After making the important decision to embrace the Common Object Request Broker(CORBA), the next important decision that you will have to make is choosing an Object Request Broker(ORB) vendor suitable for your needs. You have many options to select from: Digital's ObjectBroker, ExpertSoft's PowerBroker, HP's Orb Plus, IBM's SOM, Iona's Orbix, Sun's Joe, VisiBroker from Visigenic, etc.
Six months back, when I first worked on VisiBroker2.5, I found it a good ORB but without many supporting services. In July 1997, Visigenic announced version 3.0 of VisiBroker which made it more promising than before.
In 1991, Object Management Group (OMG) specified CORBA 1.0, an Object Management Architecture (OMA), for cross-platform distributed computing. Using ORB, a client object can transparently invoke a method on a server object sitting at any location on a network and developed in any language. CORBA enables this functionality with the help of its Interface Definition language (IDL), ORB bus, and services and facilities extending it. The key feature of CORBA is its IDL. All that the client object knows about the remote object, is its interface. CORBA provides location, language and platform transparency. The OMG left the implementation details of the specifications up to the vendors. Different vendors implemented the architecture differently. Subsequently, in December 1994 OMG specified CORBA 2.0 with Internet-Inter ORB protocol (IIOP) for the interoperability across the vendors.
I could easily install Visibroker 3.0 for Java(VBJ) on both Windows 95 and Solaris 2.5. Visigenic separately ships other CORBA components like Naming service, Event Service and VisiBroker Manager. So I repeated the installation procedure for them too. Configuration was also easy. VisiBroker is a completely dynamic system and easy to manage. VisiBroker 3.0 comes with good documentation, including an installation guide, programmer's guide and reference manual.
VBJ comes with a bunch of examples covering almost all important features of the product. It is easy to start with them. You can go through the following steps for developing CORBA based application:
Important Components and Their Functions
- Identify remote objects required for the application. Write an interface for the object using IDL.
- Then compile the interface using the IDL compiler, namely idl2java. The compiler creates an interface class, stub class, skeleton classes, and supporting classes per interface. The stub, _st_<interface name>.java, is a proxy of the remote object for the client. The proxy implementation invokes operations on the remote objects. It does this by forming a method, marshaling the data, and sending the request over ORB. The skeleton object provides the server side functionality and connects the server object to ORB. VBJ 3.0 generates skeleton classes named <interface name>ImplBase.java. It also generates two more classes, namely <interface name>Helper.java and <interface name>Holder.java. The helper class defines utility functions related to binding, etc. The holder class provides a holder for passing parameters.
- The next step is to implement the client and server interfaces. The implementation generally involves initialization of ORB, binding and registering implementation.
- Now, compile all the generated code and application. Start the VBJ specific daemon called OSAgent. You can run the server and client using a program called vbj. The vbj program actually invokes Java Virtual Machine(JVM) and runs your code. It also enables it to configure the threads and connection information. You can even plug an object debugger into it. This debugger is a GUI based interceptor of the IIOP messaging. The debugger is a very useful tool for development.
VBJ supports a full language mapping and complete implementation of mandatory CORBA features, like pseudo-interfaces, ORB, BOA, and IR. Visigenic ORB fully implements the IIOP protocol. VBJ 3.0 comes with naming and event services fully implemented in Java.
VBJ offers idl2java compiler to automatically generate Java code from IDL. The idl2java provides IDL1.0 compliant IDL to Java language binding. VBJ also contains a fully implemented Interface Repository(IR). The IR is an on-line database of meta information (object interface) about ORB interfaces. It is essential for Dynamic Invocation Interface(DII). In such cases, the clients learn about an unknown object's interface at runtime. You need to create IR using VBJ's irep program. VBJ also provides API for IR. You can populate the IR by running idl2ir on the IDL file. VBJ comes with a proprietary, distributed object location and a directory service called an OSAgent (Smart Agent). Multiple OSAgents running in the same network locate each other and automatically partition the name spaces. Thus, it offers replication and load balancing. When client application invokes the bind method on an object or when object implementations register their objects, it sends a UDP broadcast and the first osagent to respond is used. Once the OSAgent has been located, point to point UDP connection is established. If one osagent is failed, the other OSAgent takes over the client and object tasks. Thus, VBJ offers a fault tolerance. Smart Agents can use VBJ's Object Activation Daemon(OAD) to launch instances of a server process on demand. You can register or unregister the implementations using OAD's API or command-line utilities. VBJ also provides an interface to osagent called a Location Service. It is an extension of the CORBA specification to provide general purpose facilities for locating object instances. The location services query the smart agents about the instances. This information can be useful for load-balancing. This service is available via the Agent API. The API has two parts: one for query related and another for registering and de-registering the triggers. These triggers are, in fact, notifications by which clients of the Location Service can be notified of the changes of the availability of instances. Typically you can use it as a callback mechanism.
