Design Patterns

We mentioned design patterns before. Once more: What is a design pattern?

A known solution for a common problem.
Design patterns give us a system of names and ideas for common problems.

� What are the major description parts?
� Design Patterns consist of the following parts:

� - Problem Statement
� - Solution
� - Impact (Side Effects)
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There are different types and levels of design patterns.
For example, the Model-View-Controller (MVC) is the architectural level of design pattern.
Most design patterns are component level design patterns.
� The basic types are Behavior, Creational, Structural, and System design patterns.
� More types: Enterprise and SOA Design Patterns

Names are extremely important in design patterns; they should be clear and descriptive.

Christopher Alexander � construction architect � invented the main principles and published the first book on Design Patterns.
One of the first books in the software world was: "Design Patterns: Elements of Reusable Object-Oriented Software" by Erich Gamma, Richard Helm, Ralph
Johnson, John Vlissides (often called the Gang of Four or GOF � Book :-)
Among other good books: Integration-Ready Architecture and Design.

Here are several examples of Design Patterns:
Name: Service-Oriented Architecture (Approach to software architecture)
Problem: Application development or even modification require longer and longer projects.
Why: Growing applications become more complex and rigid; too firm and inflexible in spite of the name � Software
Solution: Develop applications as a set of connected standard-based reusable services to minimize duplications and maintenance.

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Impact: Extra time and efforts needed to learn and follow best practices and standards and to govern the development process.

Periodically re-visit and align technical and business architecture, carefully consider and govern creating new applications to avoid duplications and minimize maintenance.
Impact: Extra funding, time and efforts are needed for governance, while planning to retire some applications, modify and integrate others.
Name: Business-Driven Architecture (Architecture Level)
Problem: How can technology be designed to remain in alignment with changing business goals and requirements?
Solution: Business and architecture analysis is conducted as collaborative efforts on a regular basis.
Impact: To keep technology in alignment with the business that is changing over time, it will require a commitment in time and cost to govern the development process.

Name: MVC (Architecture Level)
�Problem: � Complex object involves user interface and data. Need to
simplify structure
Solution: Data in one part (Model), user View in another part (View), interaction logic in a third part (Controller)
� Model maintains state. Notifies view of changes in state.
� Controller uses state information (in Model?) and user request to determine how to handle request, tells view what to display
� View must correctly display the state of the Model
Impact: Requires separate engineering of the three parts and communication between them through interfaces

Name: Factory Method (Component Level � Creational Type)
Problem: Need to create a family of similar but different type objects, where objects are used in standard ways.
Solution: Creator class has a "getter" method which instantiate the correct subclass, for example, ConcreteProduct. Subclass is used through generic interface, for example, Product Factory.
Impact: Extra time for analysis and modeling upfront required.

Assignments:
1. Read more on Design Patterns: http://ITUniversity.us/school/public/training/SOADesignPatterns.pdf
2. Create 2 QnA related to the Design Pattern section, each with the following format where the first answer is correct and two other answers are wrong.
Question:
Answer:
Answer:
Answer:
3. Email the QnAs to dean@ituniversity.us