COM/DCOM & COM+
Qisheng Hong
Yani Mulyani
Paul Visokey
Outline
The Component Object Model - COM
DCOM
COM+
What is COM?
Build software from parts, not from scratch
A framework for creating and using
components
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Both a specification and an implementation
Makes software easier to write and reuse
Provides widest choice in services, tools,
languages, and applications
Evolved from Object Linking and Embedding
technology
Foundation in OLE Version 2, 1995
Isn’t COM about “Objects”?
No … COM sets out to solve system software
problems that arise when hooking up
randomly evolving combinations of binaries
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Benefits stem from its design an an integration and
deployment architecture for software components
Yes … from the perspective of these new
kinds of objects, also called “components”
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COM is about clients communicating with objects
COM based applications can be constructed using languageindependent components
COM Architecture
The Model
The “ORB”
The Services
COM Fundamental Principles
What the designers of COM were striving for...
Binary components
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Any programming language
Any location (in-process, cross-process, crossmachine)
No centralized authority
Object orientation
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Interoperability
Reusability
Simplest model possible
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Enable extensibility, maintainability and
adaptability
Multi-tier architecture
The Component Object Model
(COM)
Components, Components, Components
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Code reuse and off-the-shelf components
Language independent notion of an object
Rigorous Encapsulation at the binary level
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Black box -- no leakage of implementation details
All object manipulation through strict interfaces
Polymorphism
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Define several different behaviors for a single object
via multiple interfaces per class
“Discoverable”: QueryInterface()
IUnknown
IDispatch
COM
Object
IRobot
The Component Object Model
(COM)
Standard API
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Binary structure for the interface between client
and object
Compilers support Microsoft’s COM binary
structure - language independence
The point of contact is the run-time binary
representation
OS acts as central registry for objects
Applications only know the function of the
object
Wire-level standard for
location transparency
OOD and COM
Table 1: Differences in design considerations between OOD and COM
Differences in design considerations between OOD and COM
Object-Oriented Design Assumptions
Added COM Considerations
Objects typically packaged in the same
application (module) as client code
Objects and clients typically in separate
modules, both .EXEs and .DLLs
Objects and clients run in a single process
Objects and clients may run in different
processes and on different machines
Class (implementation) inheritance
Interface inheritance (no implementation
inheritance)
Single interface per object (the object's
class definition)
Multiple interfaces per object
Single client per object
Multiple simultaneous clients per object
1:1 relationships between clients and
objects typical
Many:Many relationships between clients
and objects is common
The COM Object Request
Broker
The “model” is about how the technology is
used
The “ORB” is how it’s implemented
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Microsoft calls it the COM Runtime
Secure
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Security is designed and built in. Not an option.
Inherently Distributed
Pluggable
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Security
Transports
Free! (Built-into Win32 Platforms)
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No separate client access license or expensive “developer”
version
Structure of the COM “ORB”
Components and Applications
The COM Runtime
Core Services
(Automation, Monikers, Storage, ComCat, Data Transfer, IR)
COM and Distributed COM
Registry
MS-RPC
Pluggable Security
(SSPI)
Pluggable Transports
TCP
UDP
IPX
SPX
HTTP
ETC.
NTLM DCE
Kerberos ETC.
COM Runtime Architecture
Flexible and extensible
Pluggable Transports
Proxy
Other
Component
ORB
TCP, UDP
SPX,IPX
Net BUI
HTTP
COM Runtime
COM
Client
Server Machine
COM Runtime
Client Machine
ORB
Component
COM Runtime Architecture
Flexible and extensible
Pluggable Security Providers
Proxy
Other
Component
ORB
NTLM
SSL, Certs.
NT Kerberos
DCE
COM Runtime
COM
Client
Server Machine
COM Runtime
Client Machine
ORB
Component
COM Runtime Architecture
Client
COM
run time
Security
provider
COM
run time
Security
provider
RPC
Inprocess
Protocol stack
LPC
Component
RPC
Protocol stack
Local
DCOM networkprotocol
Remote
Accessing COM Services
In the same process
Fast, direct links to
server library
Client
On the same machine
Fast, secure lightweight
RPC
Secure, reliable and
flexible DCE-RPC
based DCOM protocol
Client Process
Client
Client Machine
Across machines
Client
Component
Server Process
COM
Component
Server Machine
DCE
COM RPC COM
Component
COM Limitations
Lack of a common type system, each language
involved in COM had its own type system.
