Using the CORBA Notification Service

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Using the CosNotification Service API

This chapter describes the development steps required to create Notification Service applications using the CosNotification Service API and the C++ programming language.

This topic includes the following sections:

Note: The Oracle Tuxedo CORBA Java client and Oracle Tuxedo CORBA Java client ORB were deprecated in Tuxedo 8.1 and are no longer supported. All Oracle Tuxedo CORBA Java client and Oracle Tuxedo CORBA Java client ORB text references, associated code samples, should only be used to help implement/run third party Java ORB libraries, and for programmer reference only.
Note: Technical support for third party CORBA Java ORBs should be provided by their respective vendors. Oracle Tuxedo does not provide any technical support or documentation for third party CORBA Java ORBs.

 


Development Process

Table 4-1 outlines the development process for creating Notification Service applications.

Table 4-1 Development Process 
Step
Description
1
Designing events
2
Writing an application that posts events
3
Writing an application that subscribes to events
4
Compiling a Notification Service application

These steps are explained in detail in subsequent topics.

 


Designing Events

The design of events is basic to any notification service. The design impacts not only the volume of information that is delivered to matching subscriptions, but the efficiency and performance of the Notification Service as well. Therefore, careful planning should be done to ensure that your Notification Service will be able to handle your needs now and allow for future growth. For a discussion of event design, see "Designing Events" on page -6.

 


Step 1: Writing an Application to Post Events

The following types of CORBA applications can post events:

To post events, an application must, at a minimum, implement the following functions:

The following sections describe each of these functions.

Getting the Event Channel

Before the client application can post an event, it must get the event channel.

This development step is illustrated in Listing 4-1. Listing 4-1 is code from the Reporter.cpp file in the Introductory sample application that uses the CosNotification Service API.

To get the event channel factory object reference, the resolve_initial_references method is invoked on the Bootstrap object using the "NotificationService" environmental object. The object reference is used to get the channel factory, which is, in turn, is used to get the event channel. Listing 4-1 shows code examples in C++.

Listing 4-1 Getting the Event Channel (Reporter.cpp)
// Get the CosNotification channel factory object reference.
CORBA::Object_var channel_factory_oref =
       bootstrap.resolve_initial_references(
        "NotificationService" );
CosNotifyChannelAdmin::EventChannelFactory_var
   channel_factory =
       CosNotifyChannelAdmin::EventChannelFactory::_narrow(
          channel_factory_oref.in() );
// use the channel factory to get the default channel
CosNotifyChannelAdmin::EventChannel_var channel =
   channel_factory->get_event_channel(
       Tobj_Notification::DEFAULT_CHANNEL );

Creating and Posting Events

To post events, you must get the SupplierAdmin object, use it to create a proxy, create the event, and then post the event to the proxy.

Listing 4-2 shows how this is implemented in C++.

Listing 4-2 Creating and Posting the Event (Reporter.cpp)
// Since we are a supplier (that is, we post events),
// get the SupplierAdmin object
CosNotifyChannelAdmin::SupplierAdmin_var supplier_admin =
       channel->default_supplier_admin();
// Use the supplier admin to create a proxy. Events are posted
// to the proxy (unlike the simple events interface where events
// are posted to the channel).
CosNotifyChannelAdmin::ProxyID proxy_id;
CosNotifyChannelAdmin::ProxyConsumer_var generic_proxy_consumer =
     supplier_admin->obtain_notification_push_consumer(
       CosNotifyChannelAdmin::STRUCTURED_EVENT, proxy_id );
CosNotifyChannelAdmin::StructuredProxyPushConsumer_var
   proxy_push_consumer =
     CosNotifyChannelAdmin::StructuredProxyPushConsumer::_narrow(
       generic_proxy_consumer );
// Connect to the proxy so that we can post events.
proxy_push_consumer->connect_structured_push_supplier(
     CosNotifyComm::StructuredPushSupplier::_nil() );
...
// create an event
  CosNotification::StructuredEvent notification;
// set the domain to "News"
notification.header.fixed_header.event_type.domain_name =
    CORBA::string_dup("News");
// set the type to the news category
notification.header.fixed_header.event_type.type_name =
    CORBA::string_dup(“Sports”);
// add one field, which will contain the story, to the
// event's filterable data. set the field's name to
// "Story" and value to a string containing the story
notification.filterable_data.length(1);
notification.filterable_data[0].name =
           CORBA::string_dup("Story");
notification.filterable_data[0].value <<= “John Smith wins again”;
// post the event
// Subscribers who subscribed to events whose domain is
// "News" and whose type matches the news category will
// receive this event
proxy_push_consumer->push_structured_event(notification);
...
// Disconnect.
proxy_push_consumer->disconnect_structured_push_consumer();

