Bad Design Traits
1. It is hard to change because every change affects too many other parts of the system (Rigidity)
2. When you make a change, unexpected parts of the system break (Fragility)
3. It is hard to reuse in another application because it cannot be disentangled from the current application (Immobility)
The Cause of “Bad Design”:
What is it that makes a design rigid, fragile and immobile? It is the interdependence of the modules within that design.
A design is rigid if it cannot be easily changed. Such rigidity is due to the fact that a single change to heavily interdependent software begins a cascade of changes in dependent modules.
When the extent of that cascade of change cannot be predicted by the designers or maintainers, the impact of the change cannot be estimated. This makes the cost of the change impossible to predict. Managers, faced with such unpredictability, become reluctant to authorize changes. Thus the design becomes officially rigid.
Fragility is the tendency of a program to break in m
any places when a single change is made. Often the new problems are in areas that have no conceptual relationship with the area that was changed. Such fragility greatly decreases the credibility of the design and maintenance organization. Users and managers are unable to predict the quality of their product. Simple changes to one part of the application lead to failures in other parts that appear to be completely unrelated. Fixing those problems leads to even more problems, and the maintenance process begins to resemble a dog chasing its tail.
A design is immobile when the desirable
parts of the design are highly dependent upon other details that are not desired. Designers tasked with investigating the design to see if it can be reused
in a different application may be impressed with how well the design would do in the new application. However if the design is highly interdependent, then those designers will also be daunted by the amount of work necessary to separate the desirable portion of the design from the other portions of the design that are undesirable. In most cases, such designs are not reused because the cost of the separation is deemed to be higher than the cost of redevelopment of the design.
Good Design Traits
- HIGH LEVEL MODULES SHOULD NOT DEPEND UPON LOW LEVEL MODULES BOTH SHOULDDEPEND UPON ABSTRACTIONS
- ABSTRACTIONS SHOULD NOT DEPEND UPON DETAILS SHOULD DEPEND UPON ABSTRACTIONS
Frankly, it is because more traditional software development methods, such as Structured Analysis and design, tend to create software structures in which high level modules depend upon low level modules,
and in which abstractions depend upon details. Indeed one of the goals of these methods is to define the subprogram hierarchy that describes how the high level modules make calls to the low level modules.
Consider the implications of high level modules that depend upon low level modules. It is the high level modules that contain the important policy decisions and business models of an application. It is these models that contain the identity of the application. Yet, when these modules depend upon the lower level modules, then changes to the lower level modules can have direct effects upon them; and can force them to change.
This predicament is absurd! It is the high level modules that ought to be forcing the low level modules to change. It is the high level modules that should take precedence over the lower level modules. High level modules simply should not depend upon low level modules in any way.
Moreover, it is high level modules that we want to be able to reuse. We are already quite good at reusing low level modules in the form of subroutine libraries. When high level modules depend upon low level modules, it becomes very difficult to reuse those high level modules in different contexts. However, when the high level modules are independent of the low level modules, then the high level modules can be reused quite simply. This is the principle that is at the very heart of framework design.
Layering
Structured object-oriented architectures have clearly-defined layers, with each layer providing some coherent set of services though a well-defined and controlled interface.” A naive interpretation of this statement might lead a designer to produce a structure similar to Figure 3. In this diagram the high level policy class uses a lower level Mechanism; which in turn uses a detailed level utility class. While this may look appropriate, it has the insidious characteristic that the Policy Layer is sensitive to changes all the way down in the Utility Layer. Dependency is transitive. The Policy Layer depends upon something that depends upon the Utility Layer, thus the Policy Layer transitively depends upon the Utility Layer. This is very unfortunate.
Figure 4 shows a more appropriate model. Each of the lower level layers are represented by an abstract class. The actual layers are then derived from these abstract classes. Each of the higher level classes uses the next lowest layer through the abstract interface. Thus, none of the layers depends upon any of the other layers. Instead, the layers depend upon abstract classes. Not only is the transitive dependency of Policy Layer upon Utility Layer broken, but even the direct dependency of Policy Layer upon Mechanism Layer is broken.
Martin Fowler’s list of Best Practices of Software Development:
- Maintain a Single Source Repository.
- Automate the Build
- Make Your Build Self-Testing
- Everyone Commits Every Day
- Every Commit Should Build the Mainline on an Integration Machine
- Keep the Build Fast
- Test in a Clone of the Production Environment
- Make it Easy for Anyone to Get the Latest Executable
- Everyone can see what's happening
- Automate Deployment
Ref:
http://martinfowler.com/
Non-Software Design Patterns - http://cns2.uni.edu/~wallingf/
