CSC/ECE 517 Fall 2009/wiki3 18 301: Difference between revisions
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== Introduction == | == Introduction == | ||
Package Design is a challenging area of Software Engineering. An inefficiently designed system can be a cause of major problem, thus, it is important to have a solid understanding of various design approaches. This article discusses Stable Dependencies Principle, one of a few key package design principles that software engineers should be aware of. | |||
== Definition of Instability == | == Definition of Instability == | ||
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==Stable Dependencies Principle== | ==Stable Dependencies Principle== | ||
The Stable Dependencies Principle (SDP) states that “a package should always depend upon packages that are more stable than it is.” Ideally, the direction of dependencies should go from high volatile to more stable packages. | |||
So how do we know whether a certain component design follows the SDP? | So how do we know whether a certain component design follows the SDP? | ||
Revision as of 05:30, 18 November 2009
Topic: Stable Dependencies Principle
Introduction
Package Design is a challenging area of Software Engineering. An inefficiently designed system can be a cause of major problem, thus, it is important to have a solid understanding of various design approaches. This article discusses Stable Dependencies Principle, one of a few key package design principles that software engineers should be aware of.
Definition of Instability
The instability of any package can be calculated using the following formula.
I (instability) = Ce / ( Ca + Ce )
In this formula:
Ce – the number of other modules that our package requires in order to operate correctly. In other words, this is an indicator of package independence.
Ca – the number of packages that can be viewed as “dependents“, i.e., “depend upon classes within the package”. This is an indicator of package responsibilities.
Therefore, in order for a package to be completely independent and also stable, it should not be dependent upon other packages.
For example, let’s look at the following component diagram:
The Wheel class in this example is shown as not dependent on any other module.
Using the above formula:
I(W) = 0 / ( 3 + 0 ) = 0
I(W) has the instability of zero. In other words, even though the Wheel class is “responsible” for three classes (Ca=3), it is not dependent upon any of them (Ce = 0), thus resulting in a stable package.
Respectively, instable or irresponsible packages would have the instability of 1, as shown below:
In this case, the Car class does not have any dependent classes (Ca = 0), yet itself depends upon three classes – Wheel, Engine, and Suspension (Ce = 3).
Next, let’s calculate the instability:
I(C) = 3 / ( 0 + 3 ) = 3 / 3 = 1
The instability of 1 indicates a high level of volatility of the Car module.
Stable Dependencies Principle
The Stable Dependencies Principle (SDP) states that “a package should always depend upon packages that are more stable than it is.” Ideally, the direction of dependencies should go from high volatile to more stable packages.
So how do we know whether a certain component design follows the SDP?
The following is a more complex diagram outlining several Car components:
