1. University of Genoa Course of Software Engineering II 01-02
Elisabetta Parodi

2. Summary 1/2 Introduction COM Walking towards COM Introducing COM
Terminology
COM Binary Standard
Interface
Component Object Library
COM Server
COM Object
COM Technologies
Example

3. Summary 2/2 DCOM Global Overview References Introducing DCOM
DCOM Architecture
Location Independence
Connection Management
Security
Global Overview
Weaknesses of COM/DCOM
Comparison of DCOM, CORBA, RMI
References

4. Introduction : walking towards COM
1/2
Windows purpose : moving integration between applicative functions to O.S. level
 it needs effective communication mechanisms between applications
COM
OLE
DDE
clipboard

5. Introduction : walking towards COM
2/2
DLLs are a good starting point for modularity
problems :
physical location dependency
version management
we need a better communication mechanism. An object-based and synchronous client-server technology facilitates:
robustness
API’s homogeneity
physical location independency
splitting into components

6. Introducing COM COM : Component Object Model (1992)
component software architecture that allows applications and systems to be built from components supplied by different software vendors
supports : interoperability and reusability
benefits : maintainability and adaptability
however, in order to interact, components must adhere to a binary structure specified by Microsoft

7. COM Terminology Interface
a group of related functions that specifies a contract
Class
named, concrete implementation of one or more interfaces
Object
an instance of a COM class
Component
binary unit of code that creates COM objects

8. COM Binary Standard A standard way to :
lay out virtual function tables (vtables) in memory
call functions through the vtables
 any language that can call functions via pointers (C,  C++, Small Talk, Ada, and even Basic) can be used to write components that can interoperate with other components written into the same binary standard

9. Interface /3
collection of functions by which applications interact with each other and with the system
strongly typed contract between software components to provide a small but useful set of semantically related operations (methods) : Component Object Design
definition of an expected behavior and expected responsabilities

10. Interface 2/3 remember : considered logically immutable
reason : removing the potential for version incompatibility
new functionality : can be exposed through a different interface
is not a class
is not a component object
denotes behavior only, not state
component objects can implement multiple interfaces
is strongly typed

11. Interface 3/3 identified by an IID must derive from Iunknown :
one special interface to implement some essential functions: all component objects are required to implement it
three methods :
QueryInterface allows clients to dynamically discover whether or not an interface is supported by a component object; if so, it returns an interface pointer from a component object
AddRef
simple reference counting methods
Release

12. Component Object Library
A system component that provides the mechanics of COM:
provides the ability to make IUnknown calls across processes
encapsulates all the “legwork” associated with launching components and establishing connections between components

13. Creating a component object Application
CLSID
DLL
.EXE
Component
Object
Library
CLSID
Registration DB
CLSID

14. COM server
In general :
some piece of code that implements some component object such that the Component Object Library and its services can run that code and have it create component objects
Three ways in which a client can access COM objects provided by a server:
In-Process Server
Local Object Proxy
Remote Object Proxy

15. Local Server Process Client Process Remote Machine In-Process Object
Stub
Cient
Application
In-Process
Server
Local Server
COM
RPC
Local
Object
Proxy
Remote Machine
Remote Server Process
COM
Remote
Object
Remote
Object
Proxy
Stub
RPC
Remote Server
COM

16. COM object
In general :
some function that produces a defined output, given tah the input meets predefined parameters.
COM encapsulates the function, only giving the client of an object acces to the object interface(s).
All COM objects :
are registered with a component database
have no identity

17. 4) Call interface members
Client Application
Object
3) Get object interface pointer return to client
Server
1) Create Object
2) Locate Implementation
COM
What happens when a client wishes to create and use a COM object?

18. When an application uses a COM object:
1) initializes the system : CoInitialize
2) calls CoCreateInstance (exported by COMPOBJ.DLL) passing the CLSID of the used object; CoCreateInstance :
 uses the registry to discover which server implements the desidered class
asks the server to create an instance
 receives from the server a pointer to the IUnknown interface
3) uses IUnknown.QueryInterface to access the wanted interface
4) uses Addref and Release to manage the object lifecycle
5) in the end calls CoUninitialize

19. COM Technologies
Interfaces and API functions that expose operating system services, as well as other mechanisms necessary for a distributed environment :
Type Information
Structured Storage and Persistence
Monikers
Uniform Data Transfer
Connectable Objects

