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Joint Client/Servers

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

For either a WLE 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 WLE environmental objects to establish connections, set up security, and start transactions.

WLE 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 C++ invocation (which ends up calling code in the generated client stub). Additionally, the client programmer uses ORB interfaces defined by OMG, and WLE environmental objects that are supplied with the WLE software, to perform functions unique to WLE.

For WLE joint client/servers, the client code must be structured so that it can act as a server for callback WLE objects. Such clients do not use the TP Framework and are not subject to WLE system administration. Besides the programming implications, this means that joint client/servers do not have the same scalability and reliability as WLE 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 WLE server, rather than in a client.

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

Main Program and Server Initialization

In a WLE server, you use the buildobjserver command to create the main program for the server. That main program takes care of all WLE- and CORBA-related initialization of the server functions. The server main program allows the user to take part in server initialization and shutdown by making invocations on a user-written C++ object, the Server class.

In contrast, for a WLE joint client/server (as for a WLE 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 below.

Servants

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

The Server interface is used in WLE 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. In WLE 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.

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 WLE 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" on page 9-4 and "Preparing Callback Objects Using BEAWrapper Callbacks" on page 9-7.

Servant Inheritance from Skeletons

In a WLE client that supports callbacks, as well as in a WLE server, you write a C++ implementation class that inherits from the same skeleton class name generated by the IDL compiler (the idl command). For 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 WLE 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 WLE 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 WLE 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 WLE client that supports callbacks, you must write code that explicitly creates a servant and initializes a servant's state.

Callback Object Models Supported

WLE software 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. (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.)

Note: 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 WLE wrappers do not provide special help to do so.
Note: For WLE native joint client/servers, neither of the Persistent policies is supported, only the Transient policy.

Preparing Callback Objects Using CORBA

To set up WLE 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 WLE 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 WLE 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 following for the Transient/SystemId model. In this model, only the Root POA is needed.

// 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 following.

    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

Since 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 follows.

Note: These same wrappers are designed to be used for the WebLogic Enterprise V4.2 (Java) software, where a POA is not yet available, although aspects related to POAs do exist (notably, PortableServer.Servant). For a discussion of these for the Java software, see Java Programming Reference.

// 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);

       //shutdown 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 follows:

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_

The description of each operation in the BEAWrapper::Callbacks interface follows, in the order declared above.

Callbacks

Synopsis

Returns a reference to the Callbacks interface.
C++ Binding

BEAWrapper::Callbacks( CORBA::ORB_ptr init_orb);
Java Binding
public Callbacks(org.omg.CORBA.Object init_orb);

Argument

init_orb
The ORB to be used for all further operations.

Return Value

A reference to the Callbacks object.
Description

The constructor returns a reference to the Callbacks interface. Only one such object should be created for the process, even if multiple threads are used. Using more than one such object will result in undefined behavior.
Exception

CORBA::IMP_LIMIT
The BEAWrapper::Callbacks class has already be instantiated with an ORB pointer. Only one instance of this class can be used in a process. Users who need additional flexibility should use the POA directly.

start_transient

Synopsis

Activates an object, sets the ORB and the POA to the proper state, and returns an object reference to the activated object.
IDL

Object start_transient( in PortableServer::Servant servant,
in CORBA::RepositoryId rep_id)
raises ( ServantAlreadyActive );

C++ Binding

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

Java Binding

org.omg.CORBA.Object start_transient(
org.omg.PortableServer.Servant servant,
java.lang.String rep_id);

Arguments

servant
An instance of the C++ implementation class for the interface.
rep_id
The repository id of the interface.

Return Value

CORBA::Object_ptr
A reference to the object that was created with the ObjectId generated by the system and the rep_id provided by the user. The object reference will need to be converted to a specific object type by invoking the _narrow() operation defined for the specific object. The caller is responsible for releasing the object when the conversion is done.

Description

This operation performs the following actions:

Activates an object using the Servant supplied to service objects of the type rep_id, using an ObjectId generated by the system.

Sets the ORB and the POA into the state in which they will accept requests on that object.

Returns an object reference to the activated object. The returned object reference will be valid only until the termination of the client or until the callback servant is halted by the user via the stop_object operation; after that, invocations on that object reference are no longer valid and can never be made valid.

Exceptions

ServantAlreadyActive
The servant is already being used for a callback. A servant can be used only for a callback with a single ObjectId. To receive callbacks on objects containing different ObjectIds, you must create different servants and activate them separately. The same servant can be re-used only if a stop_object operation tells the system to stop using the servant for its original ObjectId.
CORBA::BAD_PARAM
The repository ID was a null string or the servant was a null pointer.

start_persistent_systemid

Synopsis

Activates an object, sets the ORB and the POA to the proper state, sets the output parameter stroid, and returns an object reference to the activated object.

