Part 2: Unit 9
You have been tasked with “reusing” what’s already there. You’ve already pulled out one or more components to support that functionality – catalog maintenance – how does your component measure up?
We just discussed several component coupling principles:
- Acyclic Dependencies Principle
- Stable Dependencies Principle
- Stable Abstractions Principle
We also just learned about several metrics we can use to evaluate our components:
Instability
title: InstabilityFormula —
[I = \frac{C_E}{(C_E + C_A)}]
Where:
| Symbol | Description | Term | Mnemonic |
| \(C_E\) | Efferent Coupling | Fan Out | E in Efferent: Exiting |
| \(C_E\) | Afferent Coupling | Fan In | A in Afferent Arriving |
Abstractness
title: Abstractness —
[A = \frac{N_a}{N}]
Where
| Symbol | Description |
| \(N_a\) | Number of abstract classes in component |
| \(N\) | Number of classes in component |
Distance from Main Sequence
title: DistanceFromMainSequence — (D = | A + I - 1 |)
Where:
[A = Abstractness] [I = Instability]
Original diagram
What are the values for the original diagram?
Middle Tier
\(C_E = 0\)
\[C_A = 7\] \[I = \frac{0}{0+7} = 0\] \[A = \frac{2}{14} = 0.143\] \[D = |0.143 + 0 - 1| = 0.857\]UI Tier
\[C_E = 7\] \[C_A = 0\] \[I = \frac{7}{7+0} = 1\] \[A = \frac{1}{5} = 0.2\] \[D = |1 + .2 - 1| = 0.2\]What to do next?
- Now perform these calculations for your new component configuration. How do you compare?
- What can you say about the components that have a “large” distance from the main sequence?
- Knowing what you now know, work through a second round of componentization.
Final Observations
- What is your experience with doing this kind of work?
- What tool support have you used or do you see available to assist in this work?
- Why go to all this trouble?
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