Introduction to COM and DCOM

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Introduction to COM and DCOM

• Organizational Communications and Technologies
• Prithvi N. Rao
• H. John Heinz III School of Public Policy and
  Management
• Carnegie Mellon University

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Readings

• Posting on the Class Web Site

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Objectives

• Present the basic features of COM and DCOM

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What is COM?

• Binary protocol by which software components can
  connect and communicate
• Language independent
• Platform independent
• Philosophy for architecting software components
• Programming discipline for component development
• Microsofts object technology
• Originally referred to as OLE

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What is COM?

• COM is a specification
• COM is a set of services
• COM supports modular programming
• COM is object-oriented (?)
• COM enables easy customization and upgrades to
  your application (?)
• COM supports distributed applications
• COM components can be written in a variety of
  languages

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Activex, OLE and COM
OLE
ActiveX
Embedding Linking Drag-and-drop Compound documents
Documents Controls Scripting
Uniform data transfer Persistent storage monikers
Automation
COM
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Role of COM in the Model

• COM services provide foundations
• UDT is language and program independent data
  transfer
• Persistence provides for machine-independent
  standardized object persistence
• Monikers facilitate file namespace object
  identification across machines
• Automation facilitates COM object access from
  scripting clients

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The Role of COM in the Model
COM
Uniform Data Transfer
Monikers
Persistent Storage
Automation
Component Object Model
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Object-Linking-and-Embedding

• OLE services allow off-the-shelf app data
  interoperability
• Host (client/container) can edit/store foreign
  application (server) data via pre-defined set of
  COM interfaces
• Can be user initiated
• End-user-drag-drop of data between applications

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Object-Linking-and-Embedding
In Place Activation (visual editing)
Linking
Embedding
Drag and Drop
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ActiveX Services

• Binary packaging of in-proc components
• Deployed in windows/dialogs or in web pages
• Dynamically downloaded with page
• Web-based distribution pf documents
• IIS-aware Automation-aware pages (ASP)

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Component Object Model
Interface
Server
Client (Container)
Object
Interface Pointer
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Distributed COM
Process Boundary
Network Boundary
Client (DLL or EXE)
In-Proc Object (DLL)
Local Object (EXE)
Remote Object (EXE)
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COM and DCOM

• DCOM is now a standard part of 32-bit Windows
• NT 4.0, Win98, Win95 with IE4
• DCOM being propagated to other platforms as well
• Solaris
• HPUX
• Digitals VMS (in beta)
• Provides transparency for remote interfaces,
  parameters and return values

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COM and DCOM

• Reusable code is difficult to create even in C
• Traditional static link libraries have dependency
  deficiencies
• Windows dlls have their own set of limitations
• Need for a vendor independent strata for building
  reusable components

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Lack of Reusability

• Static Libraries
• Duplicated code on disk
• Duplicated code in memory
• No way to replace library without relinking
  and/or recompiling
• Dynamic Link Libraries (a better solution)
• Code resides only on one system
• Code only needs to be loaded once in memory
• Field replacement of implementation is possible

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COM Benefits

• Component based suggests more code re-use
• Smaller distributable components
• Replaceable components
• Binary compatibility and encapsulation
• Thin layers of abstraction
• Decoupling interface from implementation
• Decoupling object identity from implementation
• Dependency reduction in large projects

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COM Fundamentals

• COM defines
• What constitutes a COM object
• How com objects expose features for other
  components to use
• How this works across processes and across
  networks
• When objects are destroyed
• No specification about implementation
• Only describes behavior/characteristics pertinent
  to connecting/communicating with other objects
• Allows flexibility
• Permits implementation stay lightweight and
  language-independent

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COM Principles

• Object services are accessed via interfaces
• Interfaces have identity
• Objects consist of interface implementation
• Objects have identity
• Objects must be registered

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COM Principles

• Objects have location independence
• Objects have implementation transparency
• Object services can be published via a type
  library

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Key COM Concepts

• Interfaces
• Objects
• Identity
• Registration
• Type Libraries

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Whats an Interface?

• An abstract data type containing pointers to a
  group of related function
• Contract between client and object
• Features/functionality only available via
  interfaces
• Objects features expressed by the interface it
  implements
• Once published never changes
• Existing interfaces may be extended
• Contract between COM and object author
• Object implements all methods in interface

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Whats an Interface?

• A pointer to a table of pointers to methods
• No limit to the number of methods in an interface
• Strongly typed
• Transparency for DCOM implementation

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COM Identity

• COM components are associated with a unique ID
• Interface (IID)
• Objects (CLSID)
• Type libraries (TLID)
• Features/service groups of categories (CATID)
• Components defined by COM have pre-assigned Ids
• Each ID distinguishes a component from another
• Prevents collisions
• Supports versioning
• Accessed via ID

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Component IDs

• Generated by component author via development
  tools
• Guaranteed to be unique
• Uses machine ID, NIC date and time information to
  create 128 bit long integer id
• Referred to as GUID
• Guaranteed to be unique on all systems on the
  planet
• Can be created in multiple ways
• CoCreateGuid from COM API function
• GUIDGEN command line tool
• Development tools do this automatically

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Registration

• System registry
• Database of information about available objects
  in system
• May contain configuration information
• Updated when object is installed
• Entries organized by IDs
• Each COM object registry entry must specify
• Objects CLSID
• Path to objects server
• Object registry entries may also specify
• Human readable name

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Type Libraries

• Binary language-independent file
• Describes COM objects, interfaces, methods, data
  types and Ids of components in a server
• Created by component author during development
• Used by client authors during client development
• Compiled form a script written in DCE Interface
  Definition Language (IDL)
• TLB provide all essential details required for
  clients to invoke objects described

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COM Principles

• Objects accessed via Interfaces
• Interfaces have identity
• Objects composed of interface implementations
• Objects have identity
• Objects have location independence

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COM Principles

• Objects have implementation transparency
• Object services can be published via a type
  library

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COM Interface Rules

• Interface methods are abstract
• Interfaces never expose data
• All methods in an interface must be implemented
• Interface methods must return HRESULT

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Instantiating a COM Object

• Client calls a COM API function with object CLSID
• CoCreateInstance
• CoGetClassObject
• COM looks up objects CLSID in registry
• COM finds the server pathname in the registry
• COM invokes the objects server
• Server requests an object instance from the
  objects class factory
• Server instantiates object

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Instantiating a COM Object

• Server returns pointer to objects default
  interface to COM
• COM returns interface pointer to client

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Interoperability Issues

• CORBA/RMI?
• CORBA/DCOM?
• RMI/DCOM?
• Multiple Inheritance (CORBA) maps to Multiple
  Interfaces in DCOM

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Summary

• Examined relationship between COM, OLE and
  ActiveX
• Examined the features of COM
• Examined the principles of COM
• Presented the activation of COM objects
• Presented issues in interoperability