IDL

Object start_persistent_systemid(
in PortableServer::Servant servant,
in CORBA::RepositoryId rep_id,
out string stroid)
raises ( ServantAlreadyActive );

C++ Binding

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

Java Binding

org.omg.CORBA.Object start_persistent_systemid(
org.omg.PortableServer.Servant servant,
java.lang.String rep_id,
java.lang.String stroid);

Arguments

servant
An instance of the C++ implementation class for the interface.
rep_id
The repository id of the interface.
stroid
This argument is set by the system and is opaque to the user. The client will use it when it reactivates the object at a later time (using restart_persistent_systemid), most likely after the client process has terminated and restarted.

Return Value

CORBA::Object_ptr
An object reference created with the ObjectId generated by the system and the rep_id provided by the user. The object reference will need to be converted to a specific object type by invoking the _narrow() operation defined for the specific object. The caller is responsible for releasing the object when the conversion is done.

Description

This operation performs the following actions:

Activates an object using the Servant supplied to service objects of the type rep_id, using an ObjectId generated by the system.

Sets the ORB and the POA into the state in which they will accept requests on that object.

Sets the output parameter stroid to the stringified version of an ObjectId assigned by the system.

Returns an object reference to the activated object. The returned object reference will be valid even after termination of the client. That is, if the client terminates, restarts again, and then activates a servant with the same rep_id and for the same ObjectId, the servant will accept requests made on that same object reference. Since the ObjectId was generated by the system, the application has to save that ObjectId.

Exceptions

ServantAlreadyActive
The servant is already being used for a callback. A servant can be used only for a callback with a single ObjectId. To receive callbacks on objects containing different ObjectIds, you must create different servants and activate them separately. The same servant can be re-used only if a stop operation tells the system to stop using the servant for its original ObjectId.
CORBA::BAD_PARAMETER
The repository ID was a null string or the servant was a null pointer.
CORBA::IMP_LIMIT
In addition to other system reasons for this exception, a reason unique to this situation is that the joint client/server was not initialized with a port number; therefore, a persistent object reference cannot be created.

restart_persistent_systemid

Synopsis

Activates an object, sets the ORB and the POA to the proper state, and returns an object reference to the activated object.
IDL

Object restart_persistent_systemid(
in PortableServer::Servant servant,
in CORBA::RepositoryId rep_id,
in string stroid)
raises (ServantAlreadyActive, ObjectAlreadyActive);

C++ Binding

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

Java Binding

org.omg.CORBA.Object restart_persistent_systemid(
org.omg.PortableServer.Servant servant,
java.lang.String rep_id,
java.lang.String stroid);

Arguments

servant
An instance of the C++ implementation class for the interface.
rep_id
The repository id of the interface.
stroid
The stringified version of the ObjectId provided by the user to be set in the object reference being created. It must have been returned from a previous call on start_persistent_systemid.

Return Value

CORBA::Object_ptr
An object reference created with the stringified ObjectId stroid and the rep_id provided by the user. The object reference will need to be converted to a specific object type by invoking the _narrow() operation defined for the specific object. The caller is responsible for releasing the object when done.

Description

This operation performs the following actions:

Activates an object using the Servant supplied to service objects of the type rep_id, using the supplied stroid (a stringified ObjectId), which must have been obtained by a previous call on start_persistent_systemid.

Sets the ORB and the POA into the state in which they will accept requests on that object.

Returns an object reference to the object activated.

The re-activation would be done using the "restart_persistent_systemid" method.

Exceptions

ServantAlreadyActive
The servant is already being used for a callback. A servant can be used only for a callback with a single ObjectId. To receive callbacks on objects containing different ObjectIds, you must create different servants and activate them separately. The same servant can be re-used only if a stop_object operation tells the system to stop using the servant for its original ObjectId.
ObjectAlreadyActive
The stringified ObjectId is already being used for a callback. A given ObjectId can have only one servant associated with it. If you wish to change to a different servant, you must first invoke stop_object with the servant currently in use.
CORBA::BAD_PARAM
The repository id was a null string or the servant was a null pointer or the ObjectId supplied was not previously assigned by the system.
CORBA::IMP_LIMIT
In addition to other system reasons for this exception, a reason unique to this situation is that the joint client/server was not initialized with a port number; therefore, a persistent object reference cannot be created.

start_persistent_userid

Synopsis

Activates an object, sets the ORB and the POA to the proper state, and returns an object reference to the activated object.

