Introduction to COM and DCOM

Organizational Communications and Technologies

Prithvi N. RaoH. John Heinz III School of Public

Policy and ManagementCarnegie Mellon University

Readings

Posting on the Class Web Site

Objectives Present the basic features of COM and DCOM

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

Microsoft’s object technology Originally referred to as OLE

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

Activex, OLE and COM

COM

OLE ActiveX

Automation

Uniform data transferPersistent storagemonikers

EmbeddingLinkingDrag-and-dropCompound documents

DocumentsControlsScripting

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

The Role of COM in the Model

Automation

Uniform Data Transfer

Persistent Storage

Monikers

Component Object Model

COM

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

Object-Linking-and-Embedding

In Place Activation(visual editing)

Linking

Embedding

Drag and Drop

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)

Component Object Model

Client (Container)

Client (Container) Object

Server

Interface Pointer

Interface

Distributed COM

Client(DLL or

EXE)

ProcessBoundary

Network Boundary

RemoteObject(EXE)

LocalObject(EXE)

In-ProcObject(DLL)

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 Digital’s VMS (in beta)

Provides transparency for remote interfaces, parameters and return values

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

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

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

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

COM Principles Object services are accessed via interfaces

Interfaces have identity

Objects consist of interface implementation

Objects have identity

Objects must be registered

COM Principles Objects have location independence

Objects have implementation transparency

Object services can be published via a type library

Key COM Concepts Interfaces

Objects

Identity

Registration

Type Libraries

What’s 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 Object’s 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

What’s 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

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

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

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 Object’s CLSID Path to object’s server

Object registry entries may also specify Human readable name

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

COM Principles Objects accessed via Interfaces

Interfaces have identity

Objects composed of interface implementations

Objects have identity

Objects have location independence

COM Principles Objects have implementation transparency

Object services can be published via a type library

COM Interface Rules Interface methods are abstract

Interfaces never expose data

All methods in an interface must be implemented

Interface methods must return HRESULT

Instantiating a COM Object Client calls a COM API function with object

CLSID 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 Server returns pointer to object’s default

interface to COM

COM returns interface pointer to client

Interoperability Issues CORBA/RMI?

CORBA/DCOM?

RMI/DCOM?

Multiple Inheritance (CORBA) maps to Multiple Interfaces in DCOM

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