java barcode generator library 8: Messaging in Java

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8: Messaging
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CERTIFICATION OBJECTIVE 8.02
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List Benef its of Synchronous and Asynchronous Messaging
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The benefits of synchronous messaging follow:
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n Because both parties must be active to participate in synchronous messaging,
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if either party is not active, the message transaction cannot be completed.
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n A message must be acknowledged before proceeding to the next message. If it
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is not acknowledged, the message cannot be considered processed. The benefits of asynchronous messaging are as follows:
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n As the volume of traffic increases, asynchronous messaging is better able to
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handle the spike in demand by keeping a backlog of requests in its queue and then operating at maximum capacity over a period of time instead of needing to service the requests instantaneously.
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n Asynchronous messaging is less affected by failures at the hardware, software,
and network levels.
n When capacities are exceeded, information is not lost; instead, it is delayed.
CERTIFICATION OBJECTIVE 8.03
Identify Scenarios That Are More Appropriate to Implementation Using Asynchronous Messaging, Rather Than Synchronous
The following scenarios are more appropriate to implementation using asynchronous messaging:
Java Message Service
SCENARIO & SOLUTION
You need to implement a messaging system in which a response is not required or not immediately required. Which messaging system is most appropriate You need a high-volume transaction processing capability for sending messages. Which type of messaging is best suited for this use You want a messaging system that uses your system hardware in an efficient manner. Which type of messaging should be used Asynchronous messaging
Asynchronous messaging Asynchronous messaging
CERTIFICATION OBJECTIVE 8.04
Identify Scenarios That Are More Appropriate to Implementation Using Synchronous Messaging, Rather Than Asynchronous
The following scenario is more appropriate to implementation using synchronous messaging:
SCENARIO & SOLUTION
You are using a credit card authorization/user login authentication system to send a message in which the response to the message is required before the transaction can be completed. Which type of messaging is most appropriate Synchronous messaging
Java Message Service
Message-oriented middleware products allow a developer to couple applications loosely together. However, these products are proprietary and quite often complex and expensive. The JMS provides a standard Java interface to these messaging middleware products, freeing developers from having to write low-level infrastructure code, or plumbing, and allowing solutions to be built quickly and easily. In short, the JMS API provides a convenient and easy way to create, send, receive, and read an enterprise messaging system s messages using Java.
8: Messaging
JMS applications can use databases to provide the storage to support message persistence that is necessary for guaranteeing delivery and order of messages. With the arrival of the Enterprise JavaBeans (EJB) 2.0 specification, the EJB messagedriven bean (MDB) has been able to receive and process messages asynchronously within the EJB container. These message-driven beans can be instantiated multiple times to provide concurrent processing (and therefore faster throughput) of a message queue. JMS provides an interface from Java applications to messaging products. JMS enables clients (or peers) to exchange data in the form of messages. Following are the major advantages of using messaging for this exchange:
n Easy integration of incompatible systems n Asynchronous communications n One-to-many communications n Guaranteed messaging n Transactional messaging
Table 8-1 shows the various components of a JMS messaging application.
Handling Exceptions
If a problem occurs, an application can be notified asynchronously via the ExceptionListener interface. This interface identifies the JMS provider problem details to the JMS client. To handle exceptions, the developer must create a listening object that implements the ExceptionListener interface and codes the onException (JMSException exception) method.
TABLE 8-1
Component
JMS provider
Function
The host application on which the JMS application runs. The JMS provider converses with JMS applications and supplies the underlying mechanisms required for a messaging application. JMS objects that are created and maintained by an administrator to be used by JMS clients. Applications that can send and/or receive messages. Bundles of information that are passed between applications. Each application defines the types of information a message can contain.
Components of a JMS Messaging Application
Administered objects Clients Messages
Java Message Service
The listening object must register itself with the JMS provider by calling the setExceptionListener (listenerobject) method on the session.
Managing Sessions
Table 8-2 describes the details of a JMS session. Unless noted otherwise, this information applies to both the publish/subscribe and point-to-point models.
TABLE 8-2
JMS Session Details
Session Detail
Transacted session
Description
A related group of messages that are treated as a single work unit. The transaction can be either committed or rolled back. When a message sender uses a transacted session and calls the commit method, the messages it produces are accepted for delivery. If it calls the rollback method, the messages it produces are destroyed. When a message receiver uses a transacted session and calls the commit method, the messages it consumes are acknowledged. If it calls the rollback method, the messages it consumes are recovered (not acknowledged). Clients send messages knowing that JMS will deliver them only once. Therefore, the JMS provider must never deliver a message more than once or deliver a copy of a message that has already been acknowledged. When a copy of a message is delivered, the message header contains a redelivery flag field that will be set, telling the client that this message may have been received before but that, for whatever reason, the JMS server did not receive the client s acknowledgment of receipt. The redelivery flag is set by the JMS provider application, usually as the result of a recovery operation. If a JMS session is transacted, messages are acknowledged automatically by the commit mechanism and recovered by the rollback mechanism. If a session is not transacted, recovery must be handled manually, and messages are acknowledged in one of three ways: AUTO_ACKNOWLEDGE: For each message, the session automatically acknowledges that a client has received the message when the client returns from a call to receive a message or the MessageListener called by the session to process the message returns successfully. CLIENT_ACKNOWLEDGE: Client acknowledges the message by calling the acknowledge method on the message. This also acknowledges all messages that were processed during the session. DUPS_OK_ACKNOWLEDGE: Because the session lazily acknowledges the delivery of messages, duplication of messages may result if the JMS provider fails. This mode should be used only if consumers can tolerate duplicate messages. This mode reduces session overhead by minimizing the work the session does to prevent duplicate messages.
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