JBoss Messaging implements fully transparent automatic failover of connections from a live to backup node, this requires no special coding for failover, and is described in a different example. Automatic failover requires server replication.
However, JBoss Messaging also supports Application-Layer failover, useful in the case that replication is not enabled on the server side.
With Application-Layer failover, it's up to the application to register a JMS ExceptionListener with JBoss Messaging which will be called by JBoss Messaging in the event that connection failure is detected.
The code in the ExceptionListener then recreates the JMS Connection, Session, etc on another node and the application can continue.
Application-Layer failover is an alternative approach to High Availabilty (HA).
Application-Layer failover differs from automatic failover in that some client side coding is required in order to implement this. Also, with Application-Layer failover, since the old Session object dies and a new is created, any uncommitted work in the old Session will be lost, and any unacknowledged messages might be redelivered.
For more information on JBoss Messaging failover and HA, and clustering in general, please see the clustering section of the user manual.
To run the example, simply type ant
from this directory
In this example, the live server is server 1, which will failover onto server 0.
The connection will initially be created to server1, server 1 will crash, and the client will carry on on server 0, the new server. With Application-Layer failover the node that is failed over onto, does not need to be specially configured as a backup server, it can be any node.
createJMSObjects(1);
connection.setExceptionListener(new ExampleListener());
final int numMessages = 10;
for (int i = 0; i < numMessages; i++)
{
TextMessage message = session.createTextMessage("This is text message " + i);
producer.send(message);
System.out.println("Sent message: " + message.getText());
}
for (int i = 0; i < numMessages; i++)
{
TextMessage message0 = (TextMessage)consumer.receive(5000);
System.out.println("Got message: " + message0.getText());
}
killServer(1);
private class ExampleListener implements ExceptionListener
{
public void onException(JMSException exception)
{
try
{
// Close the old resources
closeResources();
// Create new JMS objects on the backup server
createJMSObjects(0);
failoverLatch.countDown();
}
catch (Exception e)
{
System.err.println("Failed to handle failover");
e.printStackTrace();
}
}
}
for (int i = numMessages; i < numMessages * 2; i++)
{
TextMessage message = session.createTextMessage("This is text message " + i);
producer.send(message);
System.out.println("Sent message: " + message.getText());
}
for (int i = 0; i < numMessages; i++)
{
TextMessage message0 = (TextMessage)consumer.receive(5000);
System.out.println("Got message: " + message0.getText());
}
finally
block. Closing a JMS connection will automatically close all of its sessions, consumers, producer and browser objects
finally
{
closeResources();
}