What is DCOM? (Part 1)

So in the last post I gave you a very brief introduction to what COM actually is. In this post, I'd like to talk about DCOM – Distributed COM.

I won't concentrate on how to write a DCOM program. I will write from a theoretical angle and try to provide a simple explanation of what DCOM actually is. If you are interested in trying some hands-on development, please refer to the excellent article called "DCOM D-Mystified: A DCOM Tutorial" by Brian C Hart on codeproject.com.

So, where were we?

COM objects are self-contained components that can be used in an application and provide a certain service. One can write a COM object and this object can be used via COM APIs. This functionality is extended by the DCOM – Distributed Component Object Model. DCOM allows the COM components to reside on different computers and be accessed by an application from distance.

Imagine complicated software that is using a lot of components to manipulate different tasks and actions – this software doesn't even have to be aware that the components it's using do not reside on the same machine it is running on because DCOM "hides" this headache from the user by providing it "location transparency" that eases the development of distributed applications.

DCOM communicate with distant objects through a remote procedure call protocol that is simply called RPC (maybe more on that in the future posts), handles the "low-level" details of the networking and allows the developer to concentrate solely on his program's functionality.

You can think of DCOM as a mechanism of inter-process communication (this is actually one of the uses of COM), but in a distributed environment. Neither the COM client, nor the COM server should be aware that they do not reside on the same machine and the client's questions as well as the server's answers are carried by the DCOM network protocol back and forth.

DCOM: COM Components on different machines (Source: MSDN)

You can meet DCOM in multiuser network games, chat applications, VoIP applications, network monitoring applications and many more. In the next post I'll dig a little deeper into the guts of the whole process and its exciting features and benefits.

That's all for now. Thanks for reading!


As always I'd like to share with you a tune that I keep listening to on and on during the last week. Enjoy!

A few words about COM

I would really like to talk about Distributed Component Object Model (DCOM) a little, but to do that I first must talk aboutsome basic terms as Managed and Unmanaged code, COM (Component Object Model) andothers. So this post will deal with these terms while in the next post I'lltalk about DCOM and where you can use it.

So let's start at the very beginning.

In the Windows operating system you can write User mode or Kernel mode code. I will write posts about Kernel mode development in the future, but in this one I would like to discuss some basic staff about user space development.

Unmanaged code – a quick reminder

I know that it's probably not agood idea to explain what an unmanaged code is by telling you what is a managed code, but I guess that this would be the easier way to understand it.

Managed code is a Microsoft term for a code that the compilation and execution of which is managed by the CLR - Common Language Runtime. The CLR is an environment that is part of the .NET framework designed to manage the execution of programs. You may think of the CLR as the Java VM because here the code is also compiled to an intermediate language (CIL) that can executed later by the CLR that will convert it into machine code. 

There are numerous benefits that you gain from that. Probably the major one is the memory management that the CLR does for you by releasing objects that are no longer in use (managed data)and operating a Garbage Collector that eliminates memory leaks. But there are other benefits that reduce common programmers' errors. 

In unmanaged code you'll need to manage your own memory allocations, avoid memory leaks and free unused space in the memory. The library/executable that is resulted by the unmanaged code compilation is written in machine code (therefore running faster) and don't need the .NET framework installed.

There are several ways to manage an unmanaged code. One of them is exposing the required methods through COM and use the COM object from the managed code program - this is the most widely used method. A nice article about that can be found here.

BTW, The CLR is implemented as a COM server.

What is COM?

Well, we'll need more than one blog post to deeply understand what COM is, but let's try to keep it simple. For those of you that would like to deepen the understanding and learn how to use COM, I would recommend the "Introduction to COM" article by Michael Dunn in Codeproject.com – an oldie but a goodie.

COM is a standard that lets COM objects to interact with other object enabling sharing binary code across different applications. COM is a binary standard that applies after a program has been translated to binary machine code and specifies that the binary modules (libraries and executables) must be compiled to match a specific structure. 

This is the beauty of it. There is a little that is up to the programmer while the compiler does all the work in creating the COM objects that is easy to use later.

A COM object is made up of a set of data and the functions that manipulate the data. Access to an object's data is achieved exclusively through interface methods. COM requires that the only way to gain access to the methods of an interface is through a pointer to the interface.

COM also defines how objects work together over a distributed environment and has added security features to help provide system and component integrity – but more on that in the next post.

An example – WMI (WindowsManagement Instrumentation)

"WMI provides a uniform interface for any local or remote applications or scripts that obtain management data from a computer system, a network, or an enterprise."

(WMI Architecture,MSDN)

There is lots of cool stuff you can do with WMI and if you're not familiar with it yet, I would recommend toget familiar with it (maybe I'll write a post about WMI too) but let's focus on the fact that the way to use WMI is by its COM interface. The WMI COM API enables developers to write a WMI client or an application that uses WMI information.

Let's take a look at the example from the article "GettingWMI Data from the Local Computer" in the MSDN. First it initialize the COM parameters using the CoInitializeEx() function, then some security information is passed via CoInitializeSecurity() and then we get access to the WMI COM API by using CoCreateInstance() function with the CLSID_WbemLocator object and getting the IWbemLocator pointer.

Now we can call IWbemLocatorobject's function (like ConnectServer()).

HRESULT hres;

hres =  CoInitializeEx(0, COINIT_MULTITHREADED);

    if (FAILED(hres))

    {

        cout << "Failed to initialize COM library. Error code = 0x"

            << hex << hres <<endl;

        return 1;                  // Program has failed.

    }

    hres = CoInitializeSecurity(

        NULL,

        -1,                          // COM authentication

        NULL,                        // Authentication services

        NULL,                        // Reserved

        RPC_C_AUTHN_LEVEL_DEFAULT,   //Default authentication

        RPC_C_IMP_LEVEL_IMPERSONATE, // Default Impersonation 

        NULL,                        //Authentication info

        EOAC_NONE,                   // Additional capabilities

        NULL                         // Reserved

        );

               

    if (FAILED(hres))

    {

        cout << "Failed to initialize security. Error code = 0x"

            << hex << hres <<endl;

        CoUninitialize();

        return 1;                    // Program has failed.

    }

    IWbemLocator *pLoc = NULL;

    hres = CoCreateInstance(

        CLSID_WbemLocator,            

        0,

        CLSCTX_INPROC_SERVER,

        IID_IWbemLocator, (LPVOID *)&pLoc);

    if (FAILED(hres))

    {

        cout << "Failed to create IWbemLocator object."

            << " Err code = 0x"

            << hex << hres <<endl;

        CoUninitialize();

        return 1;                 // Program has failed.

    }

    IWbemServices *pSvc = NULL;

        

    hres = pLoc->ConnectServer(

         _bstr_t(L"ROOT\\CIMV2"), // Object path of WMI namespace

         NULL,                    // User name. NULL = current user

         NULL,                    // User password. NULL = current

         0,                       //Locale. NULL indicates current

         NULL,                    // Security flags.

         0,                       // Authority (for example, Kerberos)

         0,                       // Context object

         &pSvc                    // pointer to IWbemServices proxy

         );

   

    if (FAILED(hres))

    {

        cout << "Could not connect. Error code = 0x"

             << hex << hres<< endl;

        pLoc->Release();    

        CoUninitialize();

        return 1;                //Program has failed.

    }

Nice! Isn't it?

Let's conclude this post with a nice song by Jimi Hendrix, Love or Confusion, because that's how you probably feel right now, isn't it? J