C24 The system will work on our
existing technical infrastructure – no new
technologies will be introduced.
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C56 The system will only use the
data contained in the existing corporate database.
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C73 The system shall be available
99.99% of the time for any 24-hour period.
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C76 All master’s degree programs
must include the development of a thesis.
An interesting thing about
Figure 1 is that it contains two
constraints, C73 and C76, that could also be identified
as a
technical requirement and a
business rule respectively. Constraints can be a
little confusing because of their overlap with business
rules and technical requirements. Don’t worry about
it. The important thing is that you’ve identified the
requirement, if you happen to mis-categorize it as a
constraint instead of a business rule that’s perfectly
okay, the world isn’t going to end as a result (unless
of course you’re working on a nuclear missile guidance
system). I wouldn’t identify the same requirement as
both a business rule and a constraint, that would be
busy work, but I wouldn’t waste any time arguing over
whether something is a constraint or another type of
requirement.
As with business rules, you identify constraints as
you are developing other artifacts, such as your
use case model and
user interface.
The Object Constraint Language (OCL)
One way to describe
business rules, technical requirements, or constraints,
is via the Object Constraint Language (OCL). The OCL is
a formal language similar to structured English to
express side-effect-free constraints within UML models (Warner
and Kleppe 1999).
OCL can appear on any UML diagram or in the supporting
documentation describing a diagram, such as business
rule definitions. It can even be used on non-UML
diagrams for that matter. OCL can be used for a
wide variety of purposes, including specifying the
invariants of classes, preconditions and postconditions
on operations, and constraints on operations. The
reality is that a graphical model, such as a UML class
diagram, isn’t sufficient for a precise and unambiguous
specification. You must describe additional constraints
about the objects in the model, constraints that are
defined in your supplementary specification.
I rarely use OCL
when I’m modeling. First, few people know how to read
OCL, let alone write it, so you’re restricting the
audience of your models when you use it. Second, it’s
complex. OCL statements are depicted on UML
diagrams in the format “{constraint description},” where
the constraint description may be in any format,
including predicate calculus. I personally find that
free-form text is far more effective. Third, if I want to describe rules in such a way that I can generate
working software from them I’ll use a real programming
language such as Java or C# to do it.
The OCL was an interesting concept
but it hasn’t been adopted by the industry. Because it
shows no signs of being adopted any time soon in my
opinion it isn’t worth the effort to learn.
This artifact description is excerpted from Chapter 7 of
The Object Primer 3rd Edition: Agile Model Driven
Development with UML 2.
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The Object Primer 3rd Edition: Agile Model Driven
Development with UML 2 is an
important reference book for agile modelers,
describing how to develop 35
types of agile
models including all 13
UML 2 diagrams.
Furthermore, this book describes the techniques
of the
Full Lifecycle Object Oriented Testing
(FLOOT) methodology to give you the fundamental
testing skills which you require to succeed at
agile software development. The book also
shows how to move from your agile models to
source code (Java examples are provided) as well
as how to succeed at implementation techniques
such as
refactoring and
test-driven development
(TDD). The Object Primer also includes a
chapter overviewing the critical database
development techniques (database refactoring,
object/relational mapping,
legacy analysis, and
database access coding) from my award-winning
Agile Database Techniques
book. |
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Agile Modeling: Effective Practices for Extreme
Programming and the Unified Process is the seminal
book describing how agile software developers approach
modeling and
documentation. It describes principles and
practices which you can tailor into your existing
software process, such as
XP, the
Rational Unified Process (RUP), or the
Agile Unified Process (AUP), to streamline your
modeling and documentation efforts. Modeling and
documentation are important aspects of any software
project, including agile projects, and this book
describes in detail how to
elicit requirements,
architect, and then
design your system in an agile manner. |
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The Elements of UML 2.0 Style describes a collection
of standards, conventions, and
guidelines
for creating effective
UML diagrams. They are based on sound, proven
software engineering principles that lead to diagrams
that are easier to understand and work with. These
conventions exist as a collection of simple, concise
guidelines that if applied consistently, represent an
important first step in increasing your productivity as
a modeler. This book is oriented towards
intermediate to advanced UML modelers, although there
are numerous examples throughout the book it would not
be a good way to learn the UML (instead, consider
The Object Primer). The book is a brief 188
pages long and is conveniently pocket-sized so it's easy
to carry around. |
Translations
I actively work with clients around the world to
improve their information technology (IT) practices as
both a mentor/coach and trainer. A full
description of what I do, and how to contact me, can be
found here.
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