20. Example : AMF Bowling Center System 1/6
In 1998 AMF Bowling rearchitected their international bowling center operations software using ActiveX controls and COM components.
We’ll see :
the container supporting the AMF BOSS System, version 1.0

21. Example : AMF Bowling Center System 2/6
The AMF container

22. Example : AMF Bowling Center System 3/6
Building blocks :
Microsoft ActiveX controls
implement user interface for each primary
functional area
Component Object Model objects
 used by the controls to perform processing

23. Example : AMF Bowling Center System 4/6
The container :
needs to know enough about any ActiveX control
offers its services via interfaces
communicates with each control by using an agreed upon COM interface
manages all ActiveX controls and their relationships
supports add-in COM objects

24. Example : AMF Bowling Center System 5/6
Architectural requirements :
ability for customers to upgrade their system
isolated development of new modules
Solution :
COM deployed in a traditional client/server environment

25. Example : AMF Bowling Center System 6/6
CContainer
Handles
Messages for
Creates/Destroys
CDynDlgbar
CCentralManageActiveXControls
Manages
uses
Creates/Destroys
Creates/Destroys
CCentralManageDlgbars
uses
uses
IManageDlgbars
IManageActiveXControls

26. Introducing DCOM
An extension to COM that allows network-based component interaction
COM : processes can run on the same machine but in different address spaces
DCOM extension : allows processes to be spread across a network
DCOM (1995) is a seamless evolution of COM
 you can take advantage of your existing investment in COM-based applications, components, tools and knowledge

27. DCOM Architecture Component COM Component Object
Security
Provider
DCE RPC
Protocol Stack
DCOM network-
protocol
Component
DCOM sits right in the middle of the components fo your application; it provides the invisible glue that ties things together.

28. Location Independence
Conflicting design constraints…
… with DCOM are easy to work around, because the details of deployment are not specified in the source code : DCOM completely hides the location of a component
in practice, however, applications need to consider the two primary constraints of a network connection:
bandwidth
latency
 preferred transport protocol : UDP

29. Connection Management
DCOM :
manages connections to components that are dedicated to a single client, as well as components that are shared by multiple clients, by mantaining a reference count on each component
uses an efficient pinging protocol to detect if clients are still active
takes advantage of Windows NT support for multiprocessing for enhancing applications’ scalability

30. Security by Configuration
4) Fail or allow call
3) Is user in the list?
2) Authenticate user
DCOM obtains user name

31. Security on the Internet
DCOM uses the security framework provided by Windows NT , this supports multiple security providers including :
Windows NT NTLM authentication protocol
the Kerberos Version 5 authentication protocol
distributed password authentication
secure channel security services
a DCE-compliant security provider

32. Weaknesses of COM/DCOM
COM/DCOM is hard to use
COM is not robust enough for enterprise deployments
 Microsoft is walking to turn COM and the MTS into one programming model that will simplify the lives of developers building distributed, enterprise-wide COM applications
COM/DCOM are best supported on Windows 95 and NT platforms
low cost development tools provide the ability to build and access COM components for Windows platforms, for other platforms the prices are considerably more expensive

33. Comparison of DCOM, CORBA, RMI
Java RMI comes from JavaSoft
CORBA is a specification resulting from OMG
DCOM comes from Microsoft
Their main goal :
to provide an infrastructure to enable writing distributed applications with relative ease, although they do so in different ways

34. Java RMI Very easy to use
Remoteable interfaces have a special exception
Supports object-by-value
Versioning is built into serialization
Callbacks are blocked in synchronized methods
Not always intuitive
Not available to other languages
There are limited development tools
Performance can be slow as you scale

35. CORBA 1/2 Architecture for system composition
Has a standard terminology for concepts
Declarative interfaces separate the interface from the implementation
Provides mappings from IDL to C, C++, SmallTalk and Java
Supports :
evolvable and marshallable data
design portability
standard interoperability protocols
Scalable for large systems

36. CORBA 2/2 No inheritance for Exceptions
Objects cannot support two versions of the same interface
IDL is not internationalized
Divergent security mechanisms (Kerberos, SSL)
Few advanced services
C++ mapping has complicated memory management rules
Limited developer tools (usually just an IDL compiler)
Limited concurrency model

37. DCOM Lots of tools, books and developers
Separates interface from implementation
Good integration of automation objects with VisualBasic and Java
Good set of compound document interfaces
Microsoft depends on it working
Minimal support on non-Microsoft platforms
Hard to keep registry consistent
Reference counting is a problem
Client has to choose the interaction model

