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					            CST-201
      Component Programming




COM / DCOM

       T. V. Prasad
Assistant Director (IT Services)
Bureau of Indian Standards
      In this presentation….


• Part 1 – Introduction
• Part 2 – The Component Object
                Model (COM) Technology
• Part 3 – Distributed COM (DCOM)
• Part 4 – Microsoft Component Services
  Part 1

Introduction
    The COM Technology


COM is a proprietary technology of
       Microsoft Corporation
      introduced in 1993 and
 built around Windows platform.
COM is a Windows standard that
 defines how components must
be created, modified and loaded
into memory and finally, shared

                     Microsoft
               COM
 COM is a standard for communicating
 between objects. The basic idea is to
 have a standard that says objects will
    accept and send messages in a
  particular way, thus enabling objects
    written in disparate languages to
communicate with each other as if they
   were written in the same language
       Genesis of COM and
      change in technologies
•   Disk Operating System (DOS)
•   Object Embedding & Linking (OLE)
•   Conventional GUI based OS, e.g, Windows
•   Dynamic Data Exchange (DDE)
•   Object Oriented Programming (OOP)
• Component Object Model (COM)
• Distributed COM (DCOM)
• COM+
               Merits of COM
•   Automatic transactions
•   Configurable security
•   Automatic thread support (many users at a time)
•   Support for multiple databases and resource
    managers
•   Database connection pooling
•   Component state management (easy versioning)
•   Process isolation through packages
•   Integration with main-frame transactions
•   A broad range of development tools
     Demerits of COM/DCOM
• Programmers need to change their thinking and
  methodology from being property-oriented to
  service-oriented
• Installation of DCOM components is more
  involved since it has to be made sure that client
  knows about the services offered by server-side
  components and their location
• Components located over network can be slow
  due to network traffic
• Security could be concerning while deploying
  components
• Error handling could be cumbersome since error
  has to be sent to the client
 Applications of COM/DCOM
Manufacturing, Hospital, Airline,
 Railways, Commerce, Banking
 Industry, Telecom, Transportation,
 Tourism, Education, Science &
 Research, Government, Space,
 Defence, Law & Order
  – and many more in everyday life
Software Tools for COM

  •   Basic and Visual Basic
  •   Visual FoxPro
  •   C, C++ and Visual C++
  •   Small Talk
  •   Ada
  •   Visual J++
         Example of COM
 Some COM objects are visual in nature.
For instance, any ActiveX control (such as
calendar control) that comes with most of
 the Visual programming languages is a
 COM object. Although, it may not have
been written in that language, it has been
designed to work with any application that
   complies with the COM standard for
      accepting visual COM objects.
           Example of COM
Try invoking three Word documents through
Windows Explorer. If earlier versions of MS Office
is being used, three separate copies of Word for
separate documents would be loaded.
This is not the case with present MS Office, as one
would see that there would be three “Word like
editors” one for each document whereas only copy
of the Word component is loaded into the memory.
More speed, less space usage and many more….
          Part 2

           The
Component Object Model (COM)
       Technology
          Important terms – 1
• Object – combination of code and data that can
  be treated as a unit
• Component – reusable piece of executable code
  that can be used with other applications with
  minimum efforts; could be .EXE, .DLL or .OCX
• Interface – set of functions grouped together
  under one name; COM can support more than
  one interface
• DLL – dynamic link library is a highly reusable
  component that can communicate with objects in
  client/server application
             Object

Properties
    or
Attributes
    or
   Data

 Methods
   or
Functions
   or
  Code
             Usage of COM

Some objects are full applications in themselves,
with portions “exposed” for outside world to use.
These applications have been designed not only
for use by end users but also by developers. For
example, a faxing application can have objects
exposed for use so that a developer can automate
faxing a document. Microsoft Office applications
have portions exposed so that a developer can
use the applications by “remote control”.
             Component Object


             A

Interfaces   B           Object

             C
Interface extend toward clients



  Client                  Object
              Interface
Application   Pointer
Applications connect each other


Application
                            Object

              Interface
              Pointer

  Object
                          Application
       Important terms – 2
• Distributed environment – a network
  environment where information or COM
  components can span on server as well
  as client machines
• GUID – globally unique identifiers are
  128 bit integers used to identify every
  interface and component object class
• Business rules – the middle layer of the
  three layer COM architecture
  3-level architecture of COM

