Components, Component Models and Reuse

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Components, Component Models and Reuse

• ICS 123
• Richard N. Taylor and Eric M. Dashofy
• UC Irvine
• http//www.isr.uci.edu/classes/ics123s02/

with very special thanks to David S. Rosenblum
for the use of his materials..
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What Is Reuse? (I)

• Develop systems of components of a reasonable
  size and reuse them
• Further use or repeated use of a component
• Adapting components for use outside their
  original context
• Extend the idea beyond code to other development
  artifacts
• Goals of reuse are primarily economic
• Save cost/time/effort of redundant work, increase
  productivity
• Decrease time to market
• Improve systems by reusing both the artifact and
  the underlying engineering experience
• Economic goals achieved only when units of reuse
  reach critical mass in size, capability and
  uniformity

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What Is Reuse? (II)

• Idea originally due to Doug McIlroy
• Mass Produced Software Components, 1968 NATO
  Conference on Software Engineering
• Reusable components, component libraries
• Named as a potential silver bullet by Fred
  Brooks (1987)
• A lot of research interest in the 1980s and 1990s
• Technical and managerial barriers have prevented
  widespread success with reuse and with
  reuse-oriented technologies
• This led McIlroy to believe he had been wrong!

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What Is Reuse? (III)

• The term reuse is a misnomer
• No other engineering discipline uses the term
• Systematic design and use of standard components
  is accepted practice in other engineering
  disciplines
• The term will (eventually) become obsolete
• The important ideas behind reuse are centered on
  the notion of components
• Design of components for use in multiple contexts
• Design of families of related components
• Design of components with standardized packaging

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ReviewArchitectural Components

• A component is an architectural building block
  that is
• A unit of computation or a data store, with an
  interface specifying the services it provides
• A unit of deployment
• A unit of reuse

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The Difference Between Components and Objects

• Objects have a unique identity
• Objects have a persistent state
• Objects are instances of a class, with classes
  arranged in hierarchies according to inheritance
  relationships (object-oriented design and
  programming)
• Components may contain many classes and class
  instances
• Objects can be passed as parameters in methods
• Components vary more dramatically in size

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Component Packaging Standards

• Models and tools for building, packaging and
  interconnecting software components
• Called by many different names
• Component standards
• Component models
• Distributed component platforms
• Our focus in this course will be on the
  C2component model, although we will discuss
  others

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Key Goals of a Component Standard

• Language independence
• Interchangeability of Components
• Requires separation of interface from
  implementation
• Requires standardized interface definition
  language (IDL)
• Current IDLs primarily support syntactic
  interface specification
• Limited support for specification of semantics
  strong typing
• Customizability of Components
• Components are tailored for application use in
  well-defined, predetermined ways
• Compositionality of Components
• Easy formation of larger-grained components and
  whole applications

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Important Features of Component Standards

• Component interfaces
• Metadata
• Integrated development environments (IDEs)
• Distributed server components

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Component Interfaces

• Define facilities for interoperation with other
  components
• Properties/attributes/data members
• Entities forming component state
• Methods/operations/function members
• Entities allowing access and changes to component
  state
• Events
• Component response to internal or external
  stimuli
• Event consumers (other components) register their
  interest with a component that generates events

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Component Metadata

• Component data for the use of other components,
  rather than for the direct support of the
  components functionality
• Class name, interface elements, resources, visual
  representation, etc.
• Some component standards also support
  metaoperations
• Supports discovery of a components capabilities,
  especially at runtime
• Encapsulated in separate components or
  metaclasses
• Often called introspection or reflection
• Introspection is a misnomerimplies component
  looking at itself
• Reflection is a better termexternally available
  information reflecting internal capabilities

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Integrated Development Environments (IDEs)

• Application development based on direct
  manipulation of visually rendered components
• Drag-and-drop composition
• Color-coded syntax source code highlighting and
  checking
• Integrated source code editors, compilers,
  debuggers
• Class, object, component repositories
• Configuration management capabilities
• Versions
• Configurations
• Teams

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Distributed Server Components

• Support for enterprise-wide distribution and
  management of shared multi-user applications
• Protocols for communication with remote
  components
• Transaction services for shared data components
• Component and resource naming services
• Security services
• System management, monitoring, administration
  services
• This is the focus of distributed object
  technology and other kinds of client/server
  middleware

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Component Frameworks

• Component-based implementation of an important
  design pattern or architectural style, tailored
  to specific application domain
• Provides guidance for application construction
• Provides base architecture that can be tailored
  for specific application
• Well-known example Microsoft Foundation Classes
  (MFC)
• Implementation of Model-View-Controller (MVC)
  style for GUI development
• Supported by powerful IDE (Visual C/Visual J)
• Application generation
• Development wizards

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Leading Creators of Component Standards

• Microsoft
• OLE Object Linking and Embedding
• COM Component Object Model (component model
  underlying OLE)
• ActiveX (enhancement and generalization of OLE)
• DCOM Distributed COM
• SOAP Simple Object Access Protocol (Is this a
  component standard?)
• Sun Microsystems
• JavaBeans
• Enterprise JavaBeans
• Jini
• Object Management Group (OMG)
• OMA Object Management Architecture
• CORBA Common Object Request Broker Architecture
  (part of OMA)
• UML Unified Modeling Language
• World Wide Web Consortium (W3C)
• XML (Is this a component standard?)

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Who Will Win?

• Who knows?
• Microsoft tends to win everything
• But there is a lot of industry buy-in for Java
  and related technologies
• OMG stays afloat by reacting to whatever
  Microsoft and Sun do
• What if there isnt a winner?
• Interoperability between component standards is
  always possible
• Bridges
• Adapters
• Wrappers
• Etc.