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Joint Client/Servers
This chapter describes programming requirements for joint client/servers and the BEAWrapper Callbacks API.
For either a BEA WebLogic Enterprise 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 WebLogic Enterprise environmental objects to establish connections, set up security, and start transactions.
BEA WebLogic Enterprise 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 BEA WebLogic Enterprise environmental objects that are supplied with the BEA WebLogic Enterprise software, to perform functions unique to BEA WebLogic Enterprise.
For BEA WebLogic Enterprise joint client/servers, the client code must be structured so that it can act as a server for callback BEA WebLogic Enterprise objects. Such clients do not use the TP Framework and are not subject to BEA WebLogic Enterprise system administration. Besides the programming implications, this means that joint client/servers do not have the same scalability and reliability as BEA WebLogic Enterprise 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 WebLogic Enterprise server, rather than in a client.
The following sections describe the mechanisms you use to add callback support to a BEA WebLogic Enterprise client. In some cases, the mechanisms are contrasted with the BEA WebLogic Enterprise server mechanisms that use the TP Framework.
Main Program and Server Initialization
In a BEA WebLogic Enterprise server, you use the buildobjserver command to create the main program for the server. That main program takes care of all BEA WebLogic Enterprise- 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 BEA WebLogic Enterprise joint client/server (as for a BEA WebLogic Enterprise 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 BEA WebLogic Enterprise joint client/servers are very similar to servants for BEA WebLogic Enterprise 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 BEA WebLogic Enterprise 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 BEA WebLogic Enterprise 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 BEA WebLogic Enterprise 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 BEA WebLogic Enterprise client that supports callbacks, as well as in a BEA WebLogic Enterprise 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 BEA WebLogic Enterprise 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 BEA WebLogic Enterprise 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 BEA WebLogic Enterprise 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 BEA WebLogic Enterprise client that supports callbacks, you must write code that explicitly creates a servant and initializes a servant's state.
Callback Object Models Supported
BEA WebLogic Enterprise 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:
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).
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.)
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 BEA WebLogic Enterprise wrappers do not provide special help to do so.
Note: For BEA WebLogic Enterprise native joint client/servers, neither of the Persistent policies is supported, only the Transient policy.
Preparing Callback Objects Using CORBA
To set up BEA WebLogic Enterprise callback objects using CORBA, the client must do the following:
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 CORBA 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.
BEAWrapper Callbacks API
This API is described in the following sections.
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
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
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
Return Value
Description
This operation performs the following actions:
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
Return Value
Description
This operation performs the following actions:
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_ptr restart_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
Return Value
Description
This operation performs the following actions:
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
Return Value
Description
This operation performs the following actions:
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
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.
Note: If you do an invocation on a callback object after you call the stop_object operation, the OBJECT_NOT_EXIST exception is returned to the caller. This is because the stop_object operation, in effect, deletes the object.
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
Description
This operation returns the string version of the ObjectId of the current request.
Exceptions
~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.
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Copyright © 2000 BEA Systems, Inc. All rights reserved.
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