Front end   Business Layer   Back end

                              Oracle
 Client 1
              Products
Standard
               Object
  EXE


 Client 2
Standard                      Access
  EXE         Customer
               Object
         Accounting Package
           – A Case Study
Accounting procedure throughout the world
follows same principles for General Ledger,
Accounts Receivable, etc. but there are
numerous applications running everywhere
Enormous time is required for software
development. It is notable that over 100
million accounting packages are available in
the market
                                      Contd…
If one COM-compliant accounting module is
available, then all a programmer has to do is
write a custom GUI and processing; but leave
the accounting procedure to the software.
Clearly, the programmer do not have to think
about the accounting system at all.
Similar applications can be envisaged for
power plants, super markets, banks, etc.
This goes with the saying “Don’t reinvent
wheel” (rather use the existing technology).
Straightaway, it can be said that COM sets
standards (or business rules) of the business.
   Edge over 2-tier architecture
• Easily replaceable – entire coding or business
  rules need not be changed; small change can be
  made and get that part only replaced
• Ideal for changing business requirements –
  business logic can be localized, nicely organized
  and then distributed to clients
• Reusable – write the code once and then use it
  in many applications
• Hides complexities –management of complex
  validation part and business rules could be
  handled easily
     Support behind COM

COM is the underlying architecture
that forms the foundation for higher-
level software services, like those
provided by OLE, such as component
software, custom controls, inter-
application scripting, data transfer and
other software interactions
     Focus on Component
     Software Architecture
• Defines a binary standard for
  component interoperability
• Programming language independent
• Provided on multiple platforms viz.,
  Windows, Mac, UNIX
• Provides robust evolution of component
  based applications and systems
• Is extensible
           Binary Standard
For any given platform, COM defines a
standard way to Virtual Function Tables
(Vtables) in memory, and a standard way
to call functions through the Vtables.
Thus, any language that supports calling
functions via pointers can be used to write
components which can interoperate with
other components written to the same
binary standard
  Further, COM provides…

• Communications between components,
  even across process and network
  boundaries
• Shared memory memory management
  between components
• Error and status reporting
• Dynamic loading of components
         COM supports
        Object Orientation
• Classes can be defined
• Classes can be used to create objects
• Encapsulation by hiding its data and methods
  is supported
• Abstraction in which complexities are isolated
  and focus is primarily on necessary concepts
  thus providing software reuse
• Also supported are
  – Inheritance
  – Polymorphism
Still a step ahead…

COM is

• Language independent
• Location independent
    Part 3

Distributed COM
     (DCOM)
         Distributed COM
COM behaves like a distributed component
as, neither is it bound in the process space
of one process, nor is it stopped by remote
computer. When a call from a remote host
is marshaled, the model used is called
Distributed Component Object Model or
DCOM

DCOM was introduced in 1996
  Distributed COM


In fact, every MTS-based
application is built from COM
components, and client can
access those applications
remotely through DCOM
   Process Space in memory
Two types of processes occupy memory in
Windows: OS processes and User or any other
processes
OS is the main controller of all resources of the
computer so it is given a separate process
space; no processes can enter this space
All other processes have a separate process
space in which they keep their variables, and the
OS grants a place which is shared by all, this is
where shared code is written
     Terms related to DCOM
• Server – a piece of code that implements
  component object
• In-process – the server code executes in the
  same process space as the client (as DLL)
• Out-of-process or remote – the server code
  runs in another process on the same machine
  or in another process on a remote machine (as
  an EXE)
• Local process – the code runs in the same
  process space on the same machine
                       GUID
Every COM component is registered in the Windows
registry and has a unique 16-byte number. This
number is unique for every COM component of this
world.
Whenever the COM component or a library is called,
it is checked by its GUID in the registry.
If the COM component is a registered, then
information about its server is found, and the server is
loaded into memory. In case, there is no registered
component then an error message is generated
            Marshalling

If the COM component is a registered
component then the server that will
run the component is located and
services of the component are utilized.
       Marshalling – Case I

If another process, P2 requests reference
to the same COM component, then instead
of calling it again in P2, P2 is given a
hypothetical object of the COM component.
Even though P2 interacts with the object in
some other process but it functions as if it
has its own copy of the object
     Marshalling – Case II

If the component is registered but the
server that is required to run the
component is located on a remote
computer then the process defined in
Case I is repeated
The only difference being it is done for a
remote computer, and consequently, has
greater overhead
         COM Type Libraries
It is basically documentation about collections,
objects, methods (with their parameters), and
properties (with their data types) that are
exposed for use by COM client applications
VB uses the type libraries to give online and in-
line assistance. For instance, when calling a
method in a COM object, it displays the expected
parameters automatically
A type library can be a part of an object file, with
extension .OLB and for a standalone file
extension will be .TLB
    Creating a COM Object
CreateObject() is the mechanism for
creating an instance of a COM object, eg.
  X = CreateObject(“Word.Application”)
  Y = CreateObject(“Word.Application”)
  Z = CreateObject(“Excel.Application”)
This will open up two Word and one Excel
instances, which can be seen on the
toolbar at the bottom of Windows screen
      Getting reference of
     a running COM Object
GetObject() is similar to CreateObject(), eg.
  X = GetObject(“c:\bills.xls”)
This will instantiate an Excel object, open
the file and get a reference of it

