MDBs in a clustered Domain

Question

Hi,
Can anyone please help me with below question please?
1. Can MDBs actually behave as singleton service in a clsutered domain? How would they behave if implemented using Singleton Interface?
2. If above is possible, what would be the steps for configuring an MDB as a singleton service on a cluster?
What we need to implement is that the MDB should pick up a single message from the queue at a time and only after certain processing on the same, the next message should be taken up so as to maintain the sequence of processing the message as they arrived in the queue.
Thanks,
Himani

Answer 1

The following shows an example of how to implement the singletonservice interface
with message-driven beans.
Create your message-driven bean, for example
package model.logic;
import weblogic.cluster.singleton.SingletonService;
import javax.ejb.ActivationConfigProperty;
import javax.ejb.MessageDriven;
import javax.jms.JMSException;
import javax.jms.Message;
import javax.jms.MessageListener;
import javax.jms.TextMessage;
@MessageDriven(mappedName = "jms/SingletonMDBQueue", activationConfig = {
        @ActivationConfigProperty(propertyName = "destinationName", propertyValue = "jms/SingletonMDBQueue"),
        @ActivationConfigProperty(propertyName = "connectionFactoryJndiName", propertyValue = "jms/SingletonMDBConnectionFactory"),
        @ActivationConfigProperty(propertyName = "destinationType", propertyValue = "javax.jms.Queue")
public class SingletonMDB implements MessageListener, SingletonService {
    private String myResource;
    public SingletonMDB() {
    public void onMessage(Message message) {
        TextMessage text = (TextMessage)message;
        try {
            message.acknowledge();
            System.out.println("received the following message: " + text.getText() + myResource);
        } catch (JMSException e) {
            e.printStackTrace();
     * This method should obtain any system resources and start any services required for the singleton
     * service to begin processing requests. This method is called in the following cases:
     *  - When a newly deployed application is started
     *  - During server start
     *  - During the activation stage of service migration
    public void activate() {
        System.out.println("activating singleton service");
        myResource = "obtaining resources and start required services";
     * This method is called during server shutdown and during the deactivation stage of singleton
     * service migration. This method should release any resources obtained through the activate()
     * method. Additionally, it should stop any services that should only be available from one
     * member of a cluster.
    public void deactivate() {
        System.out.println("deactivating singleton service");
        myResource = null;
}Note that the only resource in the example is a String, which is initiated in the activate method
and released in the deactivate method. Usually, in a message-driven bean you would use the
lifecycle of an message-driven bean by using PostConstruct and PreDestroy annotations to respectivily initiate
the resources and release the resources used by the message-driven bean, for example,
    @PostConstruct
    public void activate() {
        try {
            Context context = new InitialContext();
            mailSession = (Session) context.lookup("mail/VideotheekMailSession");
        } catch (NamingException e) {
            e.printStackTrace();
        try {
            mailMessage = new MimeMessage(mailSession);
            mailMessage.setFrom(new InternetAddress(mailSession.getProperty("mail.from")));
            mailMessage.addRecipient(javax.mail.Message.RecipientType.TO,
                    new InternetAddress(mailSession.getProperty("mail.to")));
            mailMessage.setSubject("New Member Added");
        } catch (MessagingException e) {
            e.printStackTrace();
        try {
            mailTransport = mailSession.getTransport();
        } catch (NoSuchProviderException e) {
            e.printStackTrace();
    @PreDestroy
    public void deactivate() {
        try {
            if (mailTransport != null) {
                mailTransport.close();
        } catch (MessagingException e) {
            e.printStackTrace();
    }In the deployment override weblogic-ejb-jar.xml configure the free beans in the pool, for example
<weblogic-ejb-jar xmlns="http://www.bea.com/ns/weblogic/90" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                  xsi:schemaLocation="http://www.bea.com/ns/weblogic/90
