COM/DCOM & COM+
A Primer on the Evolution of a Microsoft Development Environment
The Component Object Model
The Component Object Model (COM) has its roots in OLE version 1, which was
created in 1991 and was a proprietary document integration and management
framework for the Microsoft Office suite. Microsoft later realized that document
integration is just a special case of component integration. OLE version 2,
released in 1995 was a major enhancement over its predecessor. The foundation
of OLE version 2, now called COM, provided a general-purpose mechanism for
component integration on Windows platforms. Since then additions have been
made, such as DCOM, but applications that worked then still work now.
COM, the Component Object Model, refers to both a specification and an
implementation developed by Microsoft Corporation that provides a framework
for integrating components. This framework supports interoperability and
reusability of distributed objects by allowing developers to build systems by
assembling reusable components from different vendors that communicate via
COM. By applying COM to build systems of preexisting components, developers
hope to reap benefits of maintainability and adaptability.
Objects created using the COM specification support the fundamental notions of
encapsulation, polymorphism, and reusability. Microsoft's Component Object
Model (COM) defines a language-independent notion of what an object is -- how
to create objects, how to invoke methods, and so on. This allows development of
components that programmers can use (and reuse) in a consistent way,
regardless of which languages they use to write the component and its client.
COM is an architecture for the integration and deployment of software
components, rather than a body of techniques for problem analysis. In contrast,
most Object Oriented Design methodologies were created for more monolithic
object-oriented applications. Therefore, the assumptions that you can make with
OOD don't necessarily apply to COM, and important considerations arise,
Table 1: Differences in design considerations between OOD and COM
Object-Oriented Design
Assumptions
Added COM Considerations
Objects typically packaged in the same Objects and clients typically in separate
application (module) as client code
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
Designing a component-based system in COM is not just a matter of applying an
Object Oriented Design methodology; COM introduces new considerations of
packaging, components per package, objects per component, interfaces per
object, and simultaneous clients per object.
COM is about choice. It provides the choice of the highest volume languages and
tools available, as well as the largest base of applications. COM also provides
choice in the area of security, as it provides a common interface (SSPI) where
various security providers can be plugged in. COM also provides choice of
network transport.
COM Principles
COM forces the Windows operating system to see applications as objects. The
OS takes the responsibility of creating objects when they are required, deleting
them when they are not, and handling communications between them, be it in the
same or different processes or machines. The OS creates a central registry for
the objects. One major advantage of this mechanism is versioning. If the COM
object ever changes to a new version, the applications that use that object need
not be recompiled.
All COM objects are registered with a component database. When a client
wishes to create and use a COM object:
1. It invokes the COM API to instantiate a new COM object.
2. COM locates the object implementation and initiates a server process for
the object.
3. The server process creates the object, and returns an interface pointer at
the object.
The client can then interact with the newly instantiated COM object through the
interface pointer.
COM defines a binary structure for the interface between the client and the
object. This binary structure provides the basis for interoperability between
software components written in arbitrary languages. A fully compliant COM
object can be written in any language that can produce binary compatible code.
As long as a compiler can reduce language structures down to this binary
representation, the implementation language for clients and COM objects does
not matter - the point of contact is the run-time binary representation.
COM defines an application programming interface (API) to allow for the creation
of components for use in integrating custom applications or to allow diverse
components to interact. COM components are never linked to any particular
application. The only thing that an application may know about a COM object is
what functions it may or may not support. In fact, the object model is so flexible
that applications can query the COM object at run-time as to what functionality it
provides.
As shown in Figure 1, services implemented by COM objects are exposed
through a set of interfaces that represent the only point of contact between
clients and the object.
Figure 1: Client Using COM Object Through an Interface Pointer
Garbage collection is another major advantage to using COM. When there are no
outstanding references (a.k.a. pointers) to an object, the COM object destroys
itself.
COM Runtime Architecture
COM/ DCOM is a truly distributed Object-Oriented Architecture. Components
developed using Microsoft’s COM provide a way by which two objects in different
object spaces or networks, can talk together by calling each other’s methods.
