Applying Architectural Styles and Patterns

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Applying Architectural Styles and Patterns
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Outline

• Defining Architectural Patterns and Style
• The activation model
• Styles and Quality Attributes
• Common Architectural Styles
• Dataflow systems
• Call-and-return systems
• Independent components
• Virtual machines
• Respositories

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Introduction

• Relying solely on design operators to design an
  architecture can be a slow process.
• By utilizing existing design experience from the
  software engineering community design
  productivity can be improved.
• There are two types of reusable architectural
  products
• Product families
• Architectural styles and patterns

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Architectural Style

• It is a type of metamodel that prescribes a set
  of elements and their relationships that
  characterize a system based on the style.
• These elements are expressed as components and
  connectors.
• Their relationships are the constraints on how
  components and connectors may be combined.

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Architectural Patterns

• A pattern is a solution that strikes some balance
  between competing forces.
• However, what makes it a pattern is the fact that
  it is recurring.
• If a design solution in its essential distilled
  form is found in multiple systems and is a
  successful design, then this solution is a
  pattern.

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Architectural Patterns (Contd)

• A pattern language is a set of patterns, together
  with the rules for describing how patterns are
  created and how they must be arranged with
  respect to other patterns.
• The patterns are the vocabulary of the language,
  and the rules for their implementation and
  combination are the grammar.
• Patterns in a pattern language can be constructed
  from other patterns.

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Architectural Styles

• An architectural style is a framework for a
  solution and not really a solution itself.
• The selection of a style limits the scope of the
  solution space.
• According to Ivar Jacobson, the architectural
  style of a system is the denotation of the
  modeling languages used when modeling the
  system.
• It includes the guiding principles and
  prescriptions that the other development model
  elements must satisfy.

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Architectural Styles (Contd)

• It is the foundation of a philosophy of building.
• An architectural style defines a vocabulary of
  components and connector types and a set of
  constraints on how they can be combined.
• Most applications, however, are a combination of
  styles, i.e., they have a heterogeneous
  architectural style.
• There are many parameters that we can use to
  characterize software architecture
• The way in which control is passed between
  components (activation model)
• The quality attribute emphasis (e.g.,
  maintainability and performance)

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Activation Model

• This model describes how components are activated
  and how information is passed between them (the
  control principle of the architecture).
• In classic procedural languages and their
  object-oriented derivatives, method invocation
  and method execution are bound, i.e., method
  execution follows immediately from method
  invocation.
• In languages like Smalltalk method invocation and
  method execution are decoupled.

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Activation Model (Contd)

• At the design level you can simulate these
  activation models in the language level.
• Synchronous communication means that the client
  component activates a server component and then
  waits for a response.
• A variation of this is the balking model in which
  a component may reject a request for any reason.

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Activation Model (Contd)

• Another variation is the timeout in which a
  client component assumes that the server has
  failed to complete a request after a certain
  amount of time.
• Asynchronous communication means that the client
  doesnt wait on a response from the server before
  continuing.
• In the delegating pattern, a client component
  invokes a server component and provides an
  address to which the response is to be sent.

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Activation Model Patterns of Information Passing

• Data elements can be passed directly between
  processing elements (pipes and filters).
• Data elements may also be shared in a central
  store and accessed by different processing
  elements (repository).

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Uses of the Activation Model

• To compare different architectural styles.
• To identify the appropriate architectural style
  for the entire system or the best combination of
  architectural styles.

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Styles and Quality Attributes

• Each style emphasizes a set of quality attributes
  while de-emphasizing another set.
• Pipes and filters emphasizes reusability and
  adaptability but sacrifices maintainability with
  respect to changes in data representation.
• Object-oriented systems emphasize maintainability
  but are not easily reused

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Styles and Quality Attributes (Contd)

• Repository-based systems emphasize adaptability
  and performance but are not highly reusable or
  maintainable in terms of changes to algorithms or
  data representations
• Styles can be combined in order to exhibit
  different quality attributes.

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Common Architectural Styles (Adapted from Shaw
and Garlan)

• Dataflow systems
• Batch sequential
• Pipes and filters
• Call-and-return systems
• Main program and subroutine
• Object-oriented systems
• Hierarchical layers

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Common Architectural Styles (Contd))

• Independent components
• Communicating processes
• Event systems
• Virtual machines
• Interpretors
• Rule-based systems
• Repositories
• Databases
• Hypertext systems
• Blackboards