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
Readings
Posting on the Class Web Site
Objectives
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Present the basic features of COM and DCOM
What is COM?
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Binary protocol by which software components can
connect and communicate
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Language independent
Platform independent
Philosophy for architecting software components
Programming discipline for component development
Microsoft’s object technology
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Originally referred to as OLE
What is COM?
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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
Activex, OLE and COM
OLE
Embedding
Linking
Drag-and-drop
Compound documents
ActiveX
Documents
Controls
Scripting
Uniform data transfer
Persistent storage
monikers
Automation
COM
Role of COM in the Model
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COM services provide foundations
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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
The Role of COM in the Model
Uniform Data Transfer
COM
Monikers
Persistent Storage
Automation
Component Object Model
Object-Linking-and-Embedding
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OLE services allow off-the-shelf app data
interoperability
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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
Object-Linking-and-Embedding
In Place Activation
(visual editing)
Linking
Embedding
Drag and Drop
ActiveX Services
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Binary packaging of in-proc components
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Deployed in windows/dialogs or in web pages
Dynamically downloaded with page
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Web-based distribution pf documents
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IIS-aware Automation-aware pages (ASP)
Component Object Model
Interface
Server
Client
(Container)
Object
Interface Pointer
Distributed COM
Process
Boundary
Network
Boundary
In-Proc
Object
(DLL)
Client
(DLL or
EXE)
Local
Object
(EXE)
Remote
Object
(EXE)
COM and DCOM
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DCOM is now a standard part of 32-bit Windows
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DCOM being propagated to other platforms as well
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NT 4.0+, Win98, Win95 with IE4
Solaris
HPUX
Digital’s VMS (in beta)
Provides transparency for remote interfaces,
parameters and return values
COM and DCOM
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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
Lack of Reusability
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Static Libraries
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Duplicated code on disk
Duplicated code in memory
No way to replace library without relinking and/or
recompiling
Dynamic Link Libraries (a better solution)
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Code resides only on one system
Code only needs to be loaded once in memory
Field replacement of implementation is possible
COM Benefits
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Component based suggests more code re-use
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Smaller distributable components
Replaceable components
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Binary compatibility and encapsulation
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Thin layers of abstraction
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Decoupling interface from implementation
Decoupling object identity from implementation
Dependency reduction in large projects
COM Fundamentals
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COM defines
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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
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Only describes behavior/characteristics pertinent to
connecting/communicating with other objects
Allows flexibility
Permits implementation stay lightweight and languageindependent
COM Principles
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Object services are accessed via interfaces
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Interfaces have identity
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Objects consist of interface implementation
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Objects have identity
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Objects must be registered
COM Principles
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Objects have location independence
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Objects have implementation transparency
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Object services can be published via a type library
Key COM Concepts
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Interfaces
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Objects
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Identity
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Registration
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Type Libraries
What’s an Interface?
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An abstract data type containing pointers to a group
of related function
Contract between client and object
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Once published never changes
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Features/functionality only available via interfaces
Object’s features expressed by the interface it implements
Existing interfaces may be extended
Contract between COM and object author
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Object implements all methods in interface
What’s an Interface?
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A pointer to a table of pointers to methods
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No limit to the number of methods in an interface
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Strongly typed
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Transparency for DCOM implementation
COM Identity
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COM components are associated with a unique ID
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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
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Prevents collisions
Supports versioning
Accessed via ID
Component IDs
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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
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Referred to as GUID
Guaranteed to be unique on all systems on the planet
Can be created in multiple ways
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::CoCreateGuid from COM API function
GUIDGEN command line tool
Development tools do this automatically
Registration
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System registry
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Each COM object registry entry must specify
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Database of information about available objects in system
May contain configuration information
Updated when object is installed
Entries organized by IDs
Object’s CLSID
Path to object’s server
Object registry entries may also specify
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Human readable name
Type Libraries
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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
COM Principles
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Objects accessed via Interfaces
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Interfaces have identity
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Objects composed of interface implementations
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Objects have identity
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Objects have location independence
COM Principles
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Objects have implementation transparency
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Object services can be published via a type library
COM Interface Rules
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Interface methods are abstract
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Interfaces never expose data
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All methods in an interface must be implemented
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Interface methods must return HRESULT
Instantiating a COM Object
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Client calls a COM API function with object CLSID
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CoCreateInstance
CoGetClassObject
COM looks up object’s CLSID in registry
COM finds the server pathname in the registry
COM invokes the object’s server
Server requests an object instance from the object’s
class factory
Server instantiates object
Instantiating a COM Object
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Server returns pointer to object’s default interface to
COM
COM returns interface pointer to client
Interoperability Issues
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CORBA/RMI?
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CORBA/DCOM?
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RMI/DCOM?
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Multiple Inheritance (CORBA) maps to Multiple
Interfaces in DCOM
Summary
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Examined relationship between COM, OLE and
ActiveX
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Examined the features of COM
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Examined the principles of COM
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Presented the activation of COM objects
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Presented issues in interoperability