IDL

Object start_persistent_userid( portableServer::Servant a_servant,
in CORBA::RepositoryId rep_id,
in string stroid)
raises ( ServantAlreadyActive, ObjectAlreadyActive );

C++ Binding

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

Java Binding

org.omg.CORBA.Object start_persistent_userid(
org.omg.PortableServer.Servant servant,
java.lang.String rep_id,
java.lang.String stroid);

Arguments

servant
An instance of the C++ implementation class for the interface.
rep_id
The repository id of the interface.
stroid
The stringified version of an ObjectId provided by the user to be set in the object reference being created. The stroid holds application-specific data and is opaque to the ORB.

Return Value

CORBA::Object_ptr
An object reference created with the stringified ObjectId stroid and the rep_id provided by the user. The object reference will need to be converted to a specific object type by invoking the _narrow() operation defined for the specific object. The caller is responsible for releasing the object when the conversion is done.

Description

This operation performs the following actions:

Activates an object using the Servant supplied to service objects of the type rep_id, using the object id stroid.

Sets the ORB and the POA into the state in which they will accept requests on that object.

Returns an object reference to the activated object. The returned object reference will be valid even after termination of the client. That is, if the client terminates, and restarts again, and then activates a servant with the same rep_id and for the same ObjectId, the servant will accept requests made on that same object reference.

Exceptions

ServantAlreadyActive
The servant is already being used for a callback. A servant can be used only for a callback with a single ObjectId. To receive callbacks on objects containing different ObjectIds, you must create different servants and activate them separately. The same servant can be re-used only if a stop_object operation tells the system to stop using the servant for its original ObjectId.
ObjectAlreadyActive
The stringified ObjectId is already being used for a callback. A given ObjectId can have only one servant associated with it. If you wish to change to a different servant, you must first invoke stop_object with the servant currently in use.
CORBA::BAD_PARAM
The repository ID was a null string or the servant was a null pointer.
CORBA::IMP_LIMIT
In addition to other system reasons for this exception, a reason unique to this situation is that the joint client/server was not initialized with a port number; therefore, a persistent object reference cannot be created.

stop_object

Synopsis

Tells the ORB to stop accepting requests on the object that is using the given servant.
IDL

void stop_object( in PortableServer::Servant servant);

C++ Binding

void stop_object(PortableServer::Servant servant);

Java Binding

void stop_object(org.omg.PortableServer.Servant servant);

Argument

servant
An instance of the C++ implementation class for the interface. The association between this servant and its ObjectId will be removed from the Active Object Map.

Description

This operation tells the ORB to stop accepting requests on the given servant. It does not matter what state the servant is in, activated or deactivated; no error is reported.
Return Value

None.
Exceptions

None.

stop_all_objects

Synopsis

Tells the ORB to stop accepting requests on all servants.
IDL

void stop_all_objects ();

C++ Binding

void stop_all_objects ();

Java Binding

void stop_all_objects ();

Return Value

None.
Description

This operation tells the ORB to stop accepting requests on all servants that have been set up in this process.
Usage Note

If a client calls the ORB::shutdown method, then it must not subsequently call stop_all_objects.
Exceptions

None.

get_string_oid

Synopsis

Requests the string version of the ObjectId of the current request.
IDL

string get_string_oid() raises (NotInRequest);

C++ Binding

char* get_string_oid();

Java Binding

java.lang.String get_string_oid();

Return Value

char*
The string version of the ObjectId of the current request. This is the string that was supplied when the object reference was created. The string is meaningful to users only in the case when the object reference was created by the start_persistent_userid function. (That is, the ObjectId created by start_transient and start_persistent_systemid were created by the ORB and has no relationship to the user application.)

Description

This operation returns the string version of the ObjectId of the current request.
Exceptions

NotInRequest
The function was called when the ORB was not in the context of a request (that is, not while the ORB was servicing a request in method code). Do not call this function from client code. It is legal only during the execution of a method of the callback object (that is, the servant).

~Callbacks

Synopsis

Destroys the callback object.
C++ Binding

BEAWrapper::~Callbacks( );
Java Binding
public ~Callbacks();

Arguments

None.
Return Value

None.
Description

This destructor destroys the callback object.
Usage Note

If a client wants to get rid of the wrapper, but not shut down the ORB, then the client must call the stop_all_objects method.
Exceptions

None.

Copyright � 1999 BEA Systems, Inc. All Rights Reserved.
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Last update: July 06, 1999.