com.ppt

Unit Structure



About COM



COM Components

COM Architecture

COM Data types

Introduction to Distributed Objects

Categories of Distributed object architecture

Distributed Object Architecture

Remoting

Architecture Component

About COM…



Is a platform-independent, distributed,

object-oriented system for creating binary

software components that can interact



COM is the dominant component

architecture



Microsoft created the Component Object

About COM…



Provides the communication in a

completely

transparent

fashion:

It

intercepts calls from the client and

forwards them to the component in

another process



COM objects can be created with a

variety of programming languages



COM is implemented in a

COM is not ….



COM is not a computer language



COM does not compete with or replace

DLLs.



COM is not primarily an API or a set of

functions like the Win32 API



COM is also not a C++ class library like the

COM Evolution



1990’s – Microsoft’s OLE



Microsoft created the Component Object Model (COM)

to provide the infrastructure that was needed to realize

its vision for OLE



1996 - Microsoft ActiveX



ActiveX controls enables the developer to build

sophisticated controls based on the Common Object

Model (COM).



End of 1996 - Microsoft DCOM



a set of RPC-based extensions to COM that allow COM

objects to be distributed



Now -

Benefits

_{Application Customization} Component Libraries

Distributed Components

Component A

Component B

Remoting C

Remoting D

Netwo

rk

Component C

COM Components



Consist of executable code distributed either as

Win32 dynamic link libraries (DLLs) or as

executables (EXEs).



Are fully language independent



Any language can be modified to use COM

components, including Smalltalk and Visual Basic



Can be shipped in binary form

COM

The most fundamental question COM

addresses is:

To solve this problem…

It is necessary to find solutions for 4 specific

problems



Basic Interoperability



Versioning



Language Independence

Component Object Model (COM)



The Component Object Model (COM) is a component

software architecture that allows applications and

systems to be built from components supplied by

different software vendors



These services provide distinctly different functionality

Types of Servers



In-Process server



Implemented as a dynamic linking library (DLL)

that executes within the same process space as the

application



Local Server



Executes in a separate process space on the same

computer. Communication between an application

and a local server is accomplished by the COM

runtime system



Remote Server



Executes on a remote computer. DCOM extends

Client Application

In-Process Server (DLL)

Local Server

Remote Server

Computer A Computer B

COM

DCOM

COM Fundamentals



A Binary standard for function calling between

components



A provision for strongly-typed groupings of

functions into interfaces



A Base interface providing



Reference counting to allow components to track

their own lifetime



A mechanism to uniquely identify components and

their interfaces



A "component loader" to set up component

COM defines a standard way to lay out virtual function

tables (vtables) in memory, and a standard way to call

functions through the vtables

Interface



A COM interface is a strongly-typed

contract

between software components to provide a small

but useful set of semantically related operations

(methods)



All of the functionality that a component must

implement to be a drop target is collected into the

IDropTarget

interface



All the drag source functionality is in the

IDragSource

interface.

COM Data types : Base Types

IDL C Visual Basic Java

Boolean unsigned char Unsupported char

Byte unsigned char Unsupported char

Small char Unsupported char

Short Short Integer short

Long long Long int

hyper _int64 Unsupported long

float Float Single float

double Double Double double

Char Unsigned char Unsupported char

Wchar_t Wcgar_t integer short

enum enum enum int

COM Data types : Extended types

IDL

C

Visual Basic Java

VARIANT VARIANT

Variant

ms.com.Variant

BSTR

BSTR

String

Java.lang.String

BOOL

short [-1/0]

Boolean

[True/False]

boolean

Automation Data types

COM IDL Type Description

short 16-bit signed integer long 32-bit signed integer

float 32-bit signed floating point number double 64-bit floating point number

BSTR length-prefixed wide character array

VARIANT_BOOL short integer used to indicate true or false

DATE 64-bit floating point number representing the fractional number of days

A Distributed Object Example

Checking Account class

IAccount

IAccountInit

ICheckingAccount : IAccount

COM Object

COM Client _{COM Client}

Deposit, Withdraw

Withdraw Using Cheque

 _{Both COM and CORBA rely on client-side and server-side mechanisms to}

manage issues related to remoting

 _{These mechanisms are referred to as proxies and stubs in COM. In CORBA,}

the mechanisms are referred to as stubs and skeletons

 _{In COM, the proxy and stub are packaged in a single DLL. The DLL is}

associated with the appropriate interfaces in the windows system registry.