COM services are provided in a standard way, whether those services are
required within a single running process, within two different processes on the
same machine, or on two different processes across a network using DCOM.
As a result COM and DCOM provide location transparency.
COM servers (objects) are accessed within the same process, within two
different processes on the same machine, or across the network using RPC:
1. In-process server: The client can link directly to a library containing the
server. The client and server execute in the same process.
Communication is accomplished through function calls.
2. Local Object Proxy: The client can access a server running in a different
process but on the same machine through an inter-process
communication mechanism. This mechanism is actually a lightweight
Remote Procedure Call (RPC).
3. Remote Object Proxy: The client can access a remote server running on
another machine. The network communication between client and server
is accomplished through DCE RPC. The mechanism supporting access to
remote servers is called DCOM.
Figure 2: Three Methods for Accessing COM Objects
COM and DCOM give designers three choices for packaging component code
into some executable module: in-process (same process as client), local
(separate process from client on the same machine), and remote (separate
processes on separate machines).
Table 2: Pros and Cons of COM packaging choices
Package
Pros
Type
Cons
Preferred Uses
InProcess
High speed (no
No security, no
remoting overhead),
process protectionno remoting limitations crash in component
crashes process that
loaded it, UI
synchronization
(sharing a message
pump) tricky
Add-on types of
components that
provide simple
services (like
function libraries or
child-window UI
elements) to clients
Local
Process security
(separation), process
ownership (including
threading, memory
management, etc.),
control over UI
synchronization
Slower than in-process
(remoting overhead),
remoting limitations on
interfaces, no access
security
Heavier components
that are too
expensive to load inprocess, have UI
beyond simple child
windows, or wish to
manage their own
files (such as
databases).
Remote
Process and access
security, process and
possible machine
ownership (e.g.,
managing a shared
component resource),
cross-platform
Slower than local with
additional remoting
limitations
Components that
need to run in close
proximity to a
particular resource
If the client and server are in the same process, the sharing of data between the
two is simple. However, when the server process is separate from the client
process, as in a local server or remote server, COM must format and bundle the
data in order to share it. This process of preparing the data is called marshalling.
Distributed computing purists describe marshalling as the process of packaging
and transmitting data between different address spaces, automatically resolving
pointer problems, while preserving the data’s original form and integrity.
Marshalling is accomplished through a "proxy" object and a "stub" object that
handles the cross-process communication details for any particular interface.
Even though COM objects reside in separate processes or address spaces or
even different machines, the operating system takes care of marshalling the call
and calling objects running in a different application (or address space) on a
different machine. The actual internal implementation of marshalling and unmarshalling differs depending on whether the client and server operate on the
same machine (COM) or on different machines (DCOM). Given an IDL file, the
Microsoft IDL compiler can create default proxy and stub code that performs all
necessary marshalling and un-marshalling.
Figure 3: Cross-process communication in COM
The fact that COM can access services within the same process is a huge
differentiator between COM and CORBA. Allowing for the in-process model
allows for the development of components such as AcitveX Controls or
JavaBeans. CORBA at present cannot handle in-process components and
therefore cannot participate in the component marketplace.
The IDL
Whenever a client needs some service from a remote distributed object, it
invokes a method implemented by the remote object. The service that the remote
distributed object (Server) provides is encapsulated as an object and the remote
object's interface is described in an Interface Definition Language (IDL). The
interfaces specified in the IDL file serve as a contract between a remote object
server and its clients. Clients can thus interact with these remote object servers
by invoking methods defined in the IDL. COM objects and interfaces are
specified using Microsoft Interface Definition Language (IDL), an extension of the
DCE Interface Definition Language standard. To avoid name collisions, each
object and interface must have a unique identifier. Interfaces are considered
logically immutable. Once an interface is defined, it should not be changed (new
methods should not be added and existing methods should not be modified).
