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| e-docs > Tuxedo > Using the CORBA Notification Service > CORBA Notification Service API Reference |
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Using the CORBA Notification Service
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CORBA Notification Service API Reference
This topic includes the following sections:
Introduction
The BEA Tuxedo CORBA Notification Service supports two application programming interfaces. One is based on the CORBA-based Notification Service as defined by the CORBAservices: Common Object Services Specification. This interface is referred to in this document as the CosNotification Service interface. The other interface, called the BEA Simple Events interface, is a BEA proprietary interface designed as an easier to use alternative.
Both interfaces pass structured events as defined by the CORBA-based Notification Service specification and are compatible with each other; that is, events posted using the CosNotification Service interface can be subscribed to by the BEA Simple Events interface and vice versa.
Before using the Notification Service APIs, consider the following topics:
Quality of Service
To determine the persistence of the subscription and whether or not events delivery is retried following a failed delivery, subscribers specify a Quality of Service (QoS). There are two Quality of Service settings: persistent and transient Quality of Service (QoS). The QoS is a property of the subscription.
Persistent Subscriptions
Persistent subscriptions provide strong guarantees about event delivery and the permanence of the subscription. Persistent subscriptions do come with a cost, however, as they consume more system resources (for example, disk space, CPU cycles, and so on), and require more administration (such as managing queues and detecting dead subscribers).
Persistent subscriptions exhibit the following properties:
Transient Subscriptions
Transient subscriptions provide the best performance with the least overhead and exhibit the following properties:
The subscription is in effect until a failed event delivery is detected. On detection of a failed delivery, the subscription is terminated. Normally, the Notification Service, for performance reasons, does not check whether it successfully delivered an event to a transient subscriber. However, occasionally, when the Notification Service delivers an event to a transient subscriber, it checks whether or not the event was successfully delivered. If it was not successfully delivered and the CORBA::TRANSIENT exception is not returned, the Notification Service assumes that the subscription has gone away and cancels the subscription. If the Notification Service receives the CORBA::TRANSIENT exception when an attempt to deliver fails, it assumes that the subscriber is busy and discards the event, but it does not cancel the subscription.
The automatic cancellation of dead transient subscriptions provides a cleanup mechanism for transient subscribers that forget to unsubscribe. Note, however, that the Notification Service checks for successful delivery the first time it sends an event to a subscriber, but does not perform it again until five minutes have elapsed and it delivers another event. Therefore, the interval between checks is at least five minutes, but will be longer if there is no event to deliver when five minutes have elapsed. The minimum interval of five minutes is fixed and cannot be changed. Therefore, event delivery failure is not necessarily detected on the first failed delivery attempt. It is only detected when the Notification Service checks.
Obtaining the Channel Factory
The Channel Factory is used by event poster applications and subscriber applications to find the event channel. The event channel is then used to post events and to subscribe, or create subscriptions, and unsubscribe, or cancel subscriptions.
Notification Service applications use the Bootstrap object to obtain an object reference to the event channel factory. This is done by using the Tobj_Bootstrap::resolve_initial_references operation. The Bootstrap object supports two service IDs for Notification Service applications, NotificationService and Tobj_SimpleEventsService. The NotificationService object is used in applications that use the CosNotification Service API. The Tobj_SimpleEventsService object is used in applications that use the BEA SimpleEvents API.
Note: Release 8.0 of BEA Tuxedo CORBA continues to include the BEA client environmental objects provided in previous releases of BEA WebLogic Enterprise for use with the Tuxedo 8.0 CORBA clients. BEA Tuxedo 8.0 clients should continue to use these environmental objects to resolve initial references bootstrapping, security and transaction objects. In release 8.0 of BEA Tuxedo CORBA, support has been added for using the OMG Interoperable Naming Service (INS) to resolve initial references to bootstrapping, security, and transaction objects. For information on INS, see the CORBA Programming Reference
Using Transactions
The behavior regarding transactions is the same for the BEA SimpleEvents API and the CosNotification Service API. The only operation that supports transactional behavior is push_structured_event, which is supported by the CosNotifyChannelAdmin::StructuredProxyPushConsumer and Tobj_SimpleEvents::Channel interfaces. All other operations can be used in the context of a transaction, but work the same regardless of whether they are executed in a transaction or not.
The behavior when posting an event is tied to the QoS of the subscription. If an event is posted in the context of a transaction, and the event delivery QoS of the subscription is persistent, the delivery will be affected by the outcome of the transaction; that is, if the transaction is committed, the Notification Service attempts to deliver the event to subscribers as it normally would. If the transaction is rolled back, then the Notification Service does not attempt to deliver the event.
If an event is posted in the context of a transaction, and the event delivery QoS of the subscriber's subscription is transient, one attempt will be made to deliver the event, regardless of the transaction outcome. That is, the transaction has no effect on whether the event is delivered or not, and one attempt will be made to deliver the event.
Note: There is no transaction context associated with event delivery. However, in the case of persistent subscriptions, once the poster's transaction commits, the Notification Service guarantees that the event will be delivered to the subscriber or put on the error queue to await administrative action.
