CORBA Programming Reference

Joint Client/Servers

This chapter describes programming requirements for CORBA joint client/servers and the C++ BEAWrapper Callbacks API.

Note: The BEA Tuxedo CORBA Java client and BEA Tuxedo CORBA Java client ORB were deprecated in Tuxedo 8.1 and are no longer supported in Tuxedo 9.x. All BEA Tuxedo CORBA Java client and BEA Tuxedo CORBA Java client ORB text references, associated code samples, etc. should only be used:

to help implement/run third party Java ORB libraries, and
for programmer reference only.

Technical support for third party CORBA Java ORBs should be provided by their respective vendors. BEA Tuxedo does not provide any technical support or documentation for third party CORBA Java ORBs.

This topic includes the following sections:

Introduction. This section describes:

C++ BEAWrapper Callbacks Interface API

Introduction

For either a BEA Tuxedo CORBA client or joint client/server (that is, a client that can receive and process object invocations), the programmer writes the client main(). The main() uses BEA Tuxedo CORBA environmental objects to establish connections, set up security, and start transactions.

BEA Tuxedo clients invoke operations on objects. In the case of DII, client code creates the DII Request object and then invokes one of two operations on the DII Request. In the case of static invocation, client code performs the invocation by performing what looks like an ordinary invocation (which ends up calling code in the generated client stub). Additionally, the client programmer uses ORB interfaces defined by OMG, and BEA Tuxedo CORBA environmental objects that are supplied with the BEA Tuxedo software, to perform functions unique to BEA Tuxedo.

For BEA Tuxedo joint client/server applications, the client code must be structured so that it can act as a server for callback BEA Tuxedo objects. Such clients do not use the TP Framework and are not subject to BEA Tuxedo system administration. Besides the programming implications, this means that CORBA joint client/servers do not have the same scalability and reliability as BEA Tuxedo CORBA servers, nor do they have the state management and transaction behavior available in the TP Framework. If a user wants to have those characteristics, the application must be structured in such a way that the object implementations are in a BEA Tuxedo CORBA server, rather than in a client.

The following sections describe the mechanisms you use to add callback support to a BEA Tuxedo client. In some cases, the mechanisms are contrasted with the BEA Tuxedo server mechanisms that use the TP Framework.

Main Program and Server Initialization

In a BEA Tuxedo server, you use the buildobjserver command to create the main program for the C++ server. (Java servers are not supported in release 8.0 and later of BEA Tuxedo.) Server main program takes care of all BEA Tuxedo- and CORBA-related initialization of the server functions. However, since you implement the Server object, you have an opportunity to customize the way in which the server application is initialized and shut down. The server main program automatically invokes methods on the Server object at the appropriate times.

In contrast, for a BEA Tuxedo CORBA joint client/server (as for a BEA Tuxedo CORBA client), you create the main program and are responsible for all initialization. You do not need to provide a Server object because you have complete control over the main program and you can provide initialization and shutdown code in any way that is convenient.

The specific initialization needed for a joint client/server is discussed in the section Servants.

Servants

Servants (method code) for joint client/servers are very similar to servants for servers. All business logic is written the same way. The differences result from not using the TP Framework. Therefore, the main difference is that you use CORBA functions directly instead of indirectly through the TP Framework.

The Server interface is used in BEA Tuxedo CORBA servers to allow the TP Framework to ask the user to create a servant for an object when the ORB receives a request for that object. However, in joint client/servers, the user program is responsible for creating a servant before any requests arrive; thus, the Server interface is not needed. Typically, the program creates a servant and then activates the object (using the servant and an ObjectId; the ObjectId is possibly system generated) before handing a reference to the object. Such an object might be used to handle callbacks. Thus, the servant already exists and the object is activated before a request for the object arrives.