When developing a COM-based system, it's important to get the interface down
in IDL code. In modern COM, IDL best describes COM interfaces. After
describing an interface in IDL, run the IDL through the MIDL compiler, which
produces C and C++ header files, a type library, and the source code necessary
for building a proxy-stub DLL. Interfaces must be well defined in IDL, because
the proxy and the stub need to understand exactly how to move data between
the client and the object. This is important because the client and the object
might be on different machines, and moving data from the client to the object
probably involves moving actual bits back and forth.
To invoke a remote method, the client makes a call to the client proxy. The client
side proxy packs the call parameters into a request message and invokes a wire
protocol like IIOP (in CORBA) or ORPC (in DCOM) or JRMP (in Java/RMI) to
ship the message to the server. At the server side, the wire protocol delivers the
message to the server side stub. The server side stub then unpacks the
message and calls the actual method on the object. In both CORBA and
Java/RMI, the client stub is called the stub or proxy and the server stub is called
skeleton. In DCOM, the client stub is referred to as proxy and the server stub is
referred to as stub.
The final consideration for using COM is that a single object instance may play
different roles for different simultaneous clients where each client is using a
different set of interfaces. A COM object can support any number of interfaces.
An interface provides a grouped collection of related methods. In addition, a
single piece of client code may be using many different objects polymorphically
(through the same interface). COM gives up on multiple inheritances to provide a
binary standard for object implementations. Instead of supporting multiple
inheritances, COM uses the notion of an object having multiple interfaces to
achieve the same purpose. This also allows for some flexible forms of
programming.
DCOM
Distributed COM is an extension to COM that allows network-based component
interaction. While COM processes can run on the same machine but in different
address spaces, the DCOM extension allows processes to be spread across a
network. With DCOM, components operating on a variety of platforms can
interact, as long as DCOM is available within the environment.
It is best to consider COM and DCOM as a single technology that provides a
range of services for component interaction, from services promoting component
integration on a single platform, to component interaction across heterogeneous
networks. In fact, COM and its DCOM extensions are merged into a single
runtime. This single runtime provides both local and remote access.
DCOM which is often called 'COM on the wire’ supports remoting objects by
running on a protocol called the Object Remote Procedure Call (ORPC). This
ORPC layer is built on top of DCE's RPC and interacts with COM's run-time
services. A DCOM server is a body of code that is capable of serving up objects
of a particular type at runtime. Each DCOM server object can support multiple
interfaces each representing a different behavior of the object. A DCOM client
calls into the exposed methods of a DCOM server by acquiring a pointer to one
of the server object's interfaces. The client object then starts calling the server
object's exposed methods through the acquired interface pointer as if the server
object resided in the client's address space. As specified by COM, a server
object's memory layout conforms to the C++ vtable layout. Since the COM
specification is at the binary level it allows DCOM server components to be
written in diverse programming languages like C++, Java, Object Pascal (Delphi),
Visual Basic and even COBOL. As long as a platform supports COM services,
DCOM can be used on that platform. DCOM is now heavily used on the Windows
platform.
Active X
In October of 1996 Microsoft turned over COM/DCOM, parts of OLE, and
ActiveX to the Open Group (a merger of Open Software Foundation and
X/Open). The Open Group has formed the Active Group to oversee the
transformation of the technology into an open standard. The aim of the Active
Group is to promote the technology's compatibility across systems (Windows,
UNIX, and MacOS) and to oversee future extension by creating working groups
dedicated to specific functions. However, it is unclear how much control Microsoft
will relinquish over the direction of the technology. Certainly, as the inventor and
primary advocate of COM and DCOM, Microsoft is expected to have strong
influence on the overall direction of the technology and underlying APIs.
An ActiveX control is really just another term for "OLE Object" or, more
specifically, "Component Object Model (COM) Object." In other words, a
control, at the very least, is some COM object that supports the IUnknown
interface and is also self-registering. It usually supports many more interfaces in
order to offer functionality, but all additional interfaces can be viewed as
optional and, as such, a container should not rely on any additional interfaces
being supported. This allows a control to implement as little functionality as it
needs to, instead of supporting a large number of interfaces that actually don't
do anything. Through QueryInterface a container can manage the lifetime of
the control, as well as dynamically discover the full extent of a control's
functionality based on the available interfaces.