Structured Event Fields, Types, and Filters
All events that are either pushed by posters to the Notification Service, or delivered to subscribers, are COS Structured Events; that is, they conform to the definition of Structured Events as specified by the CORBA-based Notification Service—a service which extends the CORBAservices Event Service (see Figure 2-1). If the events are to be filtered based on content (versus filtering on domain and type), or if the events are going to be subscribed to by BEA Tuxedo applications, then additional restrictions apply. The restrictions apply to data types and filtering based on event content. These restrictions are explained below.
Figure 2-1 Structured Event
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.
The Notification Service supports five levels of event design: (1) domain name, (2) type name, (3) priority, (4) filterable data, and (5) remainder of body. When designing an event, you must specify a domain name and a type name; priority and filterable data are optional. The domain name you choose can relate to your business. Hospitals, for example, are in the health care business, so for a Notification Service application for a hospital you might choose "HEALTHCARE" as a domain name. You might want to categorize the events by the type of insurance provider, so you may choose "HMO" or "UNINSURED" as the type name. You may want to further define the events by the entity responsible for payment, so you might choose to use the filterable data to identify the entity as "billing" for a specific "HMO_Account" or a specific or "Patient_Account." Listing 2-1 shows an example of this type of event design.
Listing 2-1 Event Design
domain_name = "HEALTHCARE"
type_name = "HMO"
#Filterable data name/value pairs.
filterable_data.name = "billing"
filterable_data.value = 4498
filterable_data.name = "patient_account"
filterable_data.value = 37621
Obviously, the more specific and precise you are in designing the events that you want your Notification Service application to post and receive, the fewer will be the events the Notification Service will have to process. This has a direct impact on system resources and configuration requirements. Therefore, a lot of thought should be given to event design.
Creating FML Field Table Files for Events
You must create Field Manipulation Language (FML) field table files for events only if one of the following capabilities is required; otherwise FML tables are not required.
A structured event's filterable_data field contains a list of name/value (NV) pairs. An event's data is typically stored in this list. The field names in the FML field table files must match the name in the structured event. The field type can be any allowable FML type (long, short, double, float, char, string) except carray. The value in the structured event must be the same type as defined in the field table. Table 2-1 shows the CORBA Any Types supported by BEA Tuxedo, and which ones can be used for data filtering and BEA Tuxedo interoperability.
Listing 2-2 shows an example of an FML field table file. The *base 2000 is the base number for the fields. The first entry has a field name of billing, a field number of 1 relative to the base, and a field type of long. Listing 2-2 Data Filtering FML Field Table File The following guidelines and restrictions apply to BEA Tuxedo FML field table files:
*base 2000
#Field Name Field # Field Type Flags Comments
#----------- ------- ---------- ------ --------
billing 1 long - -
stock_name 2 string - -
price_per_share 3 double - -
number_of_shares 5 long - -
For information on how to create and configure FML field table files, see field_tables in the BEA Tuxedo Command Reference and the Programming BEA Tuxedo ATMI Applications Using FML.
Interoperability with BEA Tuxedo Applications
Applications that use the BEA Tuxedo CORBA Notification Service are interoperable with BEA Tuxedo applications that use the BEA Tuxedo EventBroker. An application using the BEA Tuxedo Notification Service can post events that are delivered to BEA Tuxedo EventBroker subscribers, and can receive events that have been posted by BEA Tuxedo EventBroker.
To achieve this interoperability, it is necessary to understand the mapping between CosNotification Structured Events and the BEA Tuxedo FML buffer so that the contents of the FML field tables can be coordinated by BEA Tuxedo. There are two cases to consider: posting events that are to be received by BEA Tuxedo applications via BEA Tuxedo EventBroker; and receiving events that have been posted to the Notification Service Event Channel by BEA Tuxedo applications.
Posting Events
For a BEA Tuxedo application to subscribe to events posted by a BEA Tuxedo application, you must understand how a BEA Tuxedo structured event is mapped to FML32 and the event name at posting time. The mapping is as follows:
Receiving Events
BEA Tuxedo system events and user events can be received by BEA Tuxedo applications. System events are generated by the BEA Tuxedo system—not by applications. User events are generated by BEA Tuxedo applications. For a listing of System events see EVENTS in the BEA Tuxedo Command Reference. System events and user events are mapped in CosNotification Structured Events as follows:
The BEA Tuxedo system detects and posts certain predefined events related to system warnings and failures. For example, system-generated events report on configuration changes, state changes, connection failures, and machine partitioning. In order for a BEA Tuxedo application to receive events posted by a BEA Tuxedo application, it is necessary to understand how a FML buffer containing a BEA Tuxedo event is used to fabricate a BEA Tuxedo structured event. It is also necessary to know how the domain_name and type_name are related to the BEA Tuxedo event name. There are two cases to consider: system events and user events. Note that BEA Tuxedo uses a leading dot (".") in the event name to distinguish system-generated events from application-defined events. An example of a system event is .SysNetworkDropped. An example of a user event is eventsdropped. To subscribe to these events, the Notification Service subscriber application must define the subscription as follows:
domain_name ="TMEVT"
type_name=".SysNetworkDropped"
domain_name ="TMEVT"
type_name="eventsdropped"
When the events are received, the Notification Service subscriber application parses each event as follows:
domain_name="TMEVT"
type_name=""
event_name=""
variable_header=empty
Filterable_data=(content of the FML buffer)
Parameters Used When Creating Subscriptions
When you create subscriptions, you can specify the following parameters. These parameters support the BEA Simple Events API and the CosNotification Service API.
