Software%20System%20Design

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Software System Design

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

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Control Models

• How subsystems behavior is coordinated
• Orthogonal to system structure
• Two broad categories
• Centralized control
• Master subsystem
• Event-based control
• Each subsystem able to respond to external events

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Centralized Control

• Master subsystem has responsibility for managing
  execution of (other) subsytems
• Call-Return Model
• Manager Model

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Centralized Control

• Master subsystem has responsibility for managing
  execution of (other) subsytems
• Call-Return Model
• Subsystems invoked using traditional
  subroutine/function calls
• Applicable in sequential execution only
• Manager Model

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Call-Return Model (figure)
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Centralized Control

• Master subsystem has responsibility for managing
  execution of (other) subsytems
• Call-Return Model
• Manager Model
• Subsystems run as separate processes
• Manager starts/suspends/stops processes
• Implied in concurrent systems, can be useful in
  sequential ones

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Event-Based Control

• No master controller
• Subsystems react to external events
• Subsystems can signal events to others
• Specific examples
• Spreadsheet (cell state changes)
• Soar, Linda (AI-inspired production systems)
• Ethernet

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Event-Based ControlBroadcast Models

• Event is broadcast to all subsystems
• Appropriate subsystem responds
• Conflict resolution?
• Event Handler
• Subsystem can register interest in particular
  events
• Handler passes off to registered subsystem
• Reduces overhead on uninterested subsystems

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Event-Based ControlBroadcast Models

• Pros
• Simple to evolve (add/remove/change event
  handlers)
• Subsystems dont need to know others by name
• Cons
• Caller doesnt know if/when event will be handled
• Conflict resolution

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Event-Based ControlInterrupt-Driven Models

• Preferable to broadcast models for real-time
  reaction
• Interrupt handlers
• Hardware supported
• Triggered by raising of interrupt event
• Assume control of subsystems
• E.g. air-bag deployment fire-control systems

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Event-Based ControlInterrupt-Driven Models

• Pros
• Allows for fast responses context switches
• Cons
• Complex to program
• Difficult to validate (reproducing timings)
• Number limited by hardware

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Domain-Specific Architectures

• Can be useful to frame, guide architectural
  design for a specific domain
• Generic Models
• Generalized architecture of real applications in
  domain
• Reference Models
• Idealized architecture for an application domain

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The OSI Networking Model

• OSI Open Systems Interconnection
• A layered model for communication systems
  (similar to Abstract Machine Model)

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The OSI Networking Model

• 7. Application
• 6. Presentation
• 5. Session
• 4. Transport
• 3. Network
• 2. Data Link
• 1. Physical