CSC/ECE 517 Fall 2010/ch7 7e ab

Protected Variations Pattern

Description

The Protected Variations pattern is a specific example of a GRASP design pattern. Any object that presents a potential instability within the system due to variations of type is implemented with an interface and polymorphism (discussed in chapter 3) is used to wrap the type. The interface could be a small enough to cover only one method or a very broad piece of the total subsystem.¹

Protected Variations addresses the problem of assigning responsibilities in such a way that variations that might occur do not have undesirable effects upon other elements in the system. The solution is to identify the points of predicted variation or instability and assign responsibilities to create a stable interface around them. In this case, interface is used in the broadest sense. It might be a complete subsystem fronting a much larger set of variable subsystem.This pattern, or rule, is very broad and complex.¹

The Protected Variations pattern protects elements from the variations on other elements (objects, systems, subsystems) by wrapping the focus of instability with an interface and using polymorphism (discussed in chapter 3) to create various implementations of this interface.²

•Problem: How to design objects, subsystems and systems so that the variations or instability in these elements does not have an undesirable impact on other elements.

•Solution: Identify points of predicted variation or instability; assign responsibilities to create a stable interface (or protection mechanism) around them.

•Data encapsulation, interfaces, polymorphism, indirection and standards are motivated by PV.

•Technology like Service Lookup is an example of PV because clients are protected from variations in the location of services using the lookup service.

•Externalizing properties in a property file is another example of PV.

•Extensions required for new variations are easy to add.

•The impact of cost of changes can be lowered.

³

Protected Variations decouples clients from the changes in the system being used.⁴

Advantages

•New implementations can be introduced without affecting clients.³ •Coupling is lowered.³

References

[1] http://web.cs.wpi.edu/~gpollice/cs4233-a05/CourseNotes/maps/class4/ProtectedVariations.html

[2] http://en.wikipedia.org/wiki/GRASP_(object-oriented_design)#Protected_Variations

[3] http://www.objectsbydesign.com/books/larman_notes/6-DesignAndImplementationTechniques.html

[4] Craig Larman. Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design and Iterative Development (3rd Edition). Prentice Hall, Upper Saddle River, NJ, 2004

TO BE REMOVED (included for development reference only)

7e. Protected Variations pattern. This is one of the GRASP patterns. Wikipedia: The Protected Variations pattern protects elements from the variations on other elements (objects, systems, subsystems) by wrapping the focus of instability with an interface and using polymorphism to create various implementations of this interface. Describe the pattern, and give examples of its use.

ACM books Authors. Title. Publisher, City of Publication, Year of Publication.

Originality. Chapter is not constructed by copy-and-paste, but rather is the writer's own work. Nor does it duplicate the organization of Wikipedia's article, or any other description of the topic. Rather, it should be presented in a way that differs from each of the sources.

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Definitions. Definitions should be clear and concise, and should be supported by explanations or examples. Definitions should be just that--not descriptions. Definitions should should be composed by the author, not taken from other textbooks.

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Diagrams. If the coverage of this topic would benefit from diagrams, are sufficient diagrams included? Are they easily understandable and well explained?

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Accuracy. Is the information in this chapter consistent with other descriptions of the same material? Can any errors be identified? Is information purported to be factual indeed a matter of fact rather than opinion?