 


Step 2: Writing an Application to Subscribe to Events

The following types of CORBA applications can subscribe to events:

To subscribe to events, an application must, at a minimum, support the following functions:

Implementing the CosNotifyComm::StructuredPushConsumer Interface

In order for the callback servant object to receive events, it must implement the CosNotifyComm::StructuredPushConsumer interface that supports the push_structured_event operation. When an event occurs that has a matching subscription, the Notification Service invokes this operation on the servant callback object in the subscriber application to deliver the event to the subscriber application.

The CosNotifyComm::StructuredPushConsumer interface also defines the operations offer_change and disconnect_structured_push_consumer. The Notification Service never invokes these operations, so you should implement stubbed out versions that throw CORBA::NO_IMPLEMENT.

Listing 4-3 and Listing 4-4 show how this interface is implemented in C++.

Listing 4-3 Sample CosNotifyComm::StructuredPushConsumer Interface Implementation (NewsConsumer_i.h)
#ifndef _news_consumer_i_h
#define _news_consumer_i_h
#include "CosNotifyComm_s.h"
// For the servant class to receive news events,
// it must implement the CosNotifyComm::StructuredPushConsumer
// idl interface
class NewsConsumer_i : public POA_CosNotifyComm::StructuredPushConsumer
{
  public:
    // this method will be called when a news event occurs
    virtual void push_structured_event(
      const CosNotification::StructuredEvent& notification
    );
    // OMG's CosNotifyComm::StructuredPushConsumer idl
    // interface defines the methods "offer_change" and
    // "disconnect_structured_push_consumer". Since the
    // Notification Service never invokes these methods, just
    // have them throw a CORBA::NO_IMPLEMENT exception
    virtual void offer_change(
      const CosNotification::EventTypeSeq& added,
      const CosNotification::EventTypeSeq& removed )
    {
      throw CORBA::NO_IMPLEMENT();
    }
    virtual void disconnect_structured_push_consumer()
    {
      throw CORBA::NO_IMPLEMENT();
    }
};
#endif
Listing 4-4 Sample CosNotifyComm::StructuredPushConsumer Interface Implementation (NewsConsumer_i.cpp)
#include "NewsConsumer_i.h"
#include <iostream.h>
//-----------------------------------------------------------
// Subscriber.cpp creates a simple events subscription to "News"
// events and has the events delivered to a NewsConsumer_i
// object. When a news event occurs (this happens when a user
// runs the Reporter application and reports a news story), this
// method will be invoked:
void NewsConsumer_i::push_structured_event(
  const CosNotification::StructuredEvent& notification )
  {
// extract the story from the first field in the event's
// filterable data
char* story;
notification.filterable_data[0].value >>= story;
// for coding simplicity, assume "story" is not "null"
// print out the event
cout
    << "-----------------------------------------------------"
    << endl
    << "Category : "
    << notification.header.fixed_header.
              v            event_type.type_name.in()
    << endl
    << "Story : "
    << story
    << endl;
...
}

Getting the Event Channel, ConsumerAdmin Object, and Filter Factory Object

Before an application can create a subscription, it must get the event channel and the ConsumerAdmin and Filter Factory objects. Listing 4-5 shows how this is implemented in C++.

To get the event channel factory object reference, the resolve_initial_references method is invoked on the Bootstrap object using the "NotificationService" environmental object. The object reference is used to get the channel factory, which is, in turn, used to get the event channel. Finally, the event channel is used to get the ConsumerAdmin object and the FilterFactory object.

