Layered System Examples and Variants EEE465 2001 Reference: BMRSS96 2'2 Layers

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Layered System Examples and Variants EEE465
2001Reference BMRSS96 2.2 Layers

• Major Greg Phillips
• Royal Military College of Canada
• Electrical and Computer Engineering
• greg.phillips_at_rmc.ca
• 1-613-541-6000 ext. 6190

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Context

• Examining layered systems
• Looked at general characteristics and a design
  process
• Today, look at
• some examples
• variants
• benefits and liabilities

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The Internet Protocols

• virtual protocols key issue
• virtual protocols are the horizontal arrows why
  are these called virtual?
• inter-layer interfaces secondary concern
• application developers typically use a thin layer
  on top of TCP (e.g., sockets)
• layering is not absolute sockets access IP layer

FTP protocol
FTP
FTP
TCP protocol
TCP
TCP
IP protocol
IP
IP
Ethernet protocol
Ethernet
Ethernet
Physical connection
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The Seeheim and Arch Models
application
functional core
domain objects
application logic
application interface
functional core adapter
user interface
domain objects
dialogue controller
dialogue
presentation objects
logical interaction
interaction objects
presentation
physical interaction
Seeheim
Arch
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Adaptive Communication Enviroment (ACE)
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Java ACE
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Management Information Systems
Presentation
Application logic
Domain
Database
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Windows NT Executive
System Services
Object Manager
Security Reference Monitor
Process Manager
IO Manager
Virtual Memory Manager
Kernel
Hardware Abstraction Layer
Hardware
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Palm OS Robot Kit
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Variants

• Relaxed layered system
• each layer may use services of all layers below,
  not just adjacent layers
• some layers may be partially opaque
• gain in flexibility, performance
• loss in maintainability
• generally found in infrastructure systems with
  little maintenance, high performance requirements
• Layering through inheritance
• lower layers are base classes
• higher layers inherit from lower layers
• allows selective modification of lower layers
• closely ties layers together may suffer from the
  fragile base class problem

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Benefits

• reuse of layers
• requires well-defined abstraction, well
  documented interface
• some evidence that black-box reuse of layers can
  reduce development effort, defects
• support for standardization
• allows products from different vendors to be
  mixed
• localised dependencies, increased testability
• effect of code changes usually confined to single
  layer
• exchangeability
• can replace at design time (code changes),
  compile time (component selection) or run-time
  (adapter)

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Liabilities

• cascades of changing behaviour
• if the behaviour of a layer changes dramatically
  (e.g., becomes 10x as fast) this may have large
  ripple effect through the architecture
• lower efficiency
• multiple transformations through layers
• unnecessary work
• e.g., error correction at multiple layers
• difficulty in establishing correct granularity
  of layers
• e.g., client-server, three-tier, multi-tier

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Next ClassData Flow Architectures