Technical Publications-Whitepapers-A Detailed Comparison of CORBA, DCOM and Java/RMI (with specific code examples)

21 Sep 1998

Publications, Distributed-Computing

Introduction

Distributed object computing extends an object-oriented programming system by allowing objects to be distributed across a heterogeneous network, so that each of these distributed object components interoperate as a unified whole. These objects may be distributed on different computers throughout a network, living within their own address space outside of an application, and yet appear as though they were local to an application.

Three of the most popular distributed object paradigms are Microsoft's Distributed Component Object Model (DCOM), OMG's Common Object Request Broker Architecture (CORBA) and JavaSoft's Java/Remote Method Invocation (Java/RMI). In this article, let us examine the differences between these three models from a programmer's standpoint and an architectural standpoint. At the end of this article, you will be able to better appreciate the merits and innards of each of the distributed object paradigms.

CORBA relies on a protocol called the Internet Inter-ORB Protocol (IIOP) for remoting objects. Everything in the CORBA architecture depends on an Object Request Broker (ORB). The ORB acts as a central Object Bus over which each CORBA object interacts transparently with other CORBA objects located either locally or remotely. Each CORBA server object has an interface and exposes a set of methods. To request a service, a CORBA client acquires an object reference to a CORBA server object. The client can now make method calls on the object reference as if the CORBA server object resided in the client's address space. The ORB is responsible for finding a CORBA object's implementation, preparing it to receive requests, communicate requests to it and carry the reply back to the client. A CORBA object interacts with the ORB either through the ORB interface or through an Object Adapter - either a Basic Object Adapter (BOA) or a Portable Object Adapter (POA). Since CORBA is just a specification, it can be used on diverse operating system platforms from mainframes to UNIX boxes to Windows machines to handheld devices as long as there is an ORB implementation for that platform. Major ORB vendors like Inprise have CORBA ORB implementations through their VisiBroker product for Windows, UNIX and mainframe platforms and Iona through their Orbix product.

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. Companies like Software AG provide COM service implementations through their EntireX product for UNIX, Linux and mainframe platforms; Digital for the Open VMS platform and Microsoft for Windows and Solaris platforms.

Java/RMI relies on a protocol called the Java Remote Method Protocol (JRMP). Java relies heavily on Java Object Serialization, which allows objects to be marshaled (or transmitted) as a stream. Since Java Object Serialization is specific to Java, both the Java/RMI server object and the client object have to be written in Java. Each Java/RMI Server object defines an interface which can be used to access the server object outside of the current Java Virtual Machine(JVM) and on another machine's JVM. The interface exposes a set of methods which are indicative of the services offered by the server object. For a client to locate a server object for the first time, RMI depends on a naming mechanism called an RMIRegistry that runs on the Server machine and holds information about available Server Objects. A Java/RMI client acquires an object reference to a Java/RMI server object by doing a lookup for a Server Object reference and invokes methods on the Server Object as if the Java/RMI server object resided in the client's address space. Java/RMI server objects are named using URLs and for a client to acquire a server object reference, it should specify the URL of the server object as you would with the URL to a HTML page. Since Java/RMI relies on Java, it can be used on diverse operating system platforms from mainframes to UNIX boxes to Windows machines to handheld devices as long as there is a Java Virtual Machine (JVM) implementation for that platform. In addition to Javasoft and Microsoft, a lot of other companies have announced Java Virtual Machine ports.

Comparing Apples to Apples

CORBA 3.0 will add a middleware component model (much like MTS or EJB) to CORBA. Since it is still in a pre-spec stage, we do not know much about how the CORBA middleware component model is going to look like. As of CORBA 2.x, there is no middleware component model that CORBA defines. Though I would really like to compare MTS, EJB and CORBA 3.0's middleware component model (whatever it's going to be called) , I reserve it for a future article (Click Here...). By the way, there is a lot of comparison going on between COM and EJB. This is entirely wrong. This is like comparing apples to oranges. The competing technologies are MTS and EJB. Hence, the real comparison should be between MTS and EJB.

Application Sample - The StockMarket Server and Client

The StockMarket server reports the stock price of any given symbol. It has a method called get_price() to get the stock value of a particular symbol.