For C++ joint client/servers, instead of invoking the TP interface to perform certain operations, client servants directly invoke the ORB and POA (which is what the TP interface does internally). Alternately, since much of the interaction with the ORB and POA is the same for all applications, for ease of use, the client library provides a convenience wrapper object that does the same things, using a single operation. For a discussion of how to use the convenience wrapper object, see Callback Object Models Supported and Preparing Callback Objects Using BEAWrapper Callbacks.

Servant Inheritance from Skeletons

In a client that supports callbacks, as well as in a server, you write a implementation class that inherits from the same skeleton class name generated by the IDL compiler (the idl command).

C++ Example of Inheritance from Skeletons

The following is a C++ example, given the IDL:

interface Hospital{  };

The skeleton generated by the idl command contains a "skeleton" class, POA_Hospital, that the user-written class inherits from, as in:

class Hospital_i : public POA_Hospital { ... };

In a server, the skeleton class inherits from the TP Framework class Tobj_ServantBase, which in turn inherits from the predefined PortableServer::ServantBase.

The inheritance tree for a callback object implementation in a joint client/server is different than that in a server. The skeleton class does not inherit from the TP Framework class Tobj_ServantBase, but instead inherits directly from PortableServer::ServantBase. This behavior is achieved by specifying the -P option in the idl command.

Not having the Tobj_ServantBase class in the inheritance tree for a servant means that the servant does not have activate_object and deactivate_object methods. In a server, these methods are called by the TP Framework to dynamically initialize and save a servant's state before invoking a method on the servant. For a client that supports callbacks, you must write code that explicitly creates a servant and initializes a servant's state.

Callback Object Models Supported

BEA Tuxedo CORBA supports four kinds of callback objects and provides wrappers for the three that are most common. These objects correspond to three combinations of POA policies. The POA policies control both the types of objects and the types of object references that are possible.

The POA policies that are applicable are:

The LifeSpanPolicy, which controls how long an object reference is valid.
The IdAssignmentPolicy, which controls who assigns the ObjectId—the user or the system.

These objects are explained primarily in terms of their behavioral characteristics rather than in details about how the ORB and the POA handle them. Those details are discussed in the next sections, using either direct ORB and POA calls (which requires a little extra knowledge of CORBA servers) or using the BEAWrapper Callbacks interface, which hides the ORB and POA calls (for users who do not care about the details).

Transient/SystemId—object references are valid only for the life of the client process. The ObjectId is not assigned by the client application, but is a unique value assigned by the system. This type of object is useful for invocations that a client wants to receive only until the client terminates. (The corresponding POA LifeSpanPolicy value is TRANSIENT and the IdAssignmentPolicy is SYSTEM_ID.)
Persistent/SystemId—object references are valid across multiple activations. The ObjectId is not assigned by the client application, but is a unique value assigned by the system. This type of object and object reference is useful when the client goes up and down over a period of time. When the client is up, it can receive callback objects on that particular object reference.

Typically, the client will create the object reference once, save it in its own permanent storage area, and reactivate the servant for that object every time it comes up. If used with a BEA Tuxedo CORBA Notification Service application, for example, these are callbacks that correspond to the concept of a persistent subscription; that is, the Notification Service remembers the callback reference and delivers events any time the client is up and declares that it is again ready to receive events. This allows notification service subscriptions to survive client failures or offline-time. (The corresponding POA policy values are PERSISTENT and SYSTEM_ID.)
Persistent/UserId—this is the same as Persistent/SystemId with the exception that the ObjectId has to be assigned by the client application. Such an ObjectId might be, for example, a database key meaningful only to the client. (The corresponding POA policy values are PERSISTENT and USER_ID.)

Notes: The Transient/UserId policy combination is not considered particularly important. It is possible for users to provide for themselves by using the POA in a manner analogous to either of the persistent cases, but the BEA Tuxedo wrappers do not provide special help to do so.

For BEA Tuxedo CORBA native joint client/servers, neither of the Persistent policies is supported, only the Transient policy.

