Patterns: Serial and Parallel Processes for Process Choreography and Workflow

128 Patterns: Serial and Parallel Processes

model, for security, and for the coupling strength between requestor and

provider. The simplest effect is that parameters in Web services are

in-parameters and never ever in-out-parameters. To return a response value, you

have to use the return value of the method.

The security impact results from the fact that in the case of CORBA or RMI the

client is remotely acting in the address space of the server application.

Reference errors can lead to server-side memory access exceptions or data

corruption for example.

Furthermore, in the case of CORBA and RMI the structure of the stub objects is

identical to those on the server side. Changing the object structure on the server

side needs a refactoring of the clients. In the case of SOAP, no object structures

are exposed, except for methods that can have objects as parameters and return

values. Changing the object structure on the server side need not lead to a

refactoring of the client as a direct consequence.

7.3 Java Message Service

Messaging middleware is a popular choice for accessing existing enterprise

systems in an asynchronous manner. A standard way for using messaging

middleware from a Java application is using the Java Message Service (JMS)

interface. JMS offers Java programmers a common way to create, send, receive

and read enterprise messages. The JMS specification was developed by Sun

Microsystems with the active involvement of IBM, other enterprise messaging

vendors, transaction processing vendors, and RDBMS vendors.

In IBM WebSphere Application Server V5.0, the J2EE 1.3 specification is

implemented (including JMS 1.0 and EJB 2.0).

According to the JMS 1.0 specification, a message provider is integrated in an

application server. As shown in Figure 7-2 on page 129, the integrated message

provider makes it possible to communicate asynchronously with other

WebSphere applications without installing separate messaging software like IBM

WebSphere MQ. WebSphere Application Server’s integrated JMS server is

based on IBM WebSphere MQ.

Chapter 7. Technology options 129

Figure 7-2 Integrated JMS Provider

An important new feature of EJB 2.0 is message-driven beans (MDB).

Message-driven beans are designed specifically to handle incoming JMS

messages. Further information about message-driven beans can be found in the

IBM Redbook EJB 2.0 Development with WebSphere Studio Application

Developer, SG24-6819.

7.3.1 What messaging is

Messaging is a form of communication between two or more software

applications or components. One strength of messaging is application

integration. Messaging communication is loosely coupled, as compared to tightly

coupled technologies such as Remote Method Invocation (RMI) or Remote

Procedure Calls (RPC). The sender does not need to know anything about the

receiver for communication. The message to be delivered is sent to a destination

(queue or topic) by a sender component, and the recipient picks it up from there.

Moreover, the sender and receiver do not both have to be available at the same

time to communicate.

JMS has two messaging styles or, in other words, two domains:

򐂰 One-to-one, or point-to-point model

򐂰 Publish/subscribe model

J2EE ServerJ2EE Server

Message

PUT

Message

GET

J2EE Application

Web

Message

GET

Message

PUT

J2EE Application

Web

130 Patterns: Serial and Parallel Processes

7.3.2 JMS and IBM WebSphere MQ

When you want to integrate with an application not based on IBM WebSphere

Application Server V5.0, an external JMS Provider is needed. IBM WebSphere

MQ V5.3 includes built-in JMS Provider support with enhanced performance

features for integrating JMS applications with other applications.

WebSphere MQ enables application integration by allowing business

applications to exchange information across different platforms, sending and

receiving data as messages. WebSphere MQ takes care of network interfaces,

ensures once-and-once-only delivery of messages, deals with communications

protocols, dynamically distributes workload across available resources, and

handles recovery after system problems.

7.3.3 Advantages of JMS

The JMS standard is important because:

򐂰 It is the first enterprise messaging API that has achieved wide cross-industry

support.

򐂰 It simplifies the development of enterprise applications by providing standard

messaging concepts and conventions that apply across a wide range of

enterprise messaging systems.

򐂰 It leverages existing, enterprise-proven messaging systems.

򐂰 It allows you to extend existing message-based applications by adding new

JMS clients that are integrated fully with their existing non-JMS clients.

򐂰 Developers have to learn only one common interface for accessing diverse

messaging systems.

7.3.4 Disadvantages of JMS

Though JMS provides a common interface for Java applications to interact with

messaging systems, it lack some specific functionality offered by the messaging

vendor. In that case, you might still have to write vendor-specific code to access

such functionality.

JMS only provides asynchronous messaging, so the design is more complex

when addressing response correlation, error handling, and data synchronization.

Further information about JMS can be found in the IBM Redbook MQSeries

Programming Patterns, SG24-6506.