Overview:
To model a system the most
important aspect is to capture the dynamic behaviour. To clarify a bit in
details, dynamic behaviour means the behaviour of the system when
it is running /operating.
So only static behaviour is not
sufficient to model a system rather dynamic behaviour is more important than
static behaviour. In UML there are five diagrams available to model dynamic
nature and use case diagram is one of them. Now as we have to discuss that the
use case diagram is dynamic in nature there should be some internal or external
factors for making the interaction.
These internal and external
agents are known as actors. So use case diagrams are consists of actors, use
cases and their relationships. The diagram is used to model the
system/subsystem of an application. A single use case diagram captures a
particular functionality of a system.
So to model the entire system
numbers of use case diagrams are used.
Purpose:
The purpose of use case diagram
is to capture the dynamic aspect of a system. But this definition is too
generic to describe the purpose.
Because other four diagrams
(activity, sequence, collaboration and Statechart) are also having the same
purpose. So we will look into some specific purpose which will distinguish it
from other four diagrams.
Use case diagrams are used to
gather the requirements of a system including internal and external influences.
These requirements are mostly design requirements. So when a system is analyzed
to gather its functionalities use cases are prepared and actors are identified.
Now when the initial task is
complete use case diagrams are modelled to present the outside view.
So in brief, the purposes of
use case diagrams can be as follows:
·
Used to gather requirements of a system.
·
Used to get an outside view of a system.
·
Identify external and internal factors influencing the system.
·
Show the interacting among the requirements are actors.
How to draw Use Case Diagram?
Use case diagrams are
considered for high level requirement analysis of a system. So when the requirements
of a system are analyzed the functionalities are captured in use cases.
So we can say that uses cases
are nothing but the system functionalities written in an organized manner. Now
the second things which are relevant to the use cases are the actors. Actors
can be defined as something that interacts with the system.
The actors can be human user,
some internal applications or may be some external applications. So in a brief
when we are planning to draw an use case diagram we should have the following
items identified.
·
Functionalities to be represented as an use case
·
Actors
·
Relationships among the use cases and actors.
Use case diagrams are drawn to
capture the functional requirements of a system. So after identifying the above
items we have to follow the following guidelines to draw an efficient use case
diagram.
·
The name of a use case is very important. So the name should be
chosen in such a way so that it can identify the functionalities performed.
·
Give a suitable name for actors.
·
Show relationships and dependencies clearly in the diagram.
·
Do not try to include all types of relationships. Because the main
purpose of the diagram is to identify requirements.
·
Use note when ever required to clarify some important points.
The following is a sample use
case diagram representing the order management system. So if we look into the
diagram then we will find three use cases (Order, SpecialOrder and NormalOrder)
and one actor which is customer.
The SpecialOrder and NormalOrder use cases are extended from Order use case. So they have extends
relationship. Another important point is to identify the system boundary which
is shown in the picture. The actor Customer lies outside the system as it is an
external user of the system.
Where to Use Case Diagrams?
As we have already discussed
there are five diagrams in UML to model dynamic view of a system. Now each and
every model has some specific purpose to use. Actually these specific purposes
are different angles of a running system.
So to understand the dynamics
of a system we need to use different types of diagrams. Use case diagram is one
of them and its specific purpose is to gather system requirements and actors.
Use case diagrams specify the
events of a system and their flows. But use case diagram never describes how
they are implemented. Use case diagram can be imagined as a black box where
only the input, output and the function of the black box is known.
These diagrams are used at a
very high level of design. Then this high level design is refined again and
again to get a complete and practical picture of the system. A well structured
use case also describes the pre condition, post condition, exceptions. And
these extra elements are used to make test cases when performing the testing.
Although the use cases are not
a good candidate for forward and reverse engineering but still they are used in
a slight different way to make forward and reverse engineering. And the same is
true for reverse engineering. Still use case diagram is used differently to
make it a candidate for reverse engineering.
In forward engineering use case
diagrams are used to make test cases and in reverse engineering use cases are
used to prepare the requirement details from the existing application.
So the following are the places
where use case diagrams are used:
·
Requirement analysis and high level design.
·
Model the context of a system.
·
Reverse engineering.
·
Forward engineering.
