Robust Design Strategies II

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Robust Design Strategies II

• Gruia-Catalin Roman and Christopher Gill
• Washington University in St. Louis

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Overview

• 1. Software Architecture Fundamentals
• 2. Software Architecture Specification
• 3. Robust Design Strategies

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Robust Design Strategies

• Overview
• 3.1 Motivating factors
• 3.2 Design principles
• 3.3 Architectural styles
• 3.4 Design themes and design patterns

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3.3 Architectural Styles

• Architectural styles are abstractions for classes
  of organizational patterns encountered in
  software engineering practice
• structural patterns
• composition
• behavioral patterns
• communication
• coordination
• Themes address related design forces (specific
  concerns)
• Idioms capture frequently encountered rules of
  thumb
• Patterns capture reusable design solutions for
  frequently encountered problems
• Reference architectures relate classical and
  influential solutions proven in a specific domain

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More about Architectural Styles

• Characterization
• vocabulary
• configuration rules
• semantics
• Benefits
• design reuse
• predictability
• corporate knowledge

• Taxonomic principle
• The dominant strategy for separating of concerns
• composition
• communication
• coordination

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A Taxonomy of Styles

• Composition
• static
• layered
• controller
• client/server
• pipes and filters
• peer to peer
• proxy / object-based
• dynamic
• object-oriented
• software bus
• service provision

• Communication
• explicit
• message passing
• event propagation
• Coordination
• implicit
• blackboard
• shared tuple space
• database/futures
• Emerging styles
• new software domains
• mobile computing
• sensor networks

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Style Layered

• Components at one layer share access to resources
  provided by the layer below
• It is critical to limit or structure sharing
• Hierarchical structuring facilitates
• clean separation of constraints
• limited impact of changes

refill controller
pressure
pump
channel
D/A converter
A/D converter
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Style Controller

• System behavior
• abstracted as a set of finite state machines
• distributed among a small number of decoupled
  controllers
• limited information sharing
• synchronization limited to situations that demand
  consistency among decisions

• Complexity control
• logical and structural separation into multiple
  modes of operation
• simple and uniform transitions rules
• object encapsulation of non-finite state aspects
• partitioning of the control logic
• state reduction

heating controller
refill controller
panel controller
pressure
pump
heating element
panel
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Style Client/Server

• Natural extension of procedure calls to
  distributed computing
• Components are segregated into clients and
  servers
• Interactions are via remote procedure calls
• Servers do not need to know client identities
• Clients must know the identity of the servers

traffic manager
safety monitor
emergency controls
sector manager
trains in service
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Style Pipes and Filters

• Independent components process data in an
  incremental fashion
• Connectors are mechanisms for supporting data
  streaming between pairs of components
• Standardized and compatible interfaces are the
  key to composition
• Signal processing, UNIX shells, and image
  processing are common instances of this style
• Dataflow
• a generalization to arbitrary directed acyclic
  graphs
• image processing, workflow, etc.

satellite data reception
packet filtering
file, object, etc.
image construction
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Style Peer to Peer

• Components are active objects that know (or learn
  about) each other
• Interactions are direct
• Server support is often required for discovery
  and some coordination
• Sample applications
• music sharing
• ad hoc network interactions

Bob
Mary
video conference connection manager
Jim
Ann
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Style Proxy (Object-Based)

• Interface components decouple access to a
  resource from the resource itself
• resource updates do not affect the clients
• different clients may have different views
• communication protocols are hidden inside the
  proxy
• proxy may be remote
• proxy may be mobile code

engineer
architect
_at_ construction plan
_at_ construction plan
mechanical design
floor plan
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Style Object Oriented

• Components are objects and connectors are method
  invocations
• All architectural styles can have object oriented
  flavor
• E.g., allow substitution
• The stereotypical object oriented style entails
• web of cooperating objects
• reference dissemination is often excessive

30 msec.
cruise control
current speed
wheel
desired speed
throttle
driver
brake
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Style Software Bus

• Mechanism for decoupled access to
• distributed data
• services
• Static and dynamic object communities are
  supported
• Specialized services facilitate object
  registration, discovery, creation, and invocation
• CORBA is one standard that facilitates software
  bus construction

confidential screening
out patient
specialist
radiology
blood tests
admissions
patient records
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Style Service Provision

• Service providers register with service
  registries
• specialized ontologies facilitate understanding
• Clients
• services are discovered by querying service
  registries
• interactions with services are standardized
• proxy
• service access protocols (e.g., SOAP)

visitor
museum service registry
guide (proxy)
exhibit item
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Style Message Passing

• Connectors are message passing capabilities
• port (many to one)
• channel (one to one)
• mailbox (many to many)
• Component or connector identity is required to
  enable communication
• Bootstrapping requires some initial acquaintance
• Shared message interpretation and format
• High degree of decoupling and modularity is
  achievable
• Analysis relies on sequence and interaction
  diagrams

thermostat 243
thermostat 813
thermostat 326
control panel
thermostat 713
energy saver
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Style Event Propagation

• Publish/subscribe models
• are meant to function in distributed settings
• achieve a high level of decoupling
• client subscribes to specific events
• notifications are delivered automatically
• Explicit event propagation simplifies the design
  of reactive systems
• low level events are mapped to abstract events
• language support is available

refill controller
0.2 sec.
pressure
pump
low pressure
pump failure
channel
data ready
D/A converter
A/D converter
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Style Blackboard

• A shared data repository
• stores all key system data
• controls system activities through its own state
• Other components
• communicate with each other via the blackboard
• respond to changes in the state of the blackboard
• Sample systems
• planners
• rule-based decision
• compilers

trader
1n
stocks and bonds
regulator
1m
forecaster
1k
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Style Shared Tuple Space

• The tuple space serves as a central data
  repository
• out inserts a tuple
• Retrieval is contents-based (pattern matching)
• read obtains a tuple copy
• in removes a tuple
• High level of decoupling
• Minimal interface
• Databases
• large scale
• complex queries
• optimized processing
• reuse

checkout
1n
library catalog
search
1m
inventory
1k
powerful operations needed
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Style Mobile Computing
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Style Sensor Networks
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1
3
4
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Observations

• The architectural style impacts the properties of
  the system, but only to a certain extent
• distinct architectures may share a common style
• similar architectures may exhibit distinct
  properties
• Design rules may determine the extent to which
  resulting architectures acquire specific
  properties
• Architectural styles may be associated with
  specific application domains
• Management and study of architectural styles
• is mostly an art
• but has strong engineering implications