Listing 4-5 Getting the Event Channel and ConsumerAdmin and Filter Factory Objects (Subscriber.cpp)
// Get the CosNotification channel factory object reference.
CORBA::Object_var
  channel_factory_oref =
     bootstrap.resolve_initial_references(
          "NotificationService" );
   channel_factory =
      CosNotifyChannelAdmin::EventChannelFactory::_narrow(
          channel_factory_oref.in() );
// Use the channel factory to get the default channel.
CosNotifyChannelAdmin::EventChannel_var channel =
   channel_factory->get_event_channel(
      Tobj_Notification::DEFAULT_CHANNEL );
// Use the channel to get the consumer admin and the filter factory.
CosNotifyChannelAdmin::ConsumerAdmin_var consumer_admin =
   channel->default_consumer_admin();
CosNotifyFilter::FilterFactory_var filter_factory =
   channel->default_filter_factory();

Creating a Callback Object

To receive events, the application must also be a server; that is, the application must implement a callback object that can be invoked (called back) when an event occurs that matches the subscriber’s subscription.

Creating a callback object includes the following steps:

Note: The following steps apply to a joint client/server. Oracle Tuxedo CORBA servers can also subscribe to events.
  1. Creating a callback wrapper object. This can be implemented using either the BEAWrapper Callbacks object or the CORBA Portable Object Adaptor (POA).
  2. Creating the servant.
  3. Creating an object reference to the callback servant.

For a complete description of the BEAWrapper Callbacks object and its methods, see the Joint Client/Servers chapter in the CORBA Programming Reference.

Note: Using the BEAWrapper Callback object to create a callback object is discussed below. For a discussion of how to implement a callback object using the POA, see Using CORBA Server-to-Server Communication.

Listing 4-6 shows how to use the BEAWrapper Callbacks object to create a callback object in C++. In the code examples, the NewsConsumber_i servant is created and the start_transient method is used to create a transient object reference.

Listing 4-6 Sample Code for Creating a Callback Object with Transient Object Reference (Introductory Application Subscriber.cpp)
// Create a callback wrapper object since this client needs to
// support callbacks
BEAWrapper::Callbacks wrapper(orb.in());
NewsConsumer_i* news_consumer_impl = new NewsConsumer_i;
// Create a transient object reference to this servant.
CORBA::Object_var news_consumer_oref =
       wrapper.start_transient(
         news_consumer_impl,
         CosNotifyComm::_tc_StructuredPushConsumer->id()
          );
CosNotifyComm::StructuredPushConsumer_var
    news_consumer =
       CosNotifyComm::StructuredPushConsumer::_narrow(
           news_consumer_oref.in() );

Creating a Subscription

In order for the subscriber to receive events, it must subscribe to the Notification Service. You can create a transient subscription or a persistent subscription.

To create a subscription, the following steps must be performed:

  1. Create a notification proxy push supplier and use it to create a StructuredProxySupplier object.
  2. Set the subscription’s Quality of Service (QoS). You can set the QoS to transient or persistent.
  3. Create a filter object and assign the domain_name, type_name, and data_filter (optional) to it.
  4. Add the filter to the proxy.
  5. Connect to the proxy passing in the subscription’s callback object reference.

Listing 4-7 from the Introductory sample application, shows how to create a transient subscription in C++.

