we have couple of steps that are created in lisa to listen to the request xml

7/28/2019 We Have Couple of Steps That Are Created in Lisa to Listen to the Request XML

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What is Messaging?

Messaging is a method of communication between software components or applications.A

messaging system is a peer-to-peer facility: A messaging client can send messages to, and receive

messages from, any other client. Each client connects to a messaging agent that provides facilities

for creating, sending, receiving, and reading messages. Messaging enables distributedcommunication that is loosely coupled. A component sends a message to a destination, and the

recipient can retrieve the message from the destination. However, the sender and the receiver do

not have to be available at the same time in order to communicate. In fact, the sender does not need

to know anything about the receiver; nor does the receiver need to know anything

about the sender. The sender and the receiver need to know only what message format and what

destination to use. In this respect, messaging differs from tightly coupled technologies, such as

Remote Method Invocation (RMI) or RPC which require an application to know a remote

applications methods.

Overview

Enterprise messaging products (or as they are sometimes called, Message Oriented Middleware

products) are becoming an essential component for integrating intra-company operations. They

allow separate business components to be combined into a reliable, yet flexible, system.

In addition to the traditional MOM vendors, enterprise messaging products are also provided by

several database vendors and a number of internet related companies.Java language clients and

Java language middle tier services must be capable of using these messaging systems. JMS

provides a common way for Java language programs to access these systems.the JMS API defines

a common set of interfaces and associated semantics that allow programs written in the Java

programming language to communicate with other messaging implementations.

And thats where JMS is useful and if you add Message Driven Beans(MDB) (combination of Session

beans and JMS clients) ,introduced as part of EJB2.0 Specification,then you have an effective

asynchronous messaging system in your middletier which covers areas like security, concurrency,

transaction etc.

TheJMS APIenables communication that is not only loosely coupled but also

Asynchronous. A JMS provider can deliver messages to a client as they arrive;a client does not

have to request messages in order to receive them.

Reliable. The JMS API can ensure that a message is delivered once and only once. Lower levels of

reliability are available for applications that can afford to miss messages or to receive duplicate

messages.

Why do you need JMS ?

There are several reasons why you need JMS in a enterprise middleware application :
http://java.sun.com/products/jms/http://java.sun.com/products/jms/http://java.sun.com/products/jms/http://java.sun.com/products/jms/http://java.sun.com/products/jms/


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i)Decoupling : Different parts of an application can be developed such that they are not closely tied

to each other

ii)Flexible Integration:Conversely loosely coupled systems can be put together by using MDB for

wrapping existing systems.

iii)Separation of elements involved in messaging applications,for example a logging event can beseparated from ,say retrieving a customer details in a bank application.

iv)Delivering low-level services of an application in offline-mode,meaning although the service must

be provided, the main workflow doesnt have to wait for its completion,for example a logging service

v)Delivering same information to multiple-parties

vi)Most of the top J2EE application servers support messaging.Sun provides a list of JMS vendors

athttp://java.sun.com/products/jms/vendors.html .

Some myths & drawbacks about using JMS

1)Additional overhead in handling messages and messaging systems2)Message-based system can be a single point of failure.

Different types of Messages

There are two main ways to send messages: point-to-point and publish-subscribe.

A point-to-point messaging(queue) application has a single producer (usually) and a single

consumer. The producer produces messages while the consumer consumes them.A point-to-point

system can actually have multiple producers,but usually only a single consumer. For example a print

server,any machine on the network can send a print job to a particular print server.

The publish-subscribe messaging model (pub-sub) is more of a broadcast-oriented model. Publish-

subscribe is based on the idea of topics and typically has many consumers-and potentially many

producers as well.For example, subscription to any forum

Fundamentals in JMS API

The basic elements in JMS are administered objects(connection factories &

destinations),connections,sessions,message producers,message consumers ,queues ,topics and

messages.

A connection factory is the object a client uses to create a connection with a provider.A connection

factory encapsulates a set of connection configuration parameters that has been defined by an

administrator. A pair of connection factories come preconfigured with the J2EE SDK and are

accessible as soon as you start the service.Each connection factory is an instance of either the

QueueConnectionFactory or the TopicConnectionFactory interface.
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A message producer is an object created by a session and is used for sending messages to a

destination. The Point-to-Point form of a message producer implements the Queue-Sender interface.

The pub/sub form implements the TopicPublisher interface.

A message consumer is an object created by a session and is used for receiving messages sent to adestination. A message consumer allows a JMS client to register interest in a destination with a JMS

provider. The JMS provider manages the delivery of messages from a destination to the registered

consumers of the destination.The PTP form of message consumer implements the QueueReceiver

interface.The pub/sub form implements the TopicSubscriber interface.

Different JMS Message Types

The JMS API defines five message body formats, also called message types, which allow you to

send and to receive data in many different forms and provide compatibility with existing messaging

formats.i)TextMessage : A java.lang.String object (for example, the contents of an Extensible Markup

Language file).

ii)MapMessage : A set of name/value pairs, with names as String objects and values as primitive

types in the Java programming language. The entries can be accessed sequentially by enumerator

or randomly by name. The order of the entries is undefined.

iii)BytesMessage : A stream of uninterpreted bytes. This message type is for literally encoding a

body to match an existing message format.

iv)StreamMessage: A stream of primitive values in the Java programming language,filled and read

sequentially.

v)ObjectMessage :A Serializable object in the Java programming language.

vi)Message : Composed of header fields and properties only. This message type is useful when a

message body is not required.

we have couple of steps that are created in lisa to listen to the request xml

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