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< title > ActiveMQ JMS Failback using Static selectors Example< / title >
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< h1 > JMS Multiple Failover using Replication Example< / h1 >
< p > This example demonstrates three servers coupled as a live-backup-backup group for high availability (HA) using replication, and a client
connection failing over from live to backup when the live server is crashed and then to the second backup once the new live fails.< / p >
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< p > For more information on ActiveMQ failover and HA, and clustering in general, please see the clustering
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section of the user manual.< / p >
< h2 > Example step-by-step< / h2 >
< p > < i > To run the example, simply type < code > mvn verify< / code > from this directory< / i > < / p >
< p > In this example, the live server is server 1, and the backup server is server 0< / p >
< p > The connection will initially be created to server1, server 1 will crash, and the client will carry on
seamlessly on server 0, the backup server.< / p >
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< li > Get an initial context for looking up JNDI from server #0.< / li >
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initialContext = getContext(0);
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< li > Look up the JMS resources from JNDI on server #0.< / li >
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Queue queue = (Queue)initialContext.lookup("/queue/exampleQueue");
ConnectionFactory connectionFactory = (ConnectionFactory)initialContext.lookup("/ConnectionFactory");
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< li > Create a JMS Connection< / li >
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connection = connectionFactory.createConnection();
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< li > Create a client acknowledged Session< / li >
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Session session = connection.createSession(false, Session.CLIENT_ACKNOWLEDGE);
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< li > Start the connection to ensure delivery occurs< / li >
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connection.start();
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< li > Create a JMS MessageProducer< / li >
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MessageProducer producer = session.createProducer(queue);
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< li > Create a JMS MessageConsumer< / li >
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MessageConsumer consumer = session.createConsumer(queue);
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< li > Send some messages to server #0, the live server< / li >
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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());
}
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< li > Receive and acknowledge a third of the sent messages< / li >
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TextMessage message0 = null;
for (int i = 0; i < numMessages / 3 ; i + + )
{
message0 = (TextMessage)consumer.receive(5000);
System.out.println("Got message: " + message0.getText());
}
message0.acknowledge();
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< p > When server #0 crashes, the client automatically detects the failure and automatically
fails over from server #0 to server #1 (in your real program you wouldn't need to sleep).
< / p >
< li > Acknowledging the 2nd third of the sent messages will fail as failover to the backup server has occurred< / li >
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try
{
message0.acknowledge();
}
catch (JMSException e)
{
System.err.println("Got exception while acknowledging message: " + e.getMessage());
}
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< li > Consume again the 2nd third of the messages again. Note that they are not considered as redelivered.< / li >
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for (int i = numMessages / 3; i < (numMessages / 3) * 2; i++)
{
message0 = (TextMessage)consumer.receive(5000);
System.out.printf("Got message: %s (redelivered?: %s)\n", message0.getText(), message0.getJMSRedelivered());
}
message0.acknowledge();
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< p > now we Kill server #1 and wait for the server #2 to take over the responsibilities.< / p >
< li > Acknowledging the 3rd third of the sent messages will fail as failover to the backup server has occurred< / li >
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try
{
message0.acknowledge();
}
catch (JMSException e)
{
System.err.println("Got exception while acknowledging message: " + e.getMessage());
}
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< li > Consume again the 3rd third of the messages again. Note that they are not considered as redelivered< / li >
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for (int i = (numMessages / 3) * 2; i < numMessages ; i + + )
{
message0 = (TextMessage)consumer.receive(5000);
System.out.printf("Got message: %s (redelivered?: %s)\n", message0.getText(), message0.getJMSRedelivered());
}
message0.acknowledge();
< / pre >
< li > And finally, < strong > always< / strong > remember to close your resources after use, in a < code > finally< / code > block. Closing a JMS connection will automatically close all of its sessions, consumers, producer and browser objects< / li >
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finally
{
if (connection != null)
{
connection.close();
}
if (initialContext != null)
{
initialContext.close();
}
}
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