CSC/ECE 517 Fall 2009/wiki3 1 co

Software Design Anti-Patterns

  Not all patterns are good. Anti-patterns are patterns that initially seem effective, but over time you learn that they lead you into traps. 
      - Scott Klement, Anti-Patterns: Avoid the Programming Dark Side

In this article, we will describe several software design patterns which have been categorized as "anti patterns": Programming approaches which are not uncommon and which can lead to poor program design and performance.

What are Anti-Patterns?

Different texts define the term anti-patterns in different ways:

• "An AntiPatern is a literary form that describes a commonly occurring solution to a problem that generates decidedly negative consequences." - AntiPatterns: Refactoring Software, Architectures, and Projects in Crisis
• "Anti-patterns are patterns that initially seem effective, but over time you learn that they lead you into traps." - Anti-Patterns: Avoid the Programming Dark Side
• Over the last decade we have witnessed the maturity of both best practices...and worst practices (as manifested in the anti-patterns movement...). - Patterns of Anti-Patterns?, Journal of Object Technology

In each definition, the implementer of the anti-pattern is trying to solve a problem in a common way, however the common way is a bad design that can lead to unintended consequences. Some of the reasons cited in the AntiPatterns book as to why these anti-patterns have been used include: Haste, sloth (or laziness), ignorance, apathy, and pride. Studying anti-patterns can at least help assist the potential anti-pattern implementer in the ignorance category.

Considerations When Writing Anti-Patterns

When writing patterns, there are various templates to from which to choose, for example, the pattern template from the Gang of Four's Design Pattern book:

• Pattern Name and Classification
• Intent
• Also Known As
• Motivation
• Applicability
• Structure
• Participants
• Collaboration
• Consequences
• Implementation
• Sample Code
• Known Uses
• Related Patterns

Brown et al in their book AntiPatterns suggest 3 different possible templates for writing anti-patterns:

• Pseudo-AntiPattern Template: Only the name and problem are described
• Mini-AntiPattern Template: Includes the name, AntiPattern Problems, and Refactored Solution
• Full AntiPattern Template: Includes 18 different sections such as Root Causes, Background, Known Exceptions, and Examples.

For the purposes of brevity, this wiki page will use the Mini-AntiPattern Template form with the addition of an Example section.

Anti-Pattern Examples

This section presents six anti-patterns from the book AntiPatterns: Refactoring Software, Architectures, and Projects in Crisis.

The Blob or God Class

Anti-Pattern Problem

A design includes a class that has the majority of attributes and/or methods in the system. According to Brown et al, this anti-pattern could be the result of using a procedural design in an object-oriented language, and therefore imbuing one of the classes with the majority of the methods and seeing that class as a "controller" class. Another sign that the Blob anti-pattern may be present is if a class is designated as a "utility" class, which indicates a possible catch-all class for unrelated classes.

Refactored Solution

Brown et al suggests refactoring responsibilities into appropriate classes using the following steps:

• Identify cohesive sets of operations and attributes that relate to a common focus, behavior, or function.
• Review the system to see if there are "natural homes" for these collections in existing classes.
• Remove redundant or "far-coupled" associations.
• Migrate associates to derived classes to a common base class.
• Remove all transient associations, replacing them as appropriate with type specifiers to attributes and operations arguments.

Example

An inappropriate use of the Rails framework, where a majority of the methods reside in a single controller class or model class. To resolve the issues, helper classes or other classes could be created which would more appropriately contain related methods that were inappropriately kept in a controller or model.

Lava Flow

Anti-Pattern Problem

A design includes obsolete technologies or unused or misunderstood extensions. This problem usually occurs when prototype or research code is used as the basis for a production system, or when a code base has existed over a long period of time and the code base was not updated to reflect changes in surrounding technologies or systems.

Refactored Solution

Often times, the presence of obsolete or unused code is the result of lacking a sound design and architecture that is followed and updated throughout the system lifecycle. To resolve the issue of obsolete code, a redefinition of the architecture is necessary, with removal of "dead" code as the system is refactored to the new architecture. There is no easy fix for this issue, and it will take time to reengineer the system to remove redundancy and obsolescence.

Example

A financial program that helps customers manage their stock portfolio was developed as a web application with using HTML 2.0 and Java 1.3 servlets. Over time, the program was enhanced by building on the existing code base. Eventually the system was upgraded to serving XHTML 5 pages and using Java 6 in the server. Because of the lack of comprehensive design and architectural documentation, portions of the code use depracated methods and badly formatted HTML in places because the current development team does not understand the system to know what is "safe" to change or remove.

Poltergeists

Anti-Pattern Problem

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Refactored Solution

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Example

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Golden Hammer

Anti-Pattern Problem

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Refactored Solution

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Example

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Spaghetti Code

Anti-Pattern Problem

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Refactored Solution

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Example

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Cut-and-Paste Programming

Anti-Pattern Problem

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Refactored Solution

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Example

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Conclusion

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References

• www.antipatterns.com
• W. J. Brown et al., AntiPatterns: Refactoring Software, Architectures, and Projects in Crisis, John Wiley & Sons, Inc., New York, NY, 1998.
• S. Klement, Anti-Patterns: Avoid the Programming Dark Side, 2008
• Wikipedia entry: Anti-pattern
• Anti Patterns Catalog offered by Cunningham & Cunningham, Inc.
• What is an AntiPattern? from Sourcemaking.com
• Anti-Patterns and Worst Practices – You’re Doing it Wrong!
• M. H. Dodani,Patterns of Anti-Patterns?, Journal of Object Technology, Vol 5, No 6, pp 29-33, 2006
• E. Gamma et al,Design Patterns: Elements of Reusable Object-Oriented Software, Addison-Wesley Professional, 1994.
• G. Meszaros and J. Doble, A Pattern Language for Pattern Writing.