Architectural Design Principles

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Architectural Design Principles
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Outline

• Architectural level of design
• The design of the system in terms of components
  and connectors and their arrangements
• Architecting with design operators
• These are explicit transformations of a design
  decomposition, replication, compression,
  abstraction, and resource sharing.
• Functional design strategies
• Architectural designs can be transformed by
  applying higher-level macro operations such as
  design patterns and other design strategies.

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Architectural Level of Design

• Applying design principles
• Understanding quality attributes helps you to
  reason about specific characteristics
• Design principles guide the decisions that need
  to be made during the application of design
  methods.
• Systematic variation is a technique that helps
  the architect discover or create a solution field
  to a given problem.

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Architectural Level of Design (Contd)

• Applying design principles (contd)
• The architect can go down multiple paths forming
  a solution tree.
• At any time the architect may apply design
  evaluation techniques to test the quality
  attributes of the design.
• The architect can also apply architecture styles
  and patterns as operations.
• Operations are not necessarily transitive the
  result may depend on the order applied.

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Architectural Level of Design (Contd)

• Using Systems Thinking
• Considering the entire vies of the problem, not
  just the subproblems.
• Systems thinking requires a balance between
  looking at the big picture and looking at
  subpictures.
• Take a two pass approach
• On the first pass capture what comes to you and
  dont filter it or try to formalize it
• On the second and subsequent passes, remove,
  revise and refine as necessary.

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Architecting with Design Operators

• The application of design operators starts with a
  representation of the system as a single
  component that has externally visible
  characteristics and functions.
• The system is initially divided into two
  components, each with a well-defined purpose and
  the two components interact by the way of
  connectors.
• The system now appears as two modules that share
  a set of global design rules, which define the
  connectors.

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Common Software Design Operators

• Decomposition
• Replication
• Compression
• Abstraction
• Resource sharing

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Decomposition

• This is the operation of separating distinct
  functionality into distince components that have
  well-defined interfaces.
• It is the most important and most used principle
  in any engineering design field.
• It is used to achieve a variety of quality
  attributes including modifiability, portability,
  performance, and buildability.

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Decomposition (Contd)

• Two types of decomposition
• Part/whole
• Generalization/specialization
• Each application of the decomposition operator
  divides a component into two subcomponents.
• The choice of where to draw the line between
  components is driven by what quality attributes
  you are trying to emphasize.

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Identifying Functional Components

• Components are identified in several ways
• Interfaces The interfaces between the system
  and external entities should be associated with
  components, one per interface (or set of
  interfaces) but interfaces should not be
  fragmented across multiple components.
• Domains There are two types of domains
  application domains and solution domains. Each
  application domain (e.g., Content Authoring,
  Publishing, Marketing) may be modeled by a
  separate domain. Similarly you may identify
  components like Content Repository, Content
  Delivery, Project Management, and Royalty
  Management may be components in the solution
  domain.

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Identifying Functional Components (Contd)

• Components are identified in several ways
  (contd)
• Functional abstraction layers These decompose
  the system into a functional hierarchy. From
  this hierarchy it is possible to identify common
  functions that can be represented by shared
  components.
• Domain entities Just as a domain may be
  represented as a component, it may be useful to
  represent specific domain entities directly as
  components. In the publishing domain there is an
  entity called a Content Reviewer. It may be
  appropriate to model this entity as a component,
  or alternatively to model it in some process and
  not explicitly in the structure of the system.

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Composition/Aggregation

• This involves assembling components to form new
  components.
• The new components have behaviors that are not
  inherent in the subcomponents individually.
• This is not exactly the opposite of
  decomposition.
• The true opposite of decomposition is compression.

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Component Communication

• When a component is divided into two components
  there is an implied communications channel
  between them.
• It may be asynchronous or synchronous.
• Asynchronous communication decouples the
  processing of the two components which may
  improve performance and reliability, but it may
  add complexity.

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Compression

• This is the opposite of decomposition.
• It is the merging of components into a single
  component or removing layers or interfaces
  between components.
• Compression is used to improve performance by
  eliminating a level of indirection.
• For example, a presentation component may
  interact directly with a database instead of
  using middle-tier objects.

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Abstraction

• Abstraction hides information by introducing a
  semantically rich layer of services while hiding
  implementation details.
• Hiding the underlying implementation makes other
  components portable with respect to the
  abstracted component.
• Abstraction is also referred to as creating a
  virtual machine.
• It can be used to improve adaptability.

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Resource Sharing

• This is the encapsulating of data or services to
  share them among multiple independent client
  components.
• The result is enhanced integrability,
  portability, and modularity.
• Persistent data (such as that stored in
  databases) is a common shared resource.
• Resource sharing can simplify some aspects of
  systems configuration and management.

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Replication

• This is the operation of duplicating a component
  in order to enhance reliability and performance.
• There are two flavors of runtime replication
• Redundancy where several identical copies of a
  component execute simultaneously.
• N-version programming where there are several
  different implementations of the same
  functionality.
• A good design approach to redundancy and backups
  is to make component switching transparent to the
  other components that are using it.

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Functional Design Strategies

• Two strategies for achieving reliability
• Self-Monitoring -- This is a system that is able
  to detect certain types of failures and handle
  them appropriately. There are two approaches
• Process monitoring a layer above the
  application that watches over the system
• Component monitoring a monitor is established
  for monitoring each component
• Recovery This is a system that is able to
  restore itself after a failure.
• Instrumenting Similar to self-monitoring. A
  component can be instrumented to measure aspects
  of the execution.

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Summary

• General design methods combined with quality
  attributes and design principles yields an
  architectural design method that is heuristic and
  participative.