 _{The COM runtime system uses the registry to locate proxy-stub DLLs}

associated with an interface when marshaling of the interface is required.

Client Client Stub (COM Proxy) (CORBA Stub) Server Server Stub (COM Stub) (CORBA Skeleton) Communication Bus (DCOM or CORBA)

Remote Method Invocation

 _{A client invokes a remote method}

 _{The client stub creates a message containing information needed for the}

remote invocation. (The message creation process is referred to as

marshaling.)

 The client stub sends the message to the server stub using the

communication bus.

 _{The server stub receives the message and unpacks it. (The unpacking}

process is referred to as unmarshalling.)

 The server stub calls the appropriate server method based on the

information provided in the received message

 The server stub creates a message based on the outputs of the call to the

server method (i.e., the return value and out parameters).

 The server stub sends the result message to the client stub using the

communication bus.

 The client stub receives the result message, unpacks the message, and

Client

(Visual Basic or C++ Client)

Proxy

(ComServer.dll)

Client

(Visual C++ COM Server

Stub

(ComServer.dll)

COM/DCOM

RPC



COM uses LPC’s for communicating between

processes



LPCs are a means of communication between

different processes on the same machine based on

remote procedure call

(RPC)



RPCs allow applications on different machines to

communicate with each other using a variety of

network transport mechanisms.



Distributed COM (DCOM), uses RPCs to

communicate across the network

Client Proxy marshals parameters Component Stub unmarshals parameters P ro ce ss B ou n d ar y EXE EXE DLL DLL

Marshaling & Un-Marshaling



An interface named

IMarshal

has to be

implemented



COM queries your component for

IMarshal

as part

of its creation strategy



It then calls member functions in

IMarshal

to

marshal and unmarshal the parameters before and

after calling functions



COM library implements a standard version of

Client / Server Implementation



Creating the Component

 _{The function CreateInstance created an instance of the component}

and returned the IUnknown interface pointer to the client



Exporting a Function from a DLL

 _{The name of the Library functions has to be listed by exporting in}

the EXPORTS section of the file



Loading the DLL

 _{The files CREATE.H and CREATE.CPP implement the function}

CallCreateInstance

 _{CallCreateInstance takes the name of the DLL as a parameter, loads}



Advantages

 _{COM forces the Operating System to act as a central registry for}

objects

 _{COM components are never linked to any application}

 _{Garbage Collection is one other major advantage to using COM.}

When there are no outstanding references to an object, the COM object destroys itself

 _{COM supports Marshalling}  _{COM is a binary standard}



Disadvantages

 _Resiliency

 _{COM/ActiveX Registration}  _{Isolated Components}

AU Distance Education – DMC1754 - Middleware Technologies - Unit V

AU Distance Education – DMC1754 - Middleware Technologies - Unit V

AU Distance Education – DMC1754 - Middleware Technologies - Unit V

 _{COM - Component Object Model}

 _{Defines a binary standard for component interoperability}  _{Is programming-language-independent}

 _{Is provided on multiple platforms}

 _{Provides for robust evolution of component-based applications}

and systems

 _{Is extensible by developers in a consistent manner}

 _{Uses a single programming model for components to communicate}

within the same process, and also across process and network boundaries

 _{Allows for shared memory management between components}  _{Provides rich error and status reporting}

 _{Allows dynamic loading and unloading of components}

 _{A software architecture that allows the components made by}

different software vendors to be combined into a variety of applications