I have selected Java as the implementation language for these examples here for three reasons :

1. Java/RMI can only be implemented using Java.
2. Since I am comparing Java/RMI with other object technologies, I would have to implement the DCOM and CORBA objects too in Java.
3. Java is the best language to code CORBA and COM objects in since it keeps the implementation simple, easy to understand, and is most elegant.

Each of these implementations define an IStockMarket interface. They expose a get_price() method that returns a float value indicating the stock value of the symbol passed in. We list the sources from four files. The first set of files are the IDL and Java files that define the interface and its exposed methods. The second set of files show how the client invokes methods on these interfaces by acquiring references to the server object. The third set of files show the Server object implementations. The fourth set of files show the main program implementations that start up the Remote Server objects for CORBA and Java/RMI. No main program implementation is shown for DCOM since the JavaReg program takes up the role of invoking the DCOM Server object on the Server machine. This means you have to also ensure that JavaReg is present on your server machine.

The IDL Interface

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.

DCOM - The DCOM IDL file shows that our DCOM server implements a dual interface. COM supports both static and dynamic invocation of objects. It is a bit different than how CORBA does through its Dynamic Invocation Interface (DII) or Java does with Reflection. For the static invocation to work, The Microsoft IDL (MIDL) compiler creates the proxy and stub code when run on the IDL file. These are registered in the systems registry to allow greater flexibility of their use. This is the vtable method of invoking objects. For dynamic invocation to work, COM objects implement an interface called IDispatch. As with CORBA or Java/RMI, to allow for dynamic invocation, there has to be some way to describe the object methods and their parameters. Type libraries are files that describe the object, and COM provides interfaces, obtained through the IDispatch interface, to query an Object's type library. In COM, an object whose methods are dynamically invoked must be written to support IDispatch. This is unlike CORBA where any object can be invoked with DII as long as the object information is in the Implementation Repository. The DCOM IDL file also associates the IStockMarket interface with an object class StockMarket as shown in the coclass block. Also notice that in DCOM, each interface is assigned a Universally Unique IDentifier (UUID) called the Interface ID (IID). Similarly, each object class is assigned a unique UUID called a CLasS ID (CLSID). COM gives up on multiple inheritance to provide a binary standard for object implementations. Instead of supporting multiple inheritance, COM uses the notion of an object having multiple interfaces to achieve the same purpose. This also allows for some flexible forms of programming.

CORBA - Both CORBA and Java/RMI support multiple inheritance at the IDL or interface level. One difference between CORBA (and Java/RMI) IDLs and COM IDLs is that CORBA (and Java/RMI) can specify exceptions in the IDLs while DCOM does not. In CORBA, the IDL compiler generates type information for each method in an interface and stores it in the Interface Repository (IR). A client can thus query the IR to get run-time information about a particular interface and then use that information to create and invoke a method on the remote CORBA server object dynamically through the Dynamic Invocation Interface (DII). Similarly, on the server side, the Dynamic Skeletion Interface (DSI) allows a client to invoke an operation of a remote CORBA Server object that has no compile time knowledge of the type of object it is implementing. The CORBA IDL file shows the StockMarket interface with the get_price() method. When an IDL compiler compiles this IDL file it generates files for stubs and skeletons.

Java/RMI - Notice that unlike the other two, Java/RMI uses a .java file to define it's remote interface. This interface will ensure type consistency between the Java/RMI client and the Java/RMI Server Object. Every remotable server object in Java/RMI has to extend the java.rmi.Remote class. Similarly, any method that can be remotely invoked in Java/RMI may throw a java.rmi.RemoteException. java.rmi.RemoteException is the superclass of many more RMI specific exception classes. We define an interface called StockMarket which extends the java.rmi.Remote class. Also notice that the get_price() method throws a java.rmi.RemoteException.

How remoting works

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.

Implementing the Distributed Object Client

DCOM Client - The DCOM client shown below calls into the DCOM server object's methods by first acquiring a pointer to the server object. The new keyword here instantiates the StockMarket DCOM Server object. This leads the Microsoft JVM to use the CLSID to make a CoCreateInstance() call. The IUnknown pointer returned by CoCreateInstance() is then cast to IStockMarket, as shown below:

IStockMarket market = (IStockMarket)new simplestocks.StockMarket();

The cast to IStockMarket forces the Microsoft JVM to call the DCOM server object's QueryInterface() function to request a pointer to IStockMarket. If the interface is not supported, a ClassCastException is thrown. Reference Counting is handled automatically in Java/COM and the Microsoft JVM takes up the responsibility of calling IUnknown::AddRef() and Java's Garbage Collector automatically calls IUnknown::Release(). Once the client acquires a valid pointer to the DCOM server object, it calls into its methods as though it were a local object running in the client's address space.