Configuring Servers to Call Remote Joint Client/Server Objects

In order for a BEA Tuxedo server to call remote joint client/server objects, that is, joint client/server objects located outside the BEA Tuxedo domain, the server must be configured to enable outbound IIOP. This capability is enabled by specifying the -O (uppercase letter O) option in the IIOP Server Listener (ISL) server command. Setting the -O option enables outbound invokes (outbound IIOP) on joint client/server objects that are not connected to an IIOP Listener Handler (ISH).

You set ISL command options in the SERVERS section of the server's UBBCONFIG file. Because support for outbound IIOP requires a small amount of extra resources, the default is outbound IIOP disabled. For more information, see "Using the ISL Command to Configure Outbound IIOP" in Setting Up a BEA Tuxedo Application and "ISL(1)" in the BEA Tuxedo Command Reference.

Preparing Callback Objects Using CORBA (C++ Joint Client/Servers Only)

To set up BEA Tuxedo C++ callback objects using CORBA, the client must do the following:

Establish a connection with a POA with the appropriate policies for the callback object model. (This can be the root POA, available by default, or it may require creating a new POA.)

Create a servant (that is, an instance of the C++ implementation class for the interface).

Inform the POA that the servant is ready to accept requests on the callback object. Technically, this means the client activates the object in the POA (that is, puts the servant and the ObjectId into the POA's Active Object Map).

Tell the POA to start accepting requests from the network (that is, activate the POA itself).

Create an object reference for the callback object.

Give out the object reference. This usually happens by making an invocation on another object with the callback object reference as a parameter (that is, the parameter is a callback object). That other object will then invoke the callback object (perform a callback invocation) at some later time.

Assuming that the client already has obtained a reference to the ORB, performing this task takes four interactions with the ORB and the POA. It might look like the model show in Listing 11-1. In this model, only the Root POA is needed.

Listing 11-1 Transient/SystemId Model

// Create a servant for the callback Object 
Catcher_i* my_catcher_i = new Catcher_i();

// Get root POA reference and activate the POA
1   CORBA::Object_var oref =
              orb->resolve_initial_references("RootPOA");
2   PortableServer::POA_var root_poa =
              PortableServer::POA::_narrow(oref);
3   root_poa -> the_POAManager() -> activate();
4   PortableServer::objectId_var temp_Oid =
              root_poa ->activate_object ( my_catcher_i );
5   oref = root_poa->create_reference_with_id(
                                  temp_Oid, _tc_Catcher->id() );
6   Catcher_var my_catcher_ref = Catcher::_narrow( oref );

To use the Persistent/UserId model, there are some additional steps required when creating a POA. Further, the ObjectId is specified by the client, and this requires more steps. It might look like the model shown in Listing 11-2.

Listing 11-2 Persistent/UserId Model

    Catcher_i* my_catcher_i = new Catcher_i();
    const char* oid_str = "783";
1   PortableServer::objectId_var oid =
             PortableServer::string_to_objectId(oid_str);
// Find root POA 
2   CORBA::Object_var oref =
             orb->resolve_initial_references("RootPOA");
3   PortableServer::POA_var root_poa =
             PortableServer::POA::_narrow(oref);
// Create and activate a Persistent/UserId POA
4   CORBA::PolicyList policies(2); 
5   policies.length(2);
6   policies[0] = root_poa->create_lifespan_policy(
                                    PortableServer::PERSISTENT);
7   policies[1] = root_poa->create_id_assignment_policy(
                                     PortableServer::USER_ID );
8   PortableServer::POA_var my_poa_ref =
             root_poa->create_POA(
                   "my_poa_ref", root_poa->the_POAManager(), policies);
9   root_poa->the_POAmanager()->activate();
// Create object reference for callback Object
10 oref =  my_poa_ref -> create_reference_with_id( 
                                           oid, _tc_Catcher->id());
11 Catcher_var my_catcher_ref = Catcher::_narrow( oref );
// activate object
12 my_poa_ref -> activate_object_with_id( oid, my_catcher_i );
// Make the call passing the callback ref
    foo -> register_callback ( my_catcher_ref );

All the interfaces and operations described here are standard CORBA interfaces and operations.

