Design

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Design
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Role of Design

• Develop blueprints for implementation
• Q What does this mean?

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Design Model Hierarchy
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Difference Between Analysis and Design Models
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Example
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Design as Use Case Realization

• Collaboration within design model describes how a
  use case is realized
• Should maintain traceability to use case
  realization in analysis model
• Provides physical realization of the use case and
  handles non-functional requirements

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Design Subsystems

• Subsystems organizes system into manageable
  pieces
• Subsystems can be very large and hierarchical
• But, a subsystem should be cohesive, and they
  should be loosely coupled
• Top level subsystems normally have direct trace
  to analysis packages
• Can represent reused software products or legacy
  systems by wrapping them - means of integration

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Architectural View of Design Model

• Decomposition of design model into subsystems,
  their interfaces, and dependency between them -
  subsystems and their interfaces make up
  fundamental structure of software system
• Key design classes - those trace to
  architecturally significant analysis classes,
  those general and central (e.g. abstract
  classes), those having many relationships
• Designs that realize key use cases

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Deployment Model

• Object model describing physical distribution of
  system in terms of how functionality distributed
  among computational nodes
• Each node a computational resource, e.g.
  processor
• Nodes have relationship representing means of
  communication
• Has major impact on design and implementation
• Q. Why?

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Architectural Design

• Purpose is to outline the design and deployment
  models and their architecture by identifying
• Nodes and their network configurations
• Subsystems and their interfaces
• Architecturally significant classes
• Generic design mechanisms handling common
  requirements, e.g. performance, persistency,
  distribution
• Many possibilities to consider, need to weight
  whats more important

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I/O of Architectural Design
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Identifying Nodes and Network Configurations

• Network configuration often have major impact on
  system architecture, including active classes
  required and distribution of functionality among
  network nodes
• Which nodes involved and whats their processing
  power and memory size?
• What types of connections between the nodes, and
  what protocols
• Bandwidth, availability, quality?
• Need for redundant capacity, fault tolerance,
  process migration, backup, etc.?

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Example
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Identifying Subsystems and Interfaces
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Middleware Layer
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Identifying Subsystem Interfaces

• From subsystem dependencies
• Inherit from analysis packages
• analysis class interfaces that are referenced
  from another package

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Identifying Active Classes

• Performance, throughput, availability factors
• System distribution onto nodes
• System startup and termination, liveness,
  deadlock avoidance, starvation avoidance,
  reconfiguration of nodes, and capacity of
  connections
• Q what kind of objects most likely to be active
  objects?

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Identifying Generic Design Mechanisms

• Persistency
• Transparent object distribution
• Security features
• Error detection and recovery
• Transaction management
• Concurrency control, etc.

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Example Generic Collaboration Used in Several
Use Case Realizations
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Mapping to Invoice Buyer Use Case
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Design Use Case

• Purpose
• Identify design classes and/or subsystems whose
  instances are needed to perform the use case flow
  of events
• Distributed the behavior of the use case to
  interacting design objects and/or subsystems
• Define requirements on the operations of design
  classes and/or subsystems and their interfaces
• Capture implementation requirements for the use
  case

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Identifying Participating Design Classes

• Identify design classes that trace to those
  participating classes in the analysis model
• Identify design classes that implement
  non-functional requirements
• From analysis model
• Discovered in the design process
• Identify missing classes

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Pay Invoice Use Case Realization
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Describe Object Interactions

• When use sequence diagrams
• If there are multiple flows, use one sequence
  diagram for each flow
• Should be based on object interaction defined in
  analysis model (traceability)
• Each design class identified in the realization
  of the use case should have at least one object
  in the sequence diagram
• Focus on sequence of interaction or message
  exchange
• Should handle all relationships of the use case

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Example
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Identifying Participating Subsystems and
Interfaces
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Design a Class

• Its operations
• Attributes
• Relationship it participates in
• Methods (that realize the operations)
• Its imposed states
• Its dependency to any generic design mechanisms
• Requirements relevant to its implementation
• Correct realization of any interface it is
  required to provide

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