  Y = GetObject( ,“Excel.Application”)
This will get reference to the already
running application, and if none is running
then an OLE error is flashed
   Key benefits of ActiveX
automatically extend to DCOM

     •   Language neutral
     •   Broad tools support
     •   Broad industry support
     •   Accessible technology
      Use of Interfaces

• Ability to provide functionality in
  applications to evolve over time
• Fast and simple object interaction
• Interface reuse
• Local/remote transparency
• Programming language independent
      Attributes of Interfaces
• An interface is not a class
• An interface is not a component object
• Clients only interact with pointers to
  interface
• Components objects can implement
  multiple interfaces
• Interfaces are strongly typed using GUIDs
• Interfaces are immutable and can not be
  versioned
         Query Interface

It is a mechanism that allows clients to
dynamically discover (at runtime)
whether or not an interface is supported
by a component object

At the same time, it is this mechanism a
client uses to get an interface pointer
from a component object
      Strengths of DCOM

• DCOM is based on the most widely
  used component technology today
• DCOM is the best networking
  technology to extend component
  applications across the Internet
• DCOM is an open technology that
  runs on multiple platforms
     DCOM Benefits

• Growth path for desktop developers
• Common component model for
  clients and servers
• Multi-platform support
• Based on public, open standards
DCOM ideally positioned to become
mainstream Internet technology for
      business applications
   • Transport neutral
   • Provides distributed Java today
   • Evolutionary technology
   • Common components for
     browser and web-server
   • Security
   • Standards based
                     Local/Remote Transparency
                                                         Local server
  Client Process                    Interprocess
                                                           process
                                   Communication
                     In process
                     Component                           COM
                       object
Client Application




                                                                Local
                                                                object
                                     Cross process
                       COM        with lightweight RPC
                      Local                              Remote server
                      object                               process

                     Remote                              COM
                     object         Cross network
                                    with true RPC              Remote
                                                               object
        Local/Remote
    Transparency Benefits
• Common solutions to problems that are
  independent of distance between client
  and server
• Programmers leverage their learning
• System implementation is centralized
• Interface designers concentrate on
  design aspects
     Part 4

    Microsoft
Component Services
 Microsoft Transaction Server

MTS is a part of Windows NT server that
  helps in creating necessary infrastructure
  services for building robust enterprise
  wide applications plus standards into a
  mainstream business tool
Since introduction in 1996, MTS has
  remained the first commercial software to
  combine transactions with components
             Microsoft
   Internet Information Server
• IIS is a part of Windows NT server that
  supports web enabled applications and
  their connectivity to the databases
• IIS supports standard Internet protocols
  and services such as HTTP, SSL, CGI.
• IIS supports applications based on Active
  Server Pages (ASP), VBScript, etc.
         Active Server Pages
• ASP contains a simple program – a script –
  written in a simple language such as Visual
  Basic (Scripting Edition) or VBScript and
  executed at the server
• Benefits to the developers are
  – Access to COM objects
  – Integration with MTS-based applications
  – Transactional ASP scripts
  – Process isolation
  – Support for standard, well-known languages
  Microsoft Message Queue


MSMQ is a part of Windows NT server that
 provides support for non-blocking and
 asynchronous communication for
 distributed applications
    Features of MSMQ
• COM based access
• Integration with MTS
• Automatic message journaling
• Automatic notification
• Built-in data integrity, data privacy,
  and digital signature services
• Message priority support
• Support for multiple platforms
Microsoft COM Services
               MSMQ




  DCOM


         MTS             Database


                         Database
                 ASP
  HTTP   IIS    Script
               COM+


• The development of component services
  continues with COM+
• By enhancing and extending the existing
  services, COM+ further increases the
  value and performance of the services
  COM provides
          Features of COM+
• A publish and subscribe service – allows
  multiple clients to “subscribe” to various
  “published” events
• Queued components – allows clients to invoke
  methods on COM components using an
  asynchronous model (for unreliable networks)
• Dynamic load balancing – automatically
  spreads client requests across multiple
  equivalent COM components
• Full integration of MTS into COM – includes
  broader support for attribute based programming
  and services such as transactions, security, and
  many more…
End of presentation


    Thank you

				
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Description: Microsoft Distributed Component Object Model (DCOM) is a set of Microsoft concepts and program interfaces, the use of this interface, the client program objects can request from another computer on the network server program objects. Based on Component Object Model DCOM (COM), COM provides a set of allowed on the same computer between client and server communication interface (running on Windows95 or later version).