45720792-corba

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Introduction to CORBA • o o o o o • Distributed Applications Data are Distributed Computation is Distributed Users are Distributed Fundamental Realities of Distributed Systems Distributed Object Systems What is CORBA? o The OMG o CORBA Architecture o The ORB o CORBA as a Standard for Distributed Objects o CORBA Services o CORBA Products The Stock Application Some Objects in the Stock Application • o • Implementing a CORBA Client o CORBA Objects are Described by IDL Interfaces o Object References and Requests o IDL Type System  IDL Type Operations  Request Type Checking o IDL to Java Binding o IDL to Java Compiler o Obtaining Object References o The Client's Model of Object Creation Distributed Applications CORBA products provide a framework for the development and execution of distributed applications. But why would one want to develop a distributed application in the first place? As you will see later, distribution introduces a whole new set of difficult issues. However, sometimes there is no choice; some applications by their very nature are distributed across multiple computers because of one or more of the following reasons: • • • The data used by the application are distributed The computation is distributed The users of the application are distributed Data are Distributed Some applications must execute on multiple computers because the data that the application must access exist on multiple computers for administrative and ownership reasons. The owner may permit the data to be accessed remotely but not stored locally. Or perhaps the data cannot be co-located and must exist on multiple heterogeneous systems for historical reasons. Computation is Distributed Some applications execute on multiple computers in order to take advantage of multiple processors computing in parallel to solve some problem. Other applications may execute on multiple computers in order to take advantage of some unique feature of a particular system. Distributed applications can take advantage of the scalability and heterogeneity of the distributed system. Users are Distributed Some applications execute on multiple computers because users of the application communicate and interact with each other via the application. Each user executes a piece of the distributed application on his or her computer, and shared objects, typically execute on one or more servers. A typical architecture for this kind of application is illustrated below. Prior to designing a distributed application, it is essential to understand some of the fundamental realities of the distributed system on which it will execute. Fundamental Realities of Distributed Systems Distributed application developers must address a number of issues that can be taken for granted in a local program where all logic executes in the same operating system process. The following table summarizes some of the basic differences between objects that are co-located in the same process, and objects that interact across process or machine boundaries. The communication between objects in the same process is orders of magnitude faster than communication between objects on different machines. The implication of this is that you should avoid designing distributed applications in which two or more distributed objects have very tight interactions. If they do have tight interactions, they should be co-located. When two objects are co-located, they fail together; if the process in which they execute fails, both objects fail. The designer of the objects need not be concerned with the behavior of the application if one of the objects is available and the other one is not. But if two objects are distributed across process boundaries, the objects can fail independently. In this case, the designer of the objects must be concerned with each of the object's behavior in the event the other object has failed. Similarly, in a distributed system the network can partition and both objects can execute independently assuming the other has failed. The default mode for most local programs is to operate with a single thread of control. Single threaded programming is easy. Objects are accessed in a well-defined sequential order according to the program's algorithms, and you need not be concerned with concurrent access. If you decide to introduce multiple threads of control within a local program, you must consider the possible orderings of access to objects and use synchronization mechanisms to control concurrent access to shared objects. But at least you have a choice of introducing multiple threads of control. In a distributed application, there are necessarily multiple threads of control. Each distributed object is operating in a different thread of control. A distributed object may have multiple concurrent clients. As the developer of the object and the developer of the clients, you must consider this concurrent access to objects and use the necessary synchronization mechanisms. When two objects are co-located in the same process, you need not be concerned about security. When the objects are on different machines, you need to use security mechanisms to authenticate the identity of the other object. Distributed Object Systems Distributed object systems are distributed systems in which all entities are modeled as objects. Distributed object systems are a popular paradigm for object-oriented distributed applications. Since the application is modeled as a set of cooperating objects, it maps very naturally to the services of the distributed system. In spite of the natural mapping from object-oriented modeling to distributed object systems, do not forget the realities of distributed systems described above. Process boundaries really do matter and they will impact your design. That said, the next section of this course discusses the CORBA standard for distributed object systems. What is CORBA? CORBA, or Common Object Request Broker Architecture, is a standard architecture for distributed object systems. It allows a distributed, heterogeneous collection of objects to interoperate. The OMG The Object Management Group (OMG) is responsible for defining CORBA. The OMG comprises over 700 companies and organizations, including almost all the major vendors and developers of distributed object technology, including platform, database, and application vendors as well