Preparing Callback Objects Using BEAWrapper Callbacks

You can use the BEAWrapper callbacks API with to write either C++ joint client/servers.

Using BEAWrapper Callbacks With C++

Because the code required for callback objects is nearly identical for every client that supports callbacks, you may find it convenient to use the BEAWrappers provided in the library provided for joint client/servers.

The BEAWrappers are described in IDL, as shown in Listing 11-3.

Listing 11-3 BEAWrapper IDL

// File: BEAWrapper
#ifndef _BEA_WRAPPER _IDL_
#define _BEA_WRAPPER _IDL_
#include <orb.idl>
#include <PortableServer.idll>

#pragma prefix "beasys.com"

module BEAWrapper {
   interface Callbacks 
   {
       exception ServantAlreadyActive{ };
       exception ObjectAlreadyActive { };
       exception NotInRequest{ };

       // set up transient callback Object 
       // -- prepare POA, activate object, return objref
          Object start_transient( 
                        in PortableServer::Servant      Servant, 
                        in CORBA::RepositoryId          rep_id)
           raises (ServantAlreadyActive);

       // set up persistent/systemid callback Object 
       Object start_persistent_systemid( 
               in PortableServer::Servant      servant, 
               in CORBA::Repository            rep_id,
               out string                      stroid) 
          raises (ServantAlreadyActive);

       // reinstate set up for persistent/systemid 
       // callback object 
       Object restart_persistent_systemid( 
               in PortableServer::Servant     servant, 
               in CORBA::RepositoryId         rep_id,
               in string                      stroid)
          raises (ServantAlreadyActive, ObjectAlreadyActive);

       // set up persistent/userid callback Object
       Object start_persistent_userid( 
                in PortableServer::Servant        servant,    
                in CORBA::RepositoryId            rep_id,
                in string                       stroid)
          raises (ServantAlreadyActive, ObjectAlreadyActive);

       // stop servicing a particular callback Object
       // with the given servant
       void stop_object( in PortableServer::Servant servant);

       //Stop all callback Object processing
       void stop_all_objects();

      // get oid string for the current request
      string get_string_oid() raises (NotInRequest); 
   };
}
#endif /* _BEA_WRAPPER _IDL_ */

The BEAwrappers are described in C++ as shown in Listing 11-4.

Listing 11-4 C++ Declarations (in beawrapper.h)

#ifndef _BEAWRAPPER_H_
#define _BEAWRAPPER_H_

#include <PortableServer.h>
class BEAWrapper{
class Callbacks{
      public:
            Callbacks (CORBA::ORB_ptr init_orb);

            CORBA::Object_ptr start_transient ( 
                        PortableServer::Servant servant,
                         const char *     rep_id);

            CORBA::Object_ptr start_persistent_systemid (
                          PortableServer::Servant servant,
                          const char *     rep_id,
                           char * & stroid);

            CORBA::Object_ptr restart_persistent_systemid (
                           PortableServer::Servant servant,
                           const char *     rep_id,
                            const char *     stroid);

            CORBA::Object_ptr start_persistent_userid (
                           PortableServer::Servant servant,
                           const char *     rep_id,
                            const char *     stroid);

              void stop_object(PortableServer::Servant servant);

              char* get_string_oid ();

              void stop_all_objects();

             ~Callbacks();
        private:

              static CORBA::ORB_var orb_ptr;

              static PortableServer::POA_var root_poa;
              static PortableServer::POA_var trasys_poa;
              static PortableServer::POA_var persys_poa;
              static PortableServer::POA_var peruser_poa;
     };
};
#endif // _BEAWRAPPER_H_

C++ BEAWrapper Callbacks Interface API

This C++ BEAWrapper Callbacks interface API is described in the following sections.