Putting CORBA clients on Internet
The major market of Java is Internet applications. There are two hurdles for making CORBA applets available on the Web. One is the security restrictions from the browser. Browsers restrict applets to connect only to the domain from which they have been downloaded. Another problem comes from firewall restrictions. Firewalls deny the services, like IIOP, that are not explicitly allowed. VBJ resolves these problems with an approach called HTTP tunneling. In this mechanism, IIOP calls are wrapped in an HTTP envelope for passing through the firewalls. Then it sends all the requests to a daemon (Gatekeeper). The daemon then forwards the request to the host nominated in the object reference. VBJ's Gatekeeper runs on a Web server and enables the client program to make calls to objects that do not reside on the Web server and to receive callbacks. One of the advantages of this approach is that you can still use the same firewall that you are currently using.
In CORBA, the objects are identified by Interoperable Object References (IOR). IORs are generally clumsy. VBJ provides URL naming Service-Interface which allows you to associate a URL with an object's IOR (Interoperable Object Reference). Unlike the gatekeeper, URL naming enables you to associate with a transient object's IOR and connect to it. It also skips smart agents for locating the objects. Thus, you can locate the object by URL like: http://myhost:15000/URLNaming/myObject.ior
Comments on Compliance,
and Evaluation of CORBA
- Security: VBJ supports IIOP over Secure Socket Layer (SSL) protocol. The VBJ's SSL pack provides privacy (through encryption), integrity (through checksum), and client-server authentication.
- Scalability and Performance: VisiBroker supports scalability and performance through multi-threading. You can select either thread per client session model or thread pool model. Controlled Connection Management multiplexes and recycles all requests from the same client.
- Improved ORB API: The interceptor between, requests and enables developers to monitor or modify the request. You can use smart stub API to improve the performance by caching frequently used and non-volatile results and values.
- VisiBroker Manager: This is a GUI product that is available separately but is very useful. It provides Interface Repository Browser, Location Service Browser, and Implementation Repository Browser. It also provides a performance monitor!
I did some testing related to performance and fault tolerance but I was far from calling it a benchmark. In fact I could not find any proper bench marking suites for the same. I came across some work in that direction done by some research scholars. OMG does not certify the compliance on its own. The Open Group(TOG) has announced VSOrb, test suites, to certify the functionality conformance with CORBA specifications. DSTC's project, CORBAnet, proves interoperability between different ORBs including Visigenic's one. Patricia Carando gave some tips for the selecting and evaluating the ORBs. Visigenic satisfies almost all of them.
Now that I like the core product, when I want to make something meaningful out of it, I need more and more services. Currently, Visigenic provides the naming and event services. Visigenic does not provide services like Integrated Transaction Support, connectivity to ODBMS/RDBMS, messaging services or Trader services. Fortunately, Visigenic is working on these and they have released a plan for making them available. I would also like to see more support for tools for application development from Visigenic, either directly or through third parties. Currently Visigenic supports Java and C++ language binding. Considering the legacy systems, it would be great if Visigenic also could provide bindings for languages like smalltalk, Cobol, and Visual Basic. Probably some third parties can take up the work. Now, last but not least! The main competitor to CORBA is a COM model proposed by Microsoft. Visigenic currently supports client side support to OLE/ActiveX. In the future, we may also need a server side bridge to OLE.
VisiBroker 3.0 is a major breakthrough for Visigenic. The new functionality galvanized the ORB. The scalable, fault tolerant architecture of ORB, along with its ease in using Java development support and GUI based management tool, makes it a good choice. The existing service, along with the implementation plan of various other services, makes the picture more promising. Visigenic's technology has been licensed by Netscape, Oracle, Novell, Sybase, Borland and others. This makes the technology more suitable from a business perspective.
Editor's Note: Visibroker 3.0 also includes a java2iiop compiler and a java2Idle compiler.
Smalltalk is available now; the server-side bridge to OLE is in development.
About the Author
Khanderao Kand is a Principal Consultant at TekEdge Corp., CA. As a consultant he has been working with companies like Informix and Cisco. He can be reached at [email protected]