To use data filtering, you must set up an FML table, include filters in the subscription, filter the data, and post the event. Listing 2-3 shows an example of these tasks.
Listing 2-3 Data Filtering Requirements
//Setting up the FML Table
Field table file.
----------------
*base 2000
*Field Name Field # Field Type Flags Comments
----------- ------- --------- ------ ------
StockName 1 string - -
PricePerShare 2 double - -
CustomerId 3 long - -
CustomerName 4 string - -
//Subscription data filtering.
1) "NumberOfShares > 100 && NumberOfShares < 1000"
2) "CustomerId == 3241234"
3) "PricePerShare > 125.00"
4) "StockName == 'BEAS'"
5) "CustomerName %% '.*Jones.*'" // CustomerName contains "Jones"
6) "StockName == 'BEAS' && PricePerShare > 150.00"
//Posting the event.
// C++
CosNotification::StructuredEvent ev;
...
ev.filterable_data[0].name = CORBA::string_dup("StockName");
ev.filterable_data[0].value <<= "BEAS";
ev.filterable_data[1].name = CORBA::string_dup("PricePerShare");
ev.filterable_data[1].value <<= CORBA::Double(175.00);
ev.filterable_data[2].name = CORBA::string_dup("CustomerId");
ev.filterable_data[2].value <<= CORBA::Long(1234567);
ev.filterable_data[3].name = CORBA::string_dup("CustomerName");
ev.filterable_data[3].value <<= "Jane Jones";
// Java
StructuredEvent ev;
...
ev.filterable_data[0].name = "StockName";
ev.filterable_data[0].value.insert_string("BEAS");
ev.filterable_data[1].name = "PricePerShare";
ev.filterable_data[1].value.insert_double(175.00);
ev.filterable_data[2].name = "CustomerId";
ev.filterable_data[2].value.insert_long(1234567);
ev.filterable_data[3].name = "CustomerName";
ev.filterable_data[3].value.insert_string("Jane Jones");
Note: You can use either transient or persistent object references for the callback objects. Both QoS and application run times should be taken into consideration when deciding which type of object reference to use. For information to assist you in deciding which type of object reference to use, refer to Table 2-3.
Note: For persistent subscriptions, the Notification Service always does a two-way invoke on callback objects to deliver events. If a joint client/server does not activate a callback object (the event receiver) before it calls orb->run and then the Notification Service invokes on the callback object, as far as the POA is concerned, the callback object does not exist. In this case CORBA::OBJECT_NOT_EXIST exception is returned. If the Notification Service receives a CORBA::OBJECT_NOT_EXIST exception, it drops the subscription and the event; otherwise, the subscription is retained and the event is retried.
BEA Simple Events API
Simplicity and ease-of-use are the defining characteristics of the BEA Simple Events application programming interface (API). Its capabilities are similar to those of the BEA Tuxedo EventBroker.
The BEA Simple Events API consists of the following interfaces (see Figure 2-2):
Figure 2-2 BEA Simple Events Interfaces
The Tobj_SimpleEvents::Channel and the Tobj_SimpleEvents::ChannelFactory interfaces are implemented by the Notification Service and are described below.
The CosNotifyComm::StructuredPushConsumer interface is implemented by the subscribers. For a description of this interface, see "CosNotifyComm::StructuredPushConsumer::push_structured_event" on page 2-64.
Note: The CosNotification Service classes referred to in this section are fully described in the CosNotification Service IDL files, which are located in the tuxdir/include directory.
Note: If you use class operations that are not supported, the CORBA::NO_IMPLEMENT exception is raised.
TOBJ_SimpleEvents::Channel Interface
The Channel interface is used:
This interface provides these operations:
The CORBA IDL for this interface:
module Tobj_SimpleEvents
{
typedef long SubscriptionID;
typedef string RegularExpression;
typedef string FilterExpression;
const SubscriptionType TRANSIENT_SUBSCRIPTION = 0;
const SubscriptionType PERSISTENT_SUBSCRIPTION = 1;
interface Channel
{
void push_structured_event(
in CosNotification::StructuredEvent event);
SubscriptionID subscribe (
in string subscription_name,
in RegularExpression domain,
in RegularExpression type,
in FilterExpression data_filter,
in CosNotification::QoSProperties qos,
in CosNotifyComm::StructuredPushConsumer push_consumer);
boolean exists( in SubscriptionID id );
void unsubscribe( in SubscriptionID id );
};
};
These operations are described in the following section.
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