                  http://www.bea.com/ns/weblogic/90/weblogic-ejb-jar.xsd">
    <weblogic-enterprise-bean>
        <ejb-name>SingletonMDB</ejb-name>
        <message-driven-descriptor>
            <pool>
                <initial-beans-in-free-pool>1</initial-beans-in-free-pool>
                <max-beans-in-free-pool>1</max-beans-in-free-pool>
            </pool>
            <destination-jndi-name>jms/SingletonMDBQueue</destination-jndi-name>
        </message-driven-descriptor>
    </weblogic-enterprise-bean>
</weblogic-ejb-jar>The next step is to register the singleton service in the deployment override weblogic-application.xml, for example,
<weblogic-application xmlns="http://www.bea.com/ns/weblogic/90"
                      xmlns:j2ee="http://java.sun.com/xml/ns/j2ee"
                      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                      xsi:schemaLocation="http://www.bea.com/ns/weblogic/90 http://www.bea.com/ns/weblogic/90/weblogic-application.xsd">
    <singleton-service>
        <name>SingletonMDB</name>
        <class-name>model.logic.SingletonMDB</class-name>
    </singleton-service>
</weblogic-application>Now we need to package our singleton service in, for example, an EAR file. The singleton service must
be in either the APP-INF/lib or the APP-INF/classes directory. So we have to make sure that the EJB
JAR file is placed in the APP-INF/lib directory. We have the following structure:
APP-INF
    lib
        Model.jar (contains the message-driven bean class)
META-INF
    application.xml
    weblogic-application.xmlThe file application.xml has the following contents:
<?xml version="1.0" encoding="UTF-8"?>
<application xmlns="http://java.sun.com/xml/ns/javaee"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/application_5.xsd"
             version="5">
    <module>
        <ejb>/APP-INF/lib/Model.jar</ejb>
    </module>
</application>The file Model.jar has the following structure:
META-INF
    ejb-jar.xml
    weblogic-ejb-jar.xml
model
    logic
        SingletonMDB.classBefore you deploy the singleton service, you should configure Migration basis of the cluster.
In the admin console, click Environment, Clusters and choose the cluster to which
you want to deploy the singleton service. Click the Migration Configuration tab.
Select the candidate machine and choose , for example, Migration Basis - Consensus.
Also, adjust the Maximum Messages per Session. By default, WebLogic JMS sets this value
to 10 messages. In your situation you must change this value to 1. (Applications that
need strict ordered processing and are not using the Unit-of-Order feature
must set the value to 1. Setting it to anything else will cause messages to get out of order if
message processing fails and the message goes back to the destination for redelivery.)
The parameter can be set on the Client Configuration tab of your connection factory.
Now you can deploy the EAR file. From the documentation: Deployment of an application-scoped singleton service will happen
automatically as part of the application deployment. The candidate servers for the singleton service will be the cluster members
where the application is deployed. When you look in the logging of the respective servers in the cluster:
Log server1
####<Nov 4, 2010 11:08:53 AM CET> <Info> <Cluster> <lt1379> <Server1> <[STANDBY] ExecuteThread: '3' for queue: 'weblogic.kernel.Default (self-tuning)'> <<WLS Kernel>> <> <> <1288865333078> <BEA-000187> <The Singleton Service SingletonMDB is now registered on this server. This server may be chosen to host this service.> Log server2
####<Nov 4, 2010 11:08:53 AM CET> <Info> <Cluster> <lt1379> <Server2> <[STANDBY] ExecuteThread: '3' for queue: 'weblogic.kernel.Default (self-tuning)'> <<WLS Kernel>> <> <> <1288865333047> <BEA-000187> <The Singleton Service SingletonMDB is now registered on this server. This server may be chosen to host this service.>
####<Nov 4, 2010 11:09:02 AM CET> <Info> <Cluster> <lt1379> <Server2> <[ACTIVE] ExecuteThread: '2' for queue: 'weblogic.kernel.Default (self-tuning)'> <<WLS Kernel>> <> <> <1288865342406> <BEA-000189> <The Singleton Service SingletonMDB is now active on this server.>Some information about migration of the service can be found here:
http://download.oracle.com/docs/cd/E12840_01/wls/docs103/cluster/service_migration.html#wp1051458
and here
http://download.oracle.com/docs/cd/E12840_01/wls/docs103/cluster/service_migration.html#wp1066042