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Architecture does not have perfect knowledge of the
types.
COM is hard to use. Reference counting,
Microsoft IDL, Global Unique Identifiers
(GUID), etc. require deep knowledge of COM
specification from developers.
COM is not robust enough for enterprise
deployments. Services such as secure
transactions, reliable communications, and
load balancing are not integrated in COM.
DCOM: Multiple Network Transports
Clients
C
O
M
TCP, UDP
IPX, SPX
HTTP
C
O
M
Queued
Client Machine
Server Machine
COM
Object
DCOM: Flexible Security
Clients
C
O
M
NT4 Security
TCP,
UDP
SSL/
IPX,
SPX
Certificates
Kerberos
HTTP
IPSEC
C
O
M
Queued
Client Machine
Server Machine
COM
Object
What’s next? COM+
Evolutionary step, making it even easier to
build and use components
Infrastructure in the system, not in the
application
Extensible services
Preserves current investments in COM
Provides benefits across languages and
tools
Less coding, more productivity
COM+ Strategy
Transactions
Data Binding
Persistence
Load
Balancing
Security
In Memory
Database
Event
Infrastructure
Ubiquitous Type
Description
Dynamic Invocation
“No-Leak”
Memory Mgmt
Interception
Tools
Ubiquity
Market
Binary Standard
The COM+ MODEL
COM+ is the next step in the evolution of the
Microsoft® Component Object Model and
Microsoft Transaction Server (MTS).
COM+ handles many of the resource
management tasks such as thread allocation and
security.
COM+ automatically makes applications more
scalable by providing thread pooling, object
pooling, and just-in-time object activation.
COM+ protects data integrity by providing
transaction support, even if a transaction spans
multiple databases over a network.
The COM+ MODEL
COM+ still provides a standard library, and
objects and their clients still use it.
COM+ hides calls to this library beneath the
equivalent native functions in the programming
language.
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C++ programmers can use the standard new operator rather
than CoCreateInstance() to create a COM+ object.
COM+ developers no longer need to define
interfaces using IDL.
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Object's interfaces defined using their programming
language's syntax.
The COM+MODEL
COM+ addresses another important but
challenging problem in creating a languageindependent object model: data types.
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Different languages support different data types, which causes
problems when passing parameters between objects written in
different languages.
COM+ allows implementation inheritance
between COM+ objects running in the same
process.
COM objects do not support constructors, but
COM+ objects do.
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COM+ constructors even allow passing parameters,
better integrating COM+ objects and the objects used
by today's most popular object-oriented languages.
COM+ Philosophy
Attribute-based Programming
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Write your Logic
Set Attributes
Runtime Does all the grungy stuff
Interceptors deal with the object based on the
Attributes
Different Interceptors provide different extensible
services at Runtime
The key to extensible services:
Interception
Key concept in “COM +” run-time
Interceptors provide automatic behavior at
runtime based on the attribute set
Interceptors assure that when a transactional
object attempts to change data either all
succeed or all fail and rollback
Interception Events
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Instance creation
Method call and return, field access
Force fail pre and post call
Binding an instance to a reference
Dynamically (re)define method
COM+ and Windows DNA
For building next generation applications
The Web
•
•
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Easy to Deploy
Graphical UI
Dynamic
The Windows PC
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Rich application
services
Familiar UI for users
Consistent
management
Windows
DNA
• Integrates
client/server
and the web
• Services that
work together
• Open protocols
and published
interfaces
COM+
Applications
• Component-based
• Flexible and
dynamic
• Built-in scalability
• Preserve current
investments
• Extensible services
Summary
COM is the most widely adopted object model in
the world
COM+ builds on the success of the Component
Object Model
Provides benefits across languages and tools
Less code, more productivity
Extensible services enabled by interception
Provides the backbone to Windows DNA
Questions?
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