as software tool and corporate developers. CORBA Architecture CORBA defines an architecture for distributed objects. The basic CORBA paradigm is that of a request for services of a distributed object. Everything else defined by the OMG is in terms of this basic paradigm. The services that an object provides are given by its interface. Interfaces are defined in OMG's Interface Definition Language (IDL). Distributed objects are identified by object references, which are typed by IDL interfaces. The figure below graphically depicts a request. A client holds an object reference to a distributed object. The object reference is typed by an interface. In the figure below the object reference is typed by the Rabbit interface. The Object Request Broker, or ORB, delivers the request to the object and returns any results to the client. In the figure, a jump request returns an object reference typed by the AnotherObject interface. The ORB The ORB is the distributed service that implements the request to the remote object. It locates the remote object on the network, communicates the request to the object, waits for the results and when available communicates those results back to the client. The ORB implements location transparency. Exactly the same request mechanism is used by the client and the CORBA object regardless of where the object is located. It might be in the same process with the client, down the hall or across the planet. The client cannot tell the difference. The ORB implements programming language independence for the request. The client issuing the request can be written in a different programming language from the implementation of the CORBA object. The ORB does the necessary translation between programming languages. Language bindings are defined for all popular programming languages. CORBA as a Standard for Distributed Objects One of the goals of the CORBA specification is that clients and object implementations are portable. The CORBA specification defines an application programmer's interface (API) for clients of a distributed object as well as an API for the implementation of a CORBA object. This means that code written for one vendor's CORBA product could, with a minimum of effort, be rewritten to work with a different vendor's product. However, the reality of CORBA products on the market today is that CORBA clients are portable but object implementations need some rework to port from one CORBA product to another. CORBA 2.0 added interoperability as a goal in the specification. In particular, CORBA 2.0 defines a network protocol, called IIOP (Internet Inter-ORB Protocol), that allows clients using a CORBA product from any vendor to communicate with objects using a CORBA product from any other vendor. IIOP works across the Internet, or more precisely, across any TCP/IP implementation. Interoperability is more important in a distributed system than portability. IIOP is used in other systems that do not even attempt to provide the CORBA API. In particular, IIOP is used as the transport protocol for a version of Java RMI (so called "RMI over IIOP"). Since EJB is defined in terms of RMI, it too can use IIOP. Various application servers available on the market use IIOP but do not expose the entire CORBA API. Because they all use IIOP, programs written to these different API's can interoperate with each other and with programs written to the CORBA API. CORBA Services Another important part of the CORBA standard is the definition of a set of distributed services to support the integration and interoperation of distributed objects. As depicted in the graphic below, the services, known as CORBA Services or COS, are defined on top of the ORB. That is, they are defined as standard CORBA objects with IDL interfaces, sometimes referred to as "Object Services." There are several CORBA services. The popular ones are described in detail in another module of this course. Below is a brief description of each: Service Object life cycle Naming Events Relationships Externalization Transactions Description Defines how CORBA objects are created, removed, moved, and copied Defines how CORBA objects can have friendly symbolic names Decouples the communication between distributed objects Provides arbitrary typed n-ary relationships between CORBA objects Coordinates the transformation of CORBA objects to and from external media Coordinates atomic access to CORBA objects Concurrency Control Provides a locking service for CORBA objects in order to ensure serializable access Property Trader Query CORBA Products Supports the association of name-value pairs with CORBA objects Supports the finding of CORBA objects based on properties describing the service offered by the object Supports queries on objects CORBA is a specification; it is a guide for implementing products. Several vendors provide CORBA products for various programming languages. The CORBA products that support the Java programming language include: ORB The Java 2 ORB VisiBroker for Java Description The Java 2 ORB comes with Sun's Java 2 SDK. It is missing several features. A popular Java ORB from Inprise Corporation. VisiBroker is also embedded in other products. For example, it is the ORB that is embedded in the Netscape Communicator browser. A popular Java ORB from Iona Technologies. A popular application server with an ORB from IBM. Netscape browsers have a version of VisiBroker embedded in them. Applets can issue request on CORBA objects without downloading ORB classes into the browser. They are already there. OrbixWeb WebSphere Netscape Communicator Various free or shareware CORBA implementations for various languages are ORBs available for download on the web from various sources. Providing detailed information about all of these products is beyond the scope of this introductory course. This course will just use examples from both Sun's Java 2 ORB and Inprise's VisiBroker 3.x for Java products. Implementing a CORBA Client This section covers what you need to know to use CORBA objects from the Java programming language. It examines OMG IDL interfaces, the Java programming language binding for IDL interfaces, object references, and requests, how to obtain object references, and how, as a client, to create distributed objects. After reading this section and completing the exercises, you should be able to write a client using the Java programming language. Again, the