Question

There might be a stupid simple answer to this, but I’m trying to use ActiveMQ to pass messages between producers and consumers. I will have many producers and many consumers, but I want each message to be delivered only once among the consumers. This would seem to mean that I cannot use Topics, since they would deliver messages to all consumers who are listening, and I want only one Consumer to receive each message.

My problem is that I am able to receive messages, but the messages are not dequeued. So if I restart my consumer process, all of the messages are reprocessed. This answer seems pertinent but does not seem to apply since I can’t create durable queue consumers, only durable topic consumers (unless I’m missing something in the API docs).

My code is as follows.

TopicConnectionFactory factory = new ActiveMQConnectionFactory(props.getProperty("mq.url"));
Connection conn  = factory.createConnection();
Session session = conn.createSession(true, Session.CLIENT_ACKNOWLEDGE);
Queue queue = session.createQueue(props.getProperty("mq.source_queue"));
conn.start();
MessageConsumer consumer = session.createConsumer(queue);

Then later on

Message msg = consumer.receive();
msg.acknowledge();
if (!(msg instanceof TextMessage)) continue;
String msgStr = ((TextMessage)msg).getText();

This is my current code. I have tried with Session.AUTO_ACKNOWLEDGE and without msg.acknowledge(). Any working permutation of this code seems to retrieve the messages, but when I restart my consumer, all of the messages get received again, even if they have been received prior to the restart.

Answer 1

You created the session as a transacted Session and therefore need to call, session.commit if you want to inform the broker that all messages are now consumed and don’t need to be redelivered. If you don’t set the first argument to createSession to true then the Ack mode is respected otherwise its ignored, one of the oddities of the JMS API I’m afraid. If you do this:

ConnectionFactory factory = new ActiveMQConnectionFactory(props.getProperty("mq.url"));
Connection conn  = factory.createConnection();
Session session = conn.createSession(false, Session.CLIENT_ACKNOWLEDGE);
Queue queue = session.createQueue(props.getProperty("mq.source_queue"));
conn.start();
MessageConsumer consumer = session.createConsumer(queue);

Then this would work:

Message msg = consumer.receive();
msg.acknowledge();

Otherwise you need to do:

Message msg = consumer.receive();
session