38. So, What Shall I Use? 1/2
Requirement : no recompilation moving applications to various platforms
 the Java/CORBA solution may be more appropriate then COM
Requirement : interaction with DCOM objects
 pure DCOM solution

39. So, What Shall I Use? 2/2
Requirement : distributed application written entirely in Java
 RMI for its ease of programming
Requirement: plug-in components created by third parties
 components may be based around a DCOM or CORBA interface

40. References 1/2 http://www.microsoft.com/
Main source : terminology, technical documentation, example from official Microsoft site
www. sei.cmu.edu/str/descriptions/com_body.html
The Software Engineering Institute (SEI) is a federally funded research and development center sponsored by the U.S. Department of Defense. I found some interesting documentation in their Software Technology Review
www2.umassd.edu/coursepages/SoftwareArchitecture/paul-com/
University of Massachussets, lessons of the module of Software Architecture about COM.

41. References 2/2 http://www.whatis.techtarget.com/
An IT company which offers targeted industry-specific search engines that deliver access to relevant information on the Web. I found here useful explanation about DDE, OLE, MTS, ActiveX.
Online dictionary and search engine for computer and Internet technology.
Introduzione a COM – seminario di E. Lodolo
From a lesson at University of Genoa : introduction about COM and Windows fundamentals

42. THE END
T H E E N D

43. Clipboard
The simplest mechanism available to exchange data between applications running in hte Windows environment
A section of computer memory that temporarily stores data especially to facilitate its movement or duplication
back

44. DDE : Dynamic Data Exchange
An asynchronous message-based communication protocol
An interprocess communication (IPC) system built into the Macintosh, Windows, and OS/2 operating systems
Enables two running applications to share the same data
Example : DDE makes it possible to insert a spreadsheet chart into a document created with a word processor. Whenever the spreadsheet data changes, the chart in the document changes accordingly.
Still used by many applications
Supplanted by OLE
 provides greater control over shared data back

45. OLE : Object Linking & Embedding
A compound document standard developed by Microsoft Corporation
Enables you to create objects with one application and then link or embed them in a second application. Embedded objects retain their original format and links to the application that created them.
back

46. DLL : Dynamic Link Library
Two kinds of executables in Windows:
EXE
DLL
Functions exported from a DLL are linked at runtime,
not during compilation.
We have two linking modalities for a DLL:
static link : when application is loaded
dynamic link : at any time by API’s LoadLibrary, GetProcAddress, FreeLibrary
Advantages: modularity, code sharing,substitutability.
back

47. VMT : Virtual Method Table 1/2
Virtual methods can be redefined in derived classes.
Virtual methods calls are solved at execution time (late binding)  indirect jump  (small) overhead
In compiled object languages virtual methods are implemented by VMT : a function pointer table
one VMT for each class
instance : data structure holding an object state
 its first field is a pointer to the VMT of the class to which the object owns
all instances of the same class point to the same VMT

48. VMT : Virtual Method Table 2/2
I1 : C1
implementations
Pointer to VMT
Attribute 1
Attribute 2
Attribute 3
……………………
VMT of class C1
Pointer to function 1
Pointer to function 2
Pointer to function 3
I2 : C1
Pointer to function 4
Pointer to VMT
Attribute 1
Attribute 2
Attribute 3
……………………
back

49. MTS : Microsoft transaction Server 1/2
A container for in-process server side components.
A component-based programming model. An MTS component is a type of COM component that executes in the MTS run-time environment.
What does it do?
Allows you to define and use transactions within COM objects
Enforces modelling based on components – just as COM

50. MTS : Microsoft transaction Server 2/2
Handles communication between components through DCOM and HTTP protocols
Uses roles and packages
Uses Just In TIME (JIT) Activation
Surrogate Process (MTS.EXE)
Microsoft Distributed
Transaction
Coordinator (MS DCT)
Run-time divided into three parts :
MTS Explorer
back

51. ActiveX /2
Technology developed by Microsoft for sharing information among different applications
Outgrowth of OLE and COM
 supports new features that enable it to take advantage of the Internet
Not a programming language but a set of rules for how applications should share information

52. ActiveX 2/2 ActiveX control similar to Java applet but :
full access to the Windows operating system
limitated to Windows environments
can be developed in a variety of languages
back