Component Development

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Component Development

• Taciana Amorim Vanderlei
• tav_at_cin.ufpe.br

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Contents

• The methodology
• Problems of asynchrony
• Multithreading
• Learning from circuit design
• Living without implementation inheritance
• Nutshell classes
• Language support
• Caller encapsulation
• The enviroment
• The tools

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The methodology

• Requires support for component-oriented
  programming
• polymorphism
• modular encapsulation
• late binding and loading
• safety

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Problems of asynchrony

• Multicasting
• Event object recipients are themselves free to
  post new events.
• The set of recipients could change while a
  multicasting is in progress.
• Some of the recipients might raise exceptions
  while handling received event objects and the
  multicast is in progress.

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Multithreading

• Concept of supporting multiple sequential
  activities concurrently over the same state
  space.
• Problems
• Conflits from concurrent writes
• Deadlocks
• Degradation of performance with synchronization
• Prolonged locking of frequently shared resources
• Propagation of exceptions across thread
  boundaries
• Difficult to debug code that uses multiple
  threads and complex interlocking patterns.

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Multithreading

• A concept that would help to reduce complexity in
  many such cases is transactional programming.
• Application server technologies that are
  increasing availability of transactional
  infrastructure support for components
• COM
• EJB
• CCM
• CLR

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Learning from circuit design

• Fully asynchronous circuits
• Best performance
• Complex to design
• Synchronous circuits
• Synchronize all activities of component
  boundaries to clock signals.
• Require asynchrounous communication at the
  boundary.
• Synchronous communication at the boundary
  requires asynchronous circuit.
• Introdution of remote procedure calls in the
  software
• Tendency to support synchronous communication,
  forcing the introdution of asynchronous
  computation.

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Living without implementation inheritance

• Subclass a class introduces implementation
  overhead and runtime overhead.
• COM-style aggregation helps to avoid the
  performance implications of forwarding.
• The implementation cost can be hidden by using
  various forms of automation.
• Generate the code of a forwarders class
• Use a template mechanism.

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Nutshell classes

• It has the same interface as the object it is
  forwarding to, and all methods are implemented as
  plain forwards to the target object.
• Nutshell classes are themselves abstract.
• Recently added to Java in the form of the proxy.
• CLR offers a similar mechanism via its RealProxy
  class.

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Language support

• The programming overhead is minimal and the
  runtime overhead in time and space is equal of
  implementation inheritance schemes.
• Objective-C support dynamic inheritance from an
  object.

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Caller encapsulation

• Need to encapsule the caller rather than the
  callee.
• Caller encapsulation is put to good use in
  several places in the BlackBox component
  framework.

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The enviroment selecting target framework

• A component object cannot function outside a
  defined enviroment.
• A component object can be design to operate in
  multiple such enviroments simutaneously.
• Danger to investing heavily in solutions based on
  a single framework.

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The tools selecting programming languages

• Requirements to component programming selecting
  language
• Polymorphism
• Late binding
• Encapsulation
• Safety
• Explicity state dependencies
• Language paradigm level
• Object-oriented
• Java and C
• Functional

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Component distribution and acquisition
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Contents

• Building what sells
• Product cataloging and description
• Component location and selection
• Superdistribution
• Intermediaries

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Building what sells

• In the past was a lack of business models.
• Slowly changing with the growth of specialized
  markers of software components and
  intermediaries.
• Software services are becoming increasingly
  popular.

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Product cataloging and description

• It is not clear how to specify a software
  component.
• Component specification need to be placed in an
  architectural context that includes
• Possible embedding in component framework
• Platform assumptions
• Requirements
• Styles of composition.

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Component location and selection

• How can component services be described to
  allow for efficient retrievel and hight precision?

Ontolological approaches
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Component location and selection

• KIF, developed at standford, proposes a standard
  (logic.standford.edu/kif).
• Project called Reuse Center in Corinto, Italy
  (www.corinto.it).
• Computer Industry Project (SCIP), project at
  Standford (www.standford.edu/group/scip).

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Superdistribution

• Pay-per-use
• Send a component to everyone who might at all
  benefit.
• Works by encouraging users of a component to send
  it to friends.

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Intermediaries

• Represent bundled interests of one side to the
  other.
• Intermediary services
• Quality control
• Integration services across products
• Application service provider (APS)

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Component assembly
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Contents

• Systematic initialization and wiring
• Visual component assembly
• Compound documents to supersede visual assembly
• Componente beyond graphical user interface
  environmetns
• Managed and self-guided component assembly
• End-user assembly
• Component evolution

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Systematic initialization and wiring

• Composition
• Selecting components and interconnecting those
  using selected connectors.
• BML (bean markup language) is a JavaBeans
  composition language.

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Visual component assembly

• One way of simplifying the assembly process.
• JavaBeans and COM support general connection
  paradigms.

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Compound documents to supersede visual assembly

• Documents are applications and document editing
  is component (instance) assembly.
• Integration of build and use enviroments,
  especially in the context of compound documents.
• Strong and produtive case for application
  development.

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Components beyond graphical user interface
environments
Reusable components valuable assets
Modern graphical user interfaces
Relative regularity of user interfaces

• Server-based solutions in particular have
  inspired many of the current components software
  approaches.
• Server-side solutions
• Suns EJB
• Microsofts COM
• Suns servlets
• Microsofts VB
• Microsoft .NET

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Managed and self-guided component assembly

• Managed assembly
• Automated assembly component that implements the
  policies that govern the dynamic assembly of
  instances.
• Self-guided assembly
• Similar as above, but uses rules that are carried
  by the component instances themselves.

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End-user assembly

• Enable end users to assemble custom solutions.
• Anders Morch termed this feature distinguished
  three levels
• Customization
• Integration
• Extension
• Robert Slagter and Henri ter Hofte show an
  application with computer-suported collaboration.
• Groove Transceiver (www.groove.net)

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Component evolution

• Potential components increases with deferral of
  assembly gt Fragility increases
• Problems of version conflits
• Upgrading of operacional instances to newer
  versions is of active research.
• COM robust approach to supporting version
  coexistence.
• Java does not have a special version control
  mechanism.

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Questions?