In short, this minimal requirement for nothing more than IUnknown allows any
control to be as lightweight as it can. Other than IUnknown and selfregistration, there are no other requirements for a control. There are, however,
conventions that should be followed about what the support of an interface
means in terms of functionality provided to the container by the control. It
should never be assumed that an interface is available, and standard returnchecking conventions should always be followed. It is important for a control or
container to degrade gracefully and offer alternative functionality if a required
interface is not available.
ActiveX controls have become the primary architecture for developing
programmable software components for use in a variety of different containers,
ranging from software development tools to end-user productivity tools. For a
control to operate well in a variety of containers, the control must be able to
assume some minimum level of functionality that it can rely on in all containers.
Component Object Model+
COM+ is much younger than COM, it was announced in Sept. 23, 1997 and is a
major upgrade of Microsoft’s long-term component strategy. The production
release of COM+ is shipped with Windows 2000. The Web-centered computing
industry has begun to align itself into two technology camps-with one camp
centered around Microsoft's COM/DCOM/COM+, Internet Explorer, and ActiveX
capabilities, and the other camp championing Netscape, CORBA, and Java/J2EE
solutions. Both sides argue vociferously about the relative merits of their
approach, but at this time there is no clear technology winner.
COM+ is the next step in the evolution of the Microsoft Component Object Model
and the Microsoft Transaction Server (MTS). COM+ is the merging of the COM
and MTS programming models with the addition of several new features. COM+
handles many of the resource management tasks a developer had to program
himself, such as thread allocation and security. It automatically makes an
application more scalable by providing thread pooling, object pooling, and just-intime object activation. COM+ also protects the integrity of the data by providing
transaction support, even if a transaction spans multiple databases over a
network. COM+ has come along to unify COM, DCOM, and MTS into a coherent,
enterprise-worthy component technology. Indeed, these and other technologies
constitute Microsoft's distributed and web-oriented strategy. This strategy is
globally referred as Distributed interNet Architecture(tm) (DNA) and it comprises
a full set of products and specifications to implement net-centric applications.
COM+ integrates MTS services and message queuing into COM, and makes
COM programming easier through a closer integration with Microsoft languages
as Visual Basic, Visual C++, and J++. COM+ does not change the wire protocol
that Distributed COM (DCOM) uses, so network communication stays
unchanged. COM+ not only adds MTS-like quality of service into every COM+
object, it hides some of the complexities of coding in COM.
COM suffers from some weaknesses that have been recognized by Microsoft
and addressed in Component Object Model+. COM is hard to use. Reference
counting, Microsoft IDL, Global Unique Identifiers (GUID), etc. require deep
knowledge of the COM specification by developers. It has been estimated that up
to 30 percent of the effort in writing component-based software is spent writing
“object housekeeping” software. COM+ will handle this task for developers by
providing implementations of most of the code that deals with the object
infrastructure, leaving the developer free to write logic that deals with the problem
they are trying to solve. COM+ provides a much better component administration
environment, support for load balancing and object pooling, and an easier-to-use
event model. These changes are based on a new COM+ runtime that moves
most of the COM grunge code (e.g., IUnknown and class factory
implementations) into the OS.
COM+ consists of:
1) A runtime or execution environment
2) Extensible services, which are provided by Microsoft, which include
transactions, security, load balancing, and automatic memory management.
Third party developers will provide additional extensible services.
3) Innovation, with key concepts such as Interception, which enables many of the
extensible services which COM+ provides.
