O'Reilly - Java Message Service

Java Message Service
Figure 1.7. Integrating two different purchase order systems with an ERP system
What is interesting about this example is that the ERP Gateway is unaware that it is
receiving purchase order messages from two completely different sources. The legacy EDI
system may be an older in-house system or it could be the main system for a business
partner or a recently acquired subsidiary. In addition, the legacy EDI system would have
been added dynamically without requiring the shutdown and retooling of the entire system.
Enterprise messaging systems make this kind of flexibility possible, and JMS allows Java
clients to access many different MOMs using the same Java programming model.
1.4 RPC Versus Asynchronous Messaging
RPC (Remote Procedure Call) is a term commonly used to describe a distributed
computing model that is used today by middleware technologies such as CORBA, Java
RMI, and Microsoft's DCOM. Component-based architectures such as Enterprise
JavaBeans are built on top of this model. RPC-based technologies have been, and will
continue to be, a viable solution for many applications. However, the enterprise messaging
model is superior in certain types of distributed applications. In this section we will discuss
the pros and cons of each model. In Chapter 8, J2EE, EJB, and JMS, we will discuss a
means of combining the two.
1.4.1 Tightly Coupled RPC
One of the most successful areas of the tightly coupled RPC model has been in building 3tier,
or n -tier applications. In this model, a presentation layer (1 st tier), communicates
using RPC with business logic on the middle tier (2 nd tier), which accesses data housed on
the back end (3 rd tier). Sun Microsystems' J2EE platform and Microsoft's DNA are the
most modern examples of this architecture.
15