EJB : Message Driven Bean [ MDB ]

• What is MDB ( Message Driven Bean )?
• Messaging System Features
• Technically Message Driven bean
• Why Messaging?
• Messaging Applications
• Example of MDB

• Messaging has been around for quite a while even before JMS as a way to build reliable, flexible, and scalable communication system. Some of you might have heard the term, Message-Oriented-Middleware, or MOM in short. In fact, in the mainframe world, product like IBM’s MQ_Series has been around for quite a while.
• Messaging can be defined as a method of communication between software components and applications, which is required when we are talking about some enterprise level applications.
• A messaging system is a peer to peer facility in which a messaging client can send messages to other clients as well as receive messages from other clients. Here each client connects to a message agent that facilitates the creating, sending, receiving and reading messages.
• Messaging System Concepts
  • De-coupled (Loosely-coupled) communication
  • Asynchronous communication
  • Messages are the means of communication between applications.

Messaging System Features

Support of two messaging models

• Point-to-point -: A message is consumed by a single consumer [ Queue ]

Queue



• Publish/Subscribe -: A message is consumed by multiple consumers [ Topic ]

Topic

It has as below

• Reliability
• Transactional operations
• Distributed messaging
• Security

Technically Message Driven bean

• A Message driven bean does not have a remote or local business interface
• You can not call a message driven bean by using an object oriented remote method invocation interface.
• Message driven beans support generic listener methods for message delivery.
• Message driven bean listener methods do not have return values.
• Message driven beans might not send exceptions back to clients.
• Message driven beans are stateless in nature.
• Message driven beans are single threaded.
• Message listener interface is business interface
• Bean class implements it or designates with @MessageListener
• No requirement to implement other interfaces
• Annotations

@MessageDriven
// EJB 3.0 Message-driven bean: Bean Class
@MessageDriven public class PayrollMDB implements javax.jms.MessageListener {

	public void onMessage(Message msg) {
	...
	}
}

Life Cycle of Message Driven Bean

• Does not Exist
• Method-Ready pool State

Why Messaging?

• Platform independence
• Network location independence
• Works well in heterogeneous environments

• Anonymity

  • Who doesn’t matter
  • Where doesn’t matter
  • When doesn’t matter

• Contrast with RPC-based systems

  • CORBA
  • RMI

• Scalability

  • Handle more clients with
  • No change in the application
  • No change in the architecture
  • No degradation in system throughput
  • Increase hardware capacity of messaging system if higher scalability is desired

• Robustness

  • Receivers can fail.
  • Senders can fail.
  • Network can fail.
  • Messaging System continues to function

Messaging Applications

• Credit card transactions
• Weather reporting
• Workflow
• Network management
• Supply chain management
• Customer care
• Communications (Voice Over IP, Paging Systems, etc.)
• Many more……

Click Here to Download the Code for MDB Bean Deployment