COM+ Principles
A good number of developers see COM as more than a little challenging to
understand and use. The reason for this is simple. Using COM's languageindependent objects in any real programming language requires understanding a
new object model -- the one defined by COM. For example, a C++ programmer
knows that creating a new object requires using the language's new operator,
while getting rid of that object requires calling delete. If that same C++
programmer wants to use a COM object, however, the developer can't do things
in this familiar way. Instead, the standard COM function CoCreateInstance (or
one of a few other choices) must be called to create the object. When done with
this COM object, the programmer doesn't delete it explicitly, as in C++, but
instead invokes the object's Release method. The object relies on an internal
reference count that it maintains to determine when it has no more clients, and
thus when it's safe to destroy itself.
COM+ still provides a standard library, and objects and their clients still use it.
But in contrast to COM, COM+ hides calls to this library beneath the equivalent
native functions in the programming language. C++ programmers, for example,
can once again use the standard new operator rather than CoCreateInstance to
create a COM+ object. In doing so, they are relying on a C++ compiler that is
aware of COM+ to generate the correct code to call the COM+ library. To
accomplish this, the compiler uses the COM+ library at compile time, then
embeds calls to this same COM+ library in the generated binary. Microsoft will
provide this library, and any language tool that wants to use COM+ must rely
upon it. Unlike classic COM, where only COM objects and their clients use the
COM library, COM+ also requires compilers (or interpreters, such as those for
Visual Basic, and scripting languages, like JavaScript) to rely on a standard
library to produce the correct code.
COM+ eliminates the need for clients to call Release when they are done using
an object. COM+ also allows implementation inheritance between COM+ objects
running in the same process.
In COM+, developers no longer need to define interfaces using IDL. Instead, they
can just use their programming language's syntax to define the object's
interfaces. The compiler for that language then works with the COM+ library to
generate metadata for the object. Since every COM+ object has metadata, it's
also possible to approach marshaling consistently. Marshaling is packaging a
method call's parameters in some standard way, allowing these parameters to
move effectively between objects written in entirely different languages or
running on entirely different machines. A developer in a COM+ world just
provides the Meta data that is needed and provides the methods. Most of the
other code dealing with things such as registering components, reference
counting for memory management etc. will be handled by the system.
COM+ addresses an important but challenging problem in creating a languageindependent object model: data types. Different languages support different data
types, which causes problems when passing parameters between objects written
in different languages.
COM+ introduces the concept of attribute-based programming. A developer sets
attributes on an object that tell the system essentially how to treat it, for example,
as a transactional object. This attribute-based programming model relies on a
mechanism known as interception.
Interceptors provide services based on attributes that have been previously set
on an object by a developer. These interceptors provide automatic behavior at
runtime based on the attribute set. As an example interception allows for things
such as dynamic load balancing. Also, an interceptor makes sure that when a
transactional object attempts to change data either all succeed or all fail and
rollback.
COM+ also changes COM's persistence model. Today, the creator of a COM
object must typically implement one or more of a fairly large set of interfaces
related to persistence. A client of this object then calls various methods in those
interfaces to have the object load or save its persistent state. But the COM+
library provides standard support for persistence, removing much of the burden
from the COM+ object implementor. And by representing an object's properties in
a standard way ("serialization"), COM+ lets the developer pass objects by value.
All that's required is to send this serialized representation of an object's data to
another object of the same class.
Today, COM and MTS components place all of their configuration information in
the Windows registry. With COM+, however, most component information is
stored in a new database, currently called the COM+ Catalog. The COM+
Catalog unifies the COM and MTS registration models and provides an
administrative environment for components. A developer interacts with the COM+
Catalog using either the COM+ Explorer, which is similar to the MTS Explorer, or
through a series of new COM interfaces that expose its capabilities.
Another interesting change, support for constructors, makes COM+ objects more
like objects in a typical object-oriented programming language. Languages like
C++ and Java can define a constructor method that runs when first creating an
object. The creator of the object can then pass parameters as needed to this
constructor, allowing easy initialization. COM objects do not support constructors,
but COM+ objects do. COM+ constructors even allow passing parameters, better
integrating COM+ objects and the objects used by today's most popular objectoriented languages.
Another important COM+ feature is its support for declarative programming.
