Asynchronous Messaging

Asynchronous Messaging

In addition to supporting RMI-based distributed business objects, Enterprise JavaBeans supports asynchronous messaging. An asynchronous messaging system allows two or more applications to exchange information in the form of messages. A message, in this case, is a self-contained package of business data and network routing headers. The business data contained in a message can be anythingdepending on the business scenarioand usually contains information about some business transaction. In enterprise systems, messages inform an application of some event or occurrence in another system.

Asynchronous messages may be transmitted from one application to another on a network using message-oriented middleware (MOM). MOM products ensure that messages are properly distributed among applications. In addition, MOM usually provides fault-tolerance, load-balancing, scalability, and transactional support for enterprises that need to reliably exchange large quantities of messages. MOM vendors use different message formats and network protocols for exchanging messages, but the basic semantics are the same. An API is used to create a message, give it a payload (application data), assign it routing information, and then send the message. The same API is used to receive messages produced by other applications.

In modern enterprise-messaging systems, applications exchange messages through virtual channels called destinations . When you send a message, it's addressed to a destination, not to a specific application. Any application that subscribes to or registers an interest in that destination may receive that message. In this way, the applications that receive messages and those that send messages are decoupled. Senders and receivers are not bound to each other in any way and may send and receive messages as they see fit.

Enterprise JavaBeans integrates the functionality of MOM into its component model. This integration extends the EJB platform so that it supports both RMI and asynchronous messaging. EJB 3.0 supports asynchronous messaging through the Java Message Service (JMS) and a new component called the message-driven bean. In addition to JMS, message-driven beans can support other synchronous and asynchronous messaging systems.

Java Message Service

Each MOM vendor implements its own networking protocols, routing, and administration facilities, but the basic semantics of the developer API provided by different MOMs are the same. It's this similarity in APIs that makes the Java Message Service (JMS) possible.

JMS is a vendor-agnostic Java API that can be used with many different MOM vendors. JMS is very similar to JDBC in that an application developer can reuse the same API to access many different systems. If a vendor provides a compliant service provider for JMS, the JMS API can be used to send messages to and receive messages from that vendor. For example, you can use the same JMS API to send messages with Progress's SonicMQ as with IBM's MQSeries.

Message-Driven Beans and JCA 1.5

Enterprise JavaBeans 2.0 introduced a new kind of component, called a message-driven bean, which is a kind of standard JMS bean. It can receive and send asynchronous JMS messages, and can easily interact with other EJBs.

EJB 2.1 extended the programming model of the message-driven bean beyond JMS to any messaging system. While EJB vendors must continue to support JMS-based message-driven beans (JMS-MDBs), other types of messaging systems are also allowed. It's likely that vendors will develop new message-driven bean types to support all kinds of protocols, including SMTP for email, SNMP for device control, peer-to-peer protocols (e.g., BEEP and Jabber), and many other open and proprietary messaging systems. In addition, the message-driven bean has become an elegant option for serving connections to legacy transaction processing systems like CICS, IMS, openUTM, and others.

The expansion of message-driven beans in EJB 2.1 to other protocols was made possible by the new Java EE Connector Architecture (JCA 1.5), which defines a portable programming model for interfacing with enterprise information systems. The use of JCA in Java EE is analogous to the use of USB in computer hardware. A computer that supports USB can interface with just about any USB-compliant device. Similarly, an EJB 3.0 container that supports JCA 1.5 can interface with any JCA 1.5-compliant resource. For example, if Vendor XYZ creates a new message-driven bean component for its proprietary messaging system based on JCA 1.5, that component will be portable across all EJB 2.1 and higher-compliant servers. Figure illustrates how a JCA connector for a messaging system integrates with EJB.

EJB 3.0 message-driven beans and JCA 1.5

Message-driven beans in EJB 3.0 allow other applications to send messages that can be captured and processed by the EJB application. This feature allows EJB applications to better integrate with legacy and other proprietary systems.