Listing 4-7 Creating a Transient Subscription
// Create a new subscription (at this point, it is not complete).
CosNotifyChannelAdmin::ProxyID subscription_id;
CosNotifyChannelAdmin::ProxySupplier_var generic_subscription =
      consumer_admin->obtain_notification_push_supplier(
          CosNotifyChannelAdmin::STRUCTURED_EVENT,
          subscription_id );
CosNotifyChannelAdmin::StructuredProxyPushSupplier_var
     subscription =
       CosNotifyChannelAdmin::StructuredProxyPushSupplier::_narrow(
            generic_subscription );
      s_subscription = subscription.in();
  // Set the quality of service. This sets the subscription name
  // and subscription type (=TRANSIENT).
  CosNotification::QoSProperties qos;
  qos.length(2);
  qos[0].name =
     CORBA::string_dup(Tobj_Notification::SUBSCRIPTION_NAME);
  qos[0].value <<= subscription_name;
  qos[1].name =
     CORBA::string_dup(Tobj_Notification::SUBSCRIPTION_TYPE);
  qos[1].value <<=
     Tobj_Notification::TRANSIENT_SUBSCRIPTION;
  subscription->set_qos(qos);
  // Create a filter (used to specify domain, type and data filter).
  CosNotifyFilter::Filter_var filter =
           filter_factory->create_filter(
            Tobj_Notification::CONSTRAINT_GRAMMAR );
  s_filter = filter.in();
    // Set the filtering parameters.
    // (domain = "News", type = “Sports”, and no data filter)
    CosNotifyFilter::ConstraintExpSeq constraints;
    constraints.length(1);
    constraints[0].event_types.length(1);
    constraints[0].event_types[0].domain_name =
CORBA::string_dup("News");
    constraints[0].event_types[0].type_name =
CORBA::string_dup(“Sports”);
    constraints[0].constraint_expr =
CORBA::string_dup(""); // No data filter.
    CosNotifyFilter::ConstraintInfoSeq_var
        add_constraints_results = // ignore this returned value
            filter->add_constraints(constraints);
    // Add the filter to the subscription.
    CosNotifyFilter::FilterID filter_id =
       subscription->add_filter(filter.in());
    // Now that we have set the subscription name, type and filtering
    // parameters, complete the subscription by passing in the
    // reference of the callback object to deliver the events to.
    subscription->connect_structured_push_consumer(
         news_consumer.in() );

 


Step 3: Compiling and Running Notification Service Applications

The final step in the development of a Notification Service application is to compile, build, and run the application. To do this, you need to perform the following steps.

  1. Generate the required client stub and skeleton files to define interfaces between the Notification Service and event poster and subscriber applications. Event poster applications can be clients, joint client/servers, or servers. Event subscriber applications can be joint client/servers or servers.
  2. Compile the application code and link against the skeleton and client stub files.
  3. Build the application.
  4. Run the application.

Generating the Client Stub and Skeleton Files

To generate the client stub and skeleton files, you must execute the idl command for each of the Notification IDL files that your application uses. Table 4-2 shows the idl commands used for each type of subscriber.

Table 4-2 idl Command Requirements
Language
Oracle Tuxedo CORBA Joint Client/Server
Oracle Tuxedo CORBA Server
C++
idl -P
idl

The following is an example of an idl command:

>idl -IC:\tuxdir\include C:\tuxdir\include\CosEventComm.idl

Table 4-3 lists the IDL files required by each type of Notification Service application.

Table 4-3 IDL Files Required by Notification Service Applications
Application Type
Required OMG IDL Files
Event poster (can be a client, a joint client/server, or a server)
CosEventChannelAdmin.idl CosEventComm.idl CosNotification.idl CosNotifyChannelAdmin
CosNotifyComm.idl CosNotifyFilter Tobj_Events.idl Tobj_Notification.idl
Subscriber (can be joint client/server or a server)
CosEventChannelAdmin.idl CosEventComm.idl CosNotification.idl CosNotifyChannelAdmin CosNotifyComm.idl CosNotifyFilter Tobj_Events.idl Tobj_Notification.idl

Compiling and Linking the Application Code

The compiling and linking procedure differs depending on the type of Notification Service application you are building. Table 4-4 provides an overview of the commands and files used to compile each type of application.

Table 4-4 Application Build Requirements 
Application Type
Client
Joint Client/Server
Server
C++ Events Poster
Use the buildobjclient command to compile the application files and the IDL stubs.
Use the buildobjclient command with the -P option to compile the application files and the IDL stubs.
Use the buildobjserver command to compile the application files and the IDL client stubs.
C++ Events Subscriber
Not applicable.
Use the buildobjclient command with the -P option to compile the application files, the IDL stubs, and the IDL skeletons.
Use the buildobjserver command to compile the application files, the IDL stubs, and the IDL skeletons.

Listing 4-8 shows the commands used for a C++ Reporter application (Reporter.cpp) on a Microsoft Windows system. To form a C++ executable, the idl command is run on the required IDL file and the buildobjclient command compiles the C++ client application file and the IDL stubs.