What this means is that a programmer can develop components in a generic way
and defer many of the details until deployment time. For example, one can
develop a component that supports working in a load-balanced environment.
However, the decision of whether or not to use load balancing is deferred. Some
applications may want to use load balancing and others may not. You indicate
support by setting an attribute or declaring to use its support for load balancing.
This is done at an administrative level using the COM+ Explorer. The MTS
declarative security model is another example. Instead of handling security
programmatically, you let the administrators do it through MTS packages and its
administration model.
Windows DNA
Windows Distributed interNet Applications Architecture, or Windows DNA, is
Microsoft’s latest acronym that describes its move from workstation-based to
enterprise-level application development. Windows DNA describes those
Microsoft technologies that provide a complete, integrated n-tier development
model, and those services that developers require to build scalable and
dependable enterprise-level systems on the Windows platform.
Figure 1
Figure 1 depicts Windows DNA as it stands today. When building an application,
a developer uses several different Windows and Internet technologies based on
the application’s target user. Rich client applications are written using the Win32
API and distributed as executables, in the typical fashion. Thin client applications,
or those that target a browser, use either straight HTML or dynamic HTML at the
presentation tier.
At the middle tier, developers use DCOM, MTS, IIS, and Active Server Pages to
handle business logic and other application services. Components executing on
the middle tier access back-end data using Active Data Object (ADO) or OLE
DB. Microsoft also provides tools to access data on non-Windows platforms.
Examples include ODBC, COM services on UNIX, and the new COM Transaction Integrator (COMTI).
Figure 2
Figure 2 depicts the long-term goal of Windows DNA: a technology called
Forms+ at the GUI level, COM+ at the middle tier, and Storage+ on the data tier.
Details on Forms+ and Storage+ are sketchy, but the majority of COM+ has been
delivered with Windows 2000. The goal of the Forms+ initiative is to merge the
Win32 GUI and Web APIs. Forms+ is Microsoft’s answer to the difficulties
developers face today when deciding what presentation platform to target when
developing an application. Today, developers have to choose to either target
Windows using the Win32 API or to target the browser and use HTML or
dynamic HTML. Forms+ is a move away from the Win32 API and a move toward
DHTML for Windows presentation development. Spend some time with the
architecture of Internet Explorer 5 for a glimpse of where Forms+ is headed.
Storage+ is the future of the Windows file system and will probably look a lot like
OLE DB, but with several new features. Storage+ is the most distant technology
of Windows DNA and we won’t see much on this front until well after Windows
2000.
Conclusion
It is important to note that COM+ essentially is one of the key unifying elements
for Windows DNA, as it allows for the development of applications, which are
flexible and powerful enough to deal with the spectrum of environments found
today, from three tier client server environments to web based.
What all of this means is that Microsoft understands that the software market is
moving faster and faster toward the Internet. All of the future “killer applications”
will be developed for Web environments, be they intranets or the Internet. To
succeed in this new market, Microsoft is making it easier for developers to build
applications without a dependency on the Win32 API.
References
Jason Pritchard, PH.D., COM and CORBA Side by Side, Addison-Wesley
Longman, Inc., 1999
Doreen L. Galli, Distributed Operating Systems, Prentice Hall, 2000
Microsoft Corporation. The Component Object Model Specification, Version 0.9,
October 24, 1995 [online]. <URL: http://www.microsoft.com/oledev/> (1995).
Microsoft Corporation. Distributed Component Object Model Protocol-DCOM/1.0,
draft, November 1996 [online]. <URL: http://www.microsoft.com/oledev/> (1996).
Kirtland, Mary. “The COM+ Programming Model Makes it Easy to Write
Components in Any Language”. Microsoft System Journal. December, 1997
Object Management Group home page [online]. The site provides information
comparing DCOM (ActiveX) to CORBA. <URL: http://www.omg.org/> (1997).
Cluts, Nancy Winnick. “Creating ActiveX Components in C++”. Microsoft
Corporation. November, 1996.
By: Paul Visokey
Qisheng Hong
Yani Mulyani