Component Object Model COM and DCOM

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Component Object Model(COM) and DCOM

• Srinivas Vennam
• Sushant Batule

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Objectives

• History of COM
• COM overview
• COM specification
• Interfaces and Objects
• COM, CORBA RMI

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History Of COM
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Component Software Problem

• Interoperability
• Versioning
• Language Independence
• Transparency

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COM overview
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What is COM?

• A framework for creating and using components
• Makes software easier to write and reuse
• Provides widest choice in services, tools,
  languages, and applications
• Provides the only currently viable third party
  market for off the shelf components
• Controls, tools, and server components

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COM Features

• Services used in a standard way,
• regardless of location
• Any Programming Language
• Integrated with Development Tools
• Flexible Security
• Multiple Network Transports
• Mature specification and reference
• implementation
• Ubiquitous

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Accessing COM Services
COM Components in the same process
COM components in different processes
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Accessing COM Services
DCOM COM components on different machines
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COM specification
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COM Specification Foundations

• Reusable component objects
• Binary standard for interoperability
• True system object model
• Provides distributed capabilities

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Components Reusability

• Using Existing Components to Reduce Development
  Cost
• Future Reusability of Components being Developed

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Object Interoperability

• Stable versioning
• Rapid object interaction
• Location independence
• Language neutrality

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Location Independence

• Components that interact more should be closer to
  each other
• Some components can only be run on specific
  machines
• Small Components Easy to Deploy - Increased
  Network Traffic
• Large Components Difficult to Deploy - Reduced
  Network Traffic

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Language Neutrality

• Virtually all languages such as Java, Visual C,
  Visual Basic, Delphi, PowerBuilder, COBOL
  interact well with DCOM
• Enables Rapid Prototyping

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True System Object Model

• Globally unique identifiers (GUID)
• Reusability and inheritance
• Single programming model
• Life-cycle encapsulation
• Object-level security
  

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Security

• Provides Security at Component Level and Method
  Level
• Uses Access Control Lists (ACL) similar to
  Windows NT File System

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Distributed Architecture

• Inherently Distributed
• Flexibility
• Scalability Performance

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Scalability and Performance
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Interfaces and Objects
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Interfaces

• An interface is a named table of function
  pointers (methods)
• An interface is not a class
• An interface is not a COM component
• COM clients only interact with pointers to
  interfaces
• COM components can implement multiple interfaces
• Interfaces are strongly typed
• Interfaces are immutable

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C implementation

• COM Server
• Implement IUnknown interface
• QueryInterface, AddRef and Release
  methods
• Implement supported interfaces
• COM Client
• Call CoCreateInstance method to get a pointer to
  the COM component
• Get a ptr to the COM interface
• Invoke COM methods and release pointer.

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Platforms

• UNIX
• Apple Macintosh
• Windows
• MS JVM

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COM, CORBA RMI
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Comparison with CORBA and RMI

• DCOM fits best with Microsoft Platform i.e.
  Windows 98 and Windows NT while CORBA is quite
  general.
• RMI works only with Java but is quite suitable
  when application wants to exploit features of
  Java

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Resources

• Books
• 1. Inside COM Dale Rogerson
• 2. Essential COM Don Box
• 3. Inside OLE Microsoft Press
• Online Resources
• 1. www.microsoft.com
• 2. www.develop.com
• 3. www.comdeveloper.com

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