Listing 4-8 C++ Reporter Application Build and Run Commands
# Run the idl command.
idl -IC:\tuxdir\include C:\tuxdir\include\CosEventComm.idl \
C:\tuxdir\include\CosEventChannelAdmin \
C:\tuxdir\include\CosNotification.idl \
C:\tuxdir\include\CosNotifyComm.idl \
C:\tuxdir\include\CosNotifyFilter.idl \
C:\tuxdir\include\Tobj_Notification.idl
# Run the buildobjclient command.
buildobjclient -v -o is_reporter.exe -f ”\
    -DWIN32 \
    Reporter.cpp \
    CosEventComm_c.cpp \
    CosEventChannelAdmin_c.cpp \
    CosNotification_c.cpp \
    CosNotifyComm_c.cpp \
    CosNotifyFilter_c.cpp \
    CosNotifyChannelAdmin_c.cpp \
    Tobj_Events_c.cpp \
    Tobj_Notification_c.cpp ”
# Run the application.
is_reporter

Listing 4-9 and Listing 4-10 show the commands used for a C++ Subscriber application (Subscriber.cpp) on Microsoft Windows and UNIX, respectively. To form a C++ executable, the buildobjclient command, with the -P option, compiles the joint client/server application files (Subscriber.cpp and NewsConsumer_i.cpp), the IDL stubs, the IDL skeleton (for CosNotifyComm_s.cpp).

Listing 4-9 C++ Subscriber Application Build and Run Commands (Microsoft Windows)
# Run the idl command.
idl -P -IC:\tuxdir\include C:\tuxdir\include\CosEventComm.idl \
C:\tuxdir\include\CosEventChannelAdmin \
C:\tuxdir\include\CosNotification.idl \
C:\tuxdir\include\CosNotifyComm.idl \
C:\tuxdir\include\CosNotifyFilter.idl \
C:\tuxdir\include\CosNotifyChannelAdmin \ \C:\tuxdir\include\Tobj_Events.idl \
\C:\tuxdir\include\Tobj_Notification
# Run the buildobjclient command.
buildobjclient -v -P -o is_subscriber.exe -f " \
   -DWIN32 \
    Subscriber.cpp \
    NewsConsumer_i.cpp \
    CosEventComm_c.cpp \
    CosEventChannelAdmin_c.cpp \
    CosNotification_c.cpp \
    CosNotifyComm_c.cpp \
    CosNotifyComm_s.cpp \
    CosNotifyFilter_c.cpp \
    CosNotifyChannelAdmin_c.cpp \
    Tobj_Events_c.cpp \
    Tobj_Notification_c.cpp \
    C:\tuxdir\lib\libbeawrapper.lib \
    "
# Run the application.
is_subscriber
Listing 4-10 C++ Subscriber Application Build and Run Commands (UNIX)
# Run the idl command.
idl -P -I/usr/local/tuxdir/include /usr/local/tuxdir/include/CosEventChannelAdmin \
/usr/local/tuxdir/include/CosEventComm.idl \
/usr/local/tuxdir/include/CosNotification.idl \
/usr/local/tuxdir/include/CosNotifyComm.idl \
/usr/local/tuxdir/include/CosNotifyFilter.idl \
/usr/local/tuxdir/include/CosNotifyChannelAdmin \
/usr/local/tuxdir/include/Tobj_Events.idl \
/usr/local/tuxdir/include/Tobj_SimpleEvents.idl
# Run the buildobjclient command.
buildobjclient -v -P -o subscriber -f " \
  Subscriber.cpp \
  NewsConsumer_i.cpp \
  CosEventComm_c.cpp \
  CosEventChannelAdmin_c.cpp \
  CosNotification_c.cpp \
  CosNotifyComm_c.cpp \
  CosNotifyComm_s.cpp \
  CosNotifyFilter_c.cpp \
  CosNotifyChannelAdmin_c.cpp \
  Tobj_Events_c.cpp \
  Tobj_SimpleEvents_c.cpp \
  -lbeawrapper \
"
# Run the application.
is_subscriber

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