Asynchronous Messaging

Overview

Motivation

• Performance
  • no need to block when performing a request
  • fire-and-forget
• Reliability
  • middleman
  • guaranteed delivery possible
• Support for multiple senders and receivers

Aspects

• Java Message Service (JMS)
  • vendor-neutral API to access enterprise messaging systems
• Message-Driven Beans
  • stateless, server-side, transaction-aware components for processing asynchronous messages

Java Message Service (JMS)

Terms

• JMS client: applications that use JMS
• JMS provider: messaging system which is responsible for sending and routing of the messages
• JMS application: business system which consists of several clients and usually one provider
• MessageProducer: JMS client that sends messages
• MessageConsumer: JMS client that receives messages
• Messaging Service: is used to send messages from within EJB

Messaging Models

• Publish/Subscribe: several producers --> Topic --> several consumers
• Point-To-Point: several producers --> Queue --> one consumer

JMS API

• Locate the JMS driver: ConnectionFactory
• Create a JMS connection: Connection
• Create a JMS session: Session
• Locate the JMS destination: Destination (Queue/ Topic)
• Create JMS producer or consumer: MessageProducer/MessageConsumer
• Send or reveice message

JMS Message

• header: routing information, attributes
• properties: used for filtering (name-value pairs)
• body: the message
  • types: TextMessage, MapMessage, ObjectMessage, StreamMessage, ByteMessage

Deployment Descriptor

• declare a JMS resource
  • EJB 2.0
    • <resource-ref>: JNDI ENC name, interface type and protocol for authorisation (e.g. name=jms/TopicFactroy, type=javax.jms.TopicConnectionFactory, auth=Container)
    • <resoucre-env-ref>: name and type of the managed objects (e.g. name=jms/TicketTopic and type=javax.jms.Topic)
  • EJB 2.1
    • <message-destination-ref>: destination of the messages and usage (Produces, Consumes, ConsumesProduces)

Example

...
TopicConnection tConnection=null;
TopicSession tSession=null;
TopicPublisher tPublisher=null;
try{
 TopicConnectionFactory tConnectionFac=
 (TopicConnectionFactory)Lookup.lookup(„java:comp/env/jms“);
 tConnection=tConnectionFac.createTopicConnection();
 tSession=tConnection.createTopicSession(false,
 Session.AUTO_ACKNOWLEDGE);
 Topic topic=(Topic)Lookup.lookup(„TestTopic“); 
 tPublisher=tSession.createPublisher(topic);
 String text=“Message“;
 Message message=tSession.createTextMessage(text);
 tPublisher.send(message);
} catch (JMSException jes){ ...
} finally {
 // close()
}

Message Driven Beans

Introduction

• does not have a home interface, local home interface, remote interface, local interface
• single, weakly typed business method (onMessage())
• do not have any return values
• cannot send exceptions back to clients
• are stateless
• can be durable or nondurable subscribers

Life Cycle

Message Driven Bean Interface

package javax.ejb;
public interface MessageDrivenBean extends javax.ejb.EnterpriseBean{
 public void setMessageDrivenContext(MessageDrivenContext context)
  throws EJBException;
  public void ejbRemove() throws EJBException;
}

Message Driven Context

• extends EJBContext
• only setRollbackOnly(), getRollbackOnly() make sense
• getEJBHome() and getEJBLocalHome() throw a RuntimeException if invoked
• getCallerPrincipal() and isCallerInRole() throw a RuntimeException if invoked

Message Listener

package javax.jms;
public interface MessageListener {
 public void onMessage(Message message); 
}

Deployment Descriptor

• EJB 2.0
  • declared in <message-driven>; ejb-name, ejb-class, transaction-type as session or entity bean
  • <message-selector>: select which messages are processed
  • <acknowledge-mode>: e.g. auto-acknowledge, dups-ok-acknowledge
  • <subscription-durability>: durable or not (messages are delivered later on if a consumer is disconnected or not)
  • <message-driven-destination>: type of destination (Topic/Queue)

• EJB 2.1
  • <activation-config>: general declaration of properties (e.g. substitutes message-selector, acknowledge-mode and subscription-durability in EJB 2.0)
  • <messaging-type>: javax.jms.MessageListener für JMS
  • <messaging-destination-type>: important for COnnector-Based beans (don't use JMS)

Example

...
 public abstract class MDBExample implements MessageDrivenBean, MessageListener {
  ...
   public void ejbCreate() throws EJBException{
    ... // create Connection, Session, Queue
    qReceiver=qSession.createReceiver(queue);
    qReceiver.setMessageListener(this);
    qConnection.start();
    ...
   }
   public void onMessage(Message message){
    ...
    // if (message instanceof TextMessage){ ... }
    ...
   }
   ... // setMessageDrivenContext(), ejbRemove()
 }

MDB vs. Session & Entity Bean

• Entity and Session Beans should not receive messages
• impossible to write a session or entity bean that is driven by incoming messages
• receive()-method blocks java threads