CORBA Client - The CORBA client will first have to initialize the CORBA ORB by making a call to ORB.init(). It then instantiates a CORBA server object by binding to a server object's remote reference. Though both Inprise's VisiBroker and Iona's Orbix have a bind() method to bind and obtain a server object reference like,

StockMarket market = StockMarketHelper.bind( orb ); // this is Visibroker or Orbix specific

we will use the CORBA Naming Service to do the same thing so that we are compatible with any ORB. We first look up a NameService and obtain a CORBA object reference. We use the returned CORBA.Object to narrow down to a naming context.

NamingContext root = NamingContextHelper.narrow( orb.resolve_initial_references("NameService") );

We now create a NameComponent and narrow down to the server object reference by resolving the name in the naming context that was returned to us by the COSNaming (CORBA Object Services - Naming) helper classes.

NameComponent[] name = new NameComponent[1] ;
name[0] = new NameComponent("NASDAQ","");

StockMarket market = StockMarketHelper.narrow(root.resolve(name));

Once the client has acquired a valid remote object reference to the CORBA server object, it can call into the server object's methods as if the server object resided in the client's address space.

Java/RMI Client - The Java/RMI client first installs a security manager before doing any remote calls.You do this by making a call to System.setSecurityManager(). The RMISecurityManager provided by JavaSoft is an attempt by JavaSoft from having you to write your own implementation. However, JavaSoft does not force you to use it's own RMISecurityManager - you can write your own security manager and install it if you want to.

The Java/RMI client then instantiates a Java/RMI server object by binding to a server object's remote reference through the call to Naming.Lookup().

StockMarket market = (StockMarket)Naming.lookup("rmi://localhost/NASDAQ");

Once the client has acquired a valid object reference to the Java/RMI server object, it can call into the server object's methods as if the server object resided in the client's address space.

Implementing the Distributed Object Server

DCOM Server Object - All the classes that are required for Java/COM are defined in the com.ms.com package. The DCOM Server object shown below implements the IStockMarket interface that we defined in our IDL file. The StockMarket class and the get_price() method are declared as public so that they will be accessible from outside the package. Also notice the CLSID specified and declared as private. It is used by COM to instantiate the object through CoCreateInstance() when a DCOM client does a new remotely. The get_price() method is capable of throwing a ComException.

CORBA Server Object - All the classes that are required for CORBA are defined in the org.omg.CORBA package. The CORBA Server object shown below extends the _StockMarketImplBase class that is a skeleton class generated by our CORBA IDL compiler. The StockMarketImpl class and the get_price() method are declared as public so that they will be accessible from outside the package. The StockMarketImpl class implements all the operations declared in our CORBA IDL file. We need to provide a constructor which takes in a name of type String for our CORBA object server class since the name of the CORBA Server class has to be passed on to the _StockMarketImplBase class object, so that it can be associated with that name with all the CORBA services.

Java/RMI Server Object - All the classes that are required for Java/RMI are defined in the java.rmi package. The Java/RMI Server object shown extends the UnicastRemoteObject class that has all of Java/RMI's remoting methods defined and implements the StockMarket interface. The StockMarketImpl class and the get_price() method are declared as public so that they will be accessible from outside the package. The StockMarketImpl class implements all the operations declared in our Java/RMI interface file. We need to provide a constructor which takes in a name of type String for our Java/RMI object server class since the name of the Java/RMI Server class is used to establish a binding and associate a public name with this Java/RMI Server Object in the RMIRegistry. The get_price() method is capable of throwing a RemoteException since it is a remotable method.

