Software System Design

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Software System Design

• System Architecture
• Distributed Systems
• Object-Oriented Design
• Design with Reuse (component-based design)
• Designing User Interfaces

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Software System Design

• System Architecture
• System structuring
• Control models
• Modular decomposition
• Domain-specific architectures

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Software Architecture

• What is a Software Architecture?
• High-level description of a complex software
  system
• Includes both diagrams and text
• Simplified diagrams often used for marketing
  (e.g. Apples OS X)

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Software Architecture

• All systems have an architecture, but what are
  the benefits of designing (and specifying) one?
• Stakeholder communication
• System analysis (reasoning about non-functional
  requirements)
• Reuse
• Project management (concurrent development)

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Software Architecture

• What is a Software Architecture?
• Answers questions like
• What are the primary subsystems?
• What is the division of labor?
• How will the subsystems communicate?
• What will the subsystems communicate?

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Subsystems
A subsystem is a system in its own right whose
operation does not depend on services provided by
other systems. Subsystems are composed of
modules, and have defined interfaces used for
communication with other subsystems.
Sommerville, p. 217

• .
• Design/implementation often delegated to
  different engineering teams
• Development can proceed in parallel
• Separation of responsibility
• Minimal inter-subsystem communication

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Software Architecture

• Subsystem breakdown guided by
• Performance
• Security
• Safety
• Availability
• Maintainability

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Software Architecture

• Subsystem breakdown guided by
• Performance
• Minimize communication among subsystems
• Security
• Safety
• Availability
• Maintainability

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Software Architecture

• Subsystem breakdown guided by
• Performance
• Security
• Use layered architecture, with critical assets in
  inner layers
• Safety
• Availability
• Maintainability

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Software Architecture

• Subsystem breakdown guided by
• Performance
• Security
• Safety
• Isolate safety-critical components
• Availability
• Maintainability

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Software Architecture

• Subsystem breakdown guided by
• Performance
• Security
• Safety
• Availability
• Redundancy, replication of services
• Maintainability

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Software Architecture

• Subsystem breakdown guided by
• Performance
• Security
• Safety
• Availability
• Maintainability
• Subsystems modules with self-contained components

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Structural Models

• The Repository
• The Client-Server Model
• The Abstract Machine

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The Repository Model

• Shared data is stored in centralized location
  (the repository subsystem), and accessed by
  various (other) subsystems
• Shared data is passed from one subsystem to the
  next (pipeline architecture)
• E.g., compiler

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CASE toolset architecture
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The Repository Model

• Characteristics
• Sharing model is published as repository schema
• Pros
• Efficient way to share large amounts of data
• Data integrity localized to repository module
• Cons
• Subsystems must agree (i.e., compromise) on a
  repository data model.
• Schema evolution is difficult and expensive
• Distribution can be a problem

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Client-server architecture

• Subsystems are viewed as independent processes.
• Set of stand-alone servers which provide specific
  services such as printing, data management, etc.
• Set of clients which call on these services.
  Servers can be clients of other servers.
• Communication network

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Film and picture library
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Client-server characteristics

• Pros
• Makes effective use of networked systems. May
  require cheaper hardware
• Easy to add new servers or upgrade existing
  servers
• Availability (redundancy) may be straightforward
• Cons
• Data interchange can be hampered by different
  data layouts
• Communication may be expensive
• Data integrity functionality must be implemented
  for each server
• The naming problems

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Abstract machine model

• Organises the system into a set of layers (or
  abstract machines) each of which provide a set of
  services
• Supports the incremental development of
  sub-systems in different layers. When a layer
  interface changes, only the adjacent layer is
  affected
• Cant model all systems this way

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Abstract machine model

• Examples
• Version Management System
• Operating System (e.g., Linux)
• Runtime Language Environments (e.g., Java)

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Version management system
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