stock example is used to illustrate the client's model of CORBA. CORBA Objects are Described by IDL Interfaces The OMG Interface Definition Language IDL supports the specification of object interfaces. An object interface indicates the operations the object supports, but not how they are implemented. That is, in IDL there is no way to declare object state and algorithms. The implementation of a CORBA object is provided in a standard programming language, such as the Java programming language or C++. An interface specifies the contract between code using the object and the code implementing the object. Clients only depend on the interface. IDL interfaces are programming language neutral. IDL defines language bindings for many different programming languages. This allows an object implementor to choose the appropriate programming language for the object. Similarly, it allows the developer of the client to choose the appropriate and possibly different programming language for the client. Currently, the OMG has standardized on language bindings for the C, C++, Java, Ada, COBOL, Smalltalk, Objective C, and Lisp programming languages. So by using OMG IDL, the following can be described without regards to any particular programming language: • • • • Modularized object interfaces Operations and attributes that an object supports Exceptions raised by an operation Data types of an operation return value, its parameters, and an object's attributes The IDL data types are: • • • • Basic data types (long, short, string, float...) Constructed data types (struct, union, enum, sequence) Typed object references The any type, a dynamically typed value Object References and Requests Clients issue a request on a CORBA object using an object reference. An object reference identifies the distributed object that will receive the request. Here's a Java programming language code fragment that obtains a Stock object reference and then it uses it to obtain the current price of the stock. Note that the code fragment does not directly use CORBA types; instead it uses the Java types that have been produced by the IDL to Java compiler. Stock theStock = ... try { Quote current_quote = theStock.get_quote (); } catch (Throwable e) { } Object references can be passed around the distributed object system, i.e. as parameters to operations and returned as results of requests. For example, notice that the StockFactory interface defines a create() operation that returns an instance of a Stock. Here's a Java client code fragment that issues a request on the factory object and receives the resulting stock object reference. Note that issuing a request on a CORBA object is not all that different from issuing a request on a Java object in a local program. The main difference is that the CORBA objects can be anywhere. The CORBA system provides location transparency, which implies that the client cannot tell if the request is to an object in the same process, on the same machine, down the hall, or across the planet. Another difference from a local Java object is that the life time of the CORBA object is not tied to the process in which the client executes, nor to the process in which the CORBA object executes. Object references persist; they can be saved as a string and recreated from a string. The following Java code converts the Stock object reference to a string: String stockString = orb.object_to_string(theStock); The string can be stored or communicated outside of the distributed object system. Any client can convert the string back to an object reference and issue a request on the distributed object. This Java code converts the string back to a Stock object reference: org.omg.CORBA.Object obj = orb.string_to_object(stockString); Stock theStock = StockHelper.narrow(obj); Note that the resulting type of the string_to_object() method is Object, not Stock. The second line narrows the type of the object reference from Object to Stock. IDL supports a hierarchy of interfaces; the narrow() method call is an operation on the hierarchy. IDL Type System IDL interfaces can be defined in terms of other IDL interfaces. You previously saw a Stock interface that represents the basic behavior of a stock object. Request Type Checking The IDL compiler for Java programming language generates client-side stubs, which represent the CORBA object locally in the Java programming language. The generated code also represents in the Java programming language all of the IDL interfaces and data types used to issue requests. The client code thus depends on the generated Java code. As you previously saw, passing an object reference typed by the Stock interface to the event manager would be illegal because the Stock interface does not inherit the Reporting interface. The Java compiler, not the IDL compiler, would catch this error at compile time. CORBA System Exceptions can contain "minor codes" which may provide additional information about what went wrong. Unfortunately, these are vendor-specific, so you need to tailor your error recovery routines to the ORB you're using. User Exceptions are generated if something goes wrong inside the execution of the remote method itself. These are declared inside the IDL definition for the object, and are automatically generated by the idltojava compiler. In the stock example, Unknown is a user exception. Since User Exceptions are subclasses of java.lang.Exception, the compiler will complain if you forget to trap them (and this is as it should be). Object Implementations NOTE: the previous section discussed the client's view of CORBA, that is, how a Java client issues a request on a CORBA object. The client's view is standard across most CORBA products. Basically, the standard worked and there are only minor differences. Unfortunately, the same is not the case for the implementation view of CORBA. As such, some of the details given here might not match a particular CORBA product. Notes on different CORBA products appear as appendices. This section describes what you need to know to implement a simple CORBA object in the Java programming language. It examines the Java server-side language binding for IDL, implementing objects and servers, implementation packaging issues, and CORBA object adaptors. After completing this section, you should be able to write a simple CORBA object and server in the Java programming language. Again, the stock example is