The Server Main Programs

CORBA Server Main - The first thing that has to be done by the main program is to initialize the CORBA ORB using ORB.init(). An Object Adapter (OA) sits on top of the ORB, and is responsible for connecting the CORBA server object implementation to the CORBA ORB. Object Adapters provide services like generation and interpretation of object references, method invocation, object activation and deactivation, and mapping object references to implementations. You have to initialize either the Basic Object Adapter(BOA) or the Portable Object Adapter(POA) depending on what your ORB supports. (Note : I use Inprise's VisiBroker as my CORBA ORB and hence I conform to its implementation requirement where I need to init the BOA). You do this by calling orb.BOA_init(). We then create the CORBA Server object with the call

StockMarketImpl stockMarketImpl = new StockMarketImpl("NASDAQ");

Note that we pass in a name "NASDAQ" by which our object is identified by all CORBA services. We then inform the ORB that the Server Object is ready to receive invocations by the statement:

boa.obj_is_ready( stockMarketImpl );

Since we are using the CORBA Object Service's Naming Service for our clients to connect to us, we will have to bind our server object with a naming service, so that clients would be able to find us. The following code helps us to do that. (Note : This ensures that our code will work with any CORBA ORB. If our clients use the bind() method -specific to VisiBroker and Orbix- to connect to the server object we do not need to do this.)
org.omg.CORBA.Object object = orb.resolve_initial_references("NameService");
NamingContext root = NamingContextHelper.narrow( object ) ;
NameComponent[] name = new NameComponent[1];
name[0] = new NameComponent("NASDAQ", "");
root.rebind(name, stockMarketImpl);

The next statement ensures that our main program sleeps on a daemon thread and does not fall off and exit the program.

boa.impl_is_ready();

We now enter into an event loop and are in that loop till the main program is shut down.

Java/RMI Server Main - The Java/RMI client will first have to install a security manager before doing any remote calls. You do this by making a call to System.setSecurityManager(). We then create the Java/RMI Server object with the call

StockMarketImpl stockMarketImpl = new StockMarketImpl("NASDAQ");

and remain there till we are shut down.

DCOM - Notice that we have not provided a Main program for our DCOM Server implementation. The Java Support in Internet Explorer runs as an in-process server, and in-process servers cannot normally be remoted using the Windows NT 4.0 Distributed COM (DCOM). However, it is possible to launch a "surrogate" .EXE in its own process that then loads the in-process server. This surrogate can then be remoted using DCOM, in effect allowing the in-process server to be remoted.You can use JavaReg's /surrogate option to support remote access to a COM class implemented in Java. When first registering the class, specify the /surrogate option on the command line. For example:

javareg /register /class:StockMarket /clsid:{FE19E681-508B-11d2-A187-000000000000} /surrogate

This adds a LocalServer32 key to the registry in addition to the usual InprocServer32 key. The command line under the LocalServer32 key specifies JavaReg with the /surrogate but without the /register option.

HKEY_CLASSES_ROOT
CLSID
{BC4C0AB3-5A45-11d2-99C5-00A02414C655}
InprocServer32 = msjava.dll
LocalServer32 = javareg /clsid:{BC4C0AB3-5A45-11d2-99C5-00A02414C655} /surrogate

This causes JavaReg to act as the surrogate itself. When a remote client requests services from the COM class that you've implemented using Java, JavaReg is invoked. JavaReg then loads the Java Support in Internet Explorer with the specified Java class. (This means that when distributing your Java program, your installation program must install JavaReg along with the Java class.) You can remove the LocalServer32 key by rerunning JavaReg with the /class option, specifying the same class name, but without the /clsid or /surrogate options

javareg /register /class:StockMarket

Conclusion

The architectures of CORBA, DCOM and Java/RMI provide mechanisms for transparent invocation and accessing of remote distributed objects. Though the mechanisms that they employ to achieve remoting may be different, the approach each of them take is more or less similar.

The Sources

 

Where do we go from here...

The moment we talk of comparisons, we start comparing features one after the other. There is a lot more to these than what I have discussed above. In fact, whole new powerful models are being built based on or around these three remoting paradigms. Since the DCOM, CORBA and Java/RMI remoting models are the basis for a lot of other paradigms built on top of them, the moment we talk of these other paradigms, it is the start of a totally new and different comparison.

I could have gone on comparing MTS and EJB next, or DNA and J2EE, or MSMQ and JMS, or Jini and Universal Plug and Play... But they are entirely different comparisons. I have to stop at some point and I decided that I stop right here, giving myself an opportunity to write about other technology comparisons in future articles.