Tarea: Design the Solution
Identify the elements and devise the interactions, behavior, relations, and data necessary to realize some functionality.

Render the design visually to aid in solving the problem and communicating the solution.

Disciplinas: Analysis & Design
Objetivo

The purpose of this task is to describe the elements of the system so that they support the required behavior, are of high quality, and fit within the architecture.

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Descripción principal

This task is about designing part of the system, not the whole system.  It should be applied based upon some small subset of requirements.  The requirements driving the design could be scenario-based functional requirements, non-functional requirements, or a combination.

This task can be applied in some specific context such as the database access elements required for some scenario.  In this case the task might be applied again later to deal with a different context on the same requirements.  Keep in mind that to actually build some functionality of value to the users, all contexts will typically need to be designed and implemented. For example, to actually utilize some system capability it will have to have been designed and implemented all its context such as user interface, business rules, database access, etc.

If this task is being performed on an architecturally significant element the results of this design should be referenced by the architecture.

Pasos
Understand requirement details

Examine the relevant Use Case(s) and Supporting Requirements to understand the scope of the design task and the expectations on the Design. Work with the Stakeholder and Analyst to clarify ambiguous or missing information.

If the requirements are not represented in some sort of scenario form (for example a non-functional requirement might not have a scenario associated with it), a scenario will have to be identified that appropriately exercises the requirements under consideration.

If the requirements are determined to be incomplete or incorrect, work with the analyst to get the requirements improved and possibly submit a change request against the requirements.

Identify design elements

Identify the elements that collaborate together to provide the required behavior.  This can start with the key abstractions identified in the architecture, domain analysis, and classical analysis of the requirements (noun filtering) to derive the elements that would be required to fulfill them.

Identify elements in all perspectives being considered when performing this task.  This could include identifying user interface elements, control elements, data elements, and elements relating to interfacing to other systems or devices.

Existing elements of the design should be examined to see if they should participate in the collaboration.  It is a mistake to create all new elements in each execution of this task.

This list of candidates must be expanded to include special-purpose participants that handle particular roles in providing the required behavior such as transaction processing or adherence to some security specification.  The Entity-Control-Boundary Pattern provides a good start for identifying elements.

Determine how elements collaborate to realize the scenario
Walk through the scenario distributing responsibilities to the participating elements. These responsibilities can be simple statements of behavior assigned to elements; they need not be detailed operation specifications with parameters, etc. This step is about ensuring that a quality model is being created that is robust enough to support the requirements. 

Identify the required relationships between the elements based on the walkthrough of the scenario examining how the elements initiate each other's behavior. An element that requests behavior from another element is represented as a relationship between those elements. As with the responsibilities, these relationships can just be defined at this step.

Look to the architecture and previous design work to create a consistent collaboration. Use the Architect to explain the details and motivations of the architecture. Look to reuse existing behavior and relations or to apply similar structure to simplify the design of the overall system.

Additional elements might be identified as behavior is found that cannot appropriately be assigned to any of the existing elements.

Refine design decisions

Refine the design to an appropriate level of detail to drive implementation and to ensure that it fits into the architecture.  In this step the design can take into consideration the actual implementation language and other technical decisions.  Revisit the identification of the elements and the collaborations that realize the scenarios if necessary as this refinement takes into consideration details at a lower level of abstraction. Discuss testability issues, such as design elements that are difficult to test or critical performance areas, with the Tester and Architect.

In particular make decisions in regard to the considerations below:

  • Specific details of relationships between the elements such as multiplicity and navigability
  • Operation detail such as parameters and return values
  • Existence and detail of data attributes necessary
  • Usage of inheritance and interfaces to improve the design

Incorporate Design Mechanisms from the architecture. Apply consistent structure of the elements and organization of the behavior as in other areas of the design and use patterns identified in the architecture.

Design internals (for large or complex elements)

Design large or complex elements or some complex internal behavior in more detail.  

This might just involve devising an algorithm that could be performed to produce the desired behavior. Add additional operations, attributes, and relationships to support the expectations of an element. Discuss testability issues, such as design elements that are difficult to test or critical performance areas, with the Tester and Architect.

The state of the element managed over the course of its lifetime can be designed to ensure proper behavior in various usages.

Communicate the design

Communicate the design to those who need to understand it. Though this is described here toward the end of the task, communication should be going on throughout the steps. Working collaboratively is always better than reviewing the work after it is complete.

Here are some ways to communicate the design:

  • Formal models specified in UML .
  • Informal diagrams that render static structure and capture dynamic behavior.
  • Annotated code that communicates information about the static structure supplemented with textual descriptions of dynamic behavior across code modules .
  • Physical data models to describe the database schema.

Here are some examples of individuals who will need to understand the design:

  • Developers who will implement a solution based on the design.
  • An architect who can review the design to ensure that it conforms to the architecture or who might examine the design for opportunities to improve the architecture.
  • Other designers who can examine the design for applicability to other parts of the system.
  • Developers or other designers who will be working on other parts of the system that will depend on the elements designed in this task.
  • Other reviewers who will review the design for quality and adherence to standards.
Understand the architecture

Understand how the architecture applies to this part of the design, and how this design work fits with the rest of the system. Incorporate any applicable interfaces, key abstractions, mechanisms and patterns into the design work. Discuss possible refinements to the architecture and new areas of potential re-use with the architect.

Design the database schema

If the system has persistent data, the design will need to address the database (or file) schema.  For a relational database schema, identify the following:

  • The structure of tables and views
  • Relationships between tables
  • Triggers which enforce referential integrity
  • Constraints on columns
  • Default values for columns
  • Stored procedures and functions
Evaluate the design

Evaluate the object design for coupling, cohesion, and other quality design measurements.

Evaluate the database/file design for performance, referential integrity, and normalization.

Consider the design from various angles to ensure that it is a high-quality, communicable design.  Work with other technical team members; an independent party can provide a fresh perspective. Use the Tester and Architect to provide perspectives on design quality and adherence to the architecture. However, when identifying potential reviewers keep in mind that if someone can add value by reviewing the design, then perhaps they could have added even more value by actively participating in the design effort itself.

If design flaws are identified, improve the design.

Factores clave

Each step in this task can cause all previous steps to be revisited in light of new information and decisions.  For example, while determining how elements collaborate you might find a gap in the requirements that causes you to go back to the beginning after collaborating with the analyst, or when evaluating the design a reviewer could note that a reusable element being used doesn't work as expected and that could cause you to identify new elements to take its place.

Consider the architecture while performing this task.  All design work must be done while regarding the architecture within which the design exists.  Furthermore, certain design elements will be deemed architecturally significant; those elements will require updates to the architecture.

This task will be applied numerous times.  Design is best performed in small chunks.

Even when starting the design for a particular project it is expected that there will be existing frameworks and reusable elements.  Every step of this task must give attention to the existing design and existing implementation, utilizing existing elements when possible and emulating or improving existing elements as appropriate while designing this portion of the solution.

Apply patterns throughout this task.  Patterns represent proven designs and their usage promotes quality and consistency across the design.

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