used to illustrate the implementation model of CORBA. CORBA object implementations are completely invisible to their clients. A client can only depend on the IDL interface. In the Java programming language, or C++, this is not the case. The user of an object declares variables by a class name; doing so makes the code depend on much more than just the interface. The client depends on the object implementation programming language, the name of the class, the implementation class hierarchy, and, in C++, even the object layout. The complete encapsulation for CORBA objects means the object implementor has much more freedom. Object implementations can be provided in a number of supported programming languages. This is not necessarily the same one the clients are written in. (Of course, here everything is in the Java programming language, but CORBA does notrequire this.) The same interface can be implemented in multiple ways. There is no limit. In the stock example, the following are possible implementations of the Stock interface: A stock implementation class written in the Java programming language that obtains values from a commercial feed • A stock implementation class written in C++ that accesses a database on the Internet • A stock implementation written in Smalltalk that guesses stock prices • Implementation Type Checking Just as type checking is done at the client for the request to a distributed object, type checking is also done for the object implementation. The IDL compiler for the Java programming language generates object skeletons and Java code to represent all of the IDL interfaces and data types used in the interface definition. The implementation code thus depends on the generated Java code. If there are any type errors in the object implementation, the Java compiler, not the IDL compiler, catches the errors at compile time. Thus, in the example, suppose the developer erroneously implemented the get_quote() operation to return a double instead of the structure that is declared in the IDL: Quote StockImpl.get_quote() { double price = ...; return price; } The Java compiler would detect this error at compile time. Implementing a Server Using the Java 2 ORB You previously saw how to provide an implementation of a CORBA object in the Java programming language. The remaining task is to define a server that when run makes the services of its objects available to clients. A server that will run with the Java 2 ORB needs to do the following: • • • • • Define a main method Initialize the ORB Instantiate at least one object Connect each object to the orb Wait for requests The server must instantiate at least one object since objects are the only way to offer services in CORBA systems. Here's an implementation of the stock objects server. This code depends on the Java 2 ORB. Notice that the server does a new on the StockImpl class implementing the Stock interface and then passes it to the ORB using the connect() call, indicating that the object is ready to accept requests. Finally, the server waits for requests. Implementing a Server Using VisiBroker 3.x You previously saw how to provide a server using the Java 2 ORB. If you are using Inprise's VisiBroker 3.x for Java ORB you need to do the following: • • • • • Define a main method Initialize the ORB and the BOA (basic object adapter) Instantiate at least one object Let the BOA know that the object is ready to provide service Let the BOA know that the server is ready The server must instantiate at least one object since objects are the only way to offer services in CORBA systems. Here's an implementation of the stock objects server. This code depends on VisiBroker 3.x:. Notice that the server does a new on the StockImpl class implementing the Stock interface and then passes it to the BOA, indicating that the object is ready to accept requests. Finally, the server calls the BOA to indicate that it is ready. At this point, the implementation will be called when requests arrive. Differences Between Server Implementations The following summarizes the differences between implementing a transient CORBA server using the Java 2 ORB and implementing a transient server using Inprise's VisiBroker 3.x: These are the only differences for transient object servers. There are further API differences of CORBA products due to persistence and automatic activation of servers. Packaging Object Implementations As illustrated above, you should separate the implementations of your objects from the implementation of the server. This allows you to mix and match object implementations in a server. The object implementation does not depend on the server. The server, of course depends on the object implementations that it contains. Another advantage of carefully isolating object implementation code from server code is portability. Most of the product-specific code exists in the server, not in the object implementation. A good strategy is to package an object implementation with its generated stubs and skeletons as a JavaBean component. This allows the implementation to be manipulated by JavaBean design tools. Object Adapters The CORBA specification defines the concept of an object adapter. An object adapter is a framework for implementing CORBA objects. It provides an API that object implementations use for various low level services. According to the CORBA specification, an object adapter is responsible for the following functions: • • • • • • Generation and interpretation of object references Method invocation Security of interactions Object and implementation activation and deactivation Mapping object references to the corresponding object implementations Registration of implementations The architecture supports the definition of many kinds of object adapters. The specification includes the definition of the basic object adapter (BOA). In the previous section, you saw some server code that uses the services of VisiBroker's implementation of the BOA. The BOA has been implemented in various CORBA products. Unfortunately, since the specification of the BOA was not complete, the various BOA implementations differ in some significant ways. This has compromised server portability. To address this shortcoming, an entirely new object adapter was added, the portable object adapter (POA). Unfortunately, the POA is not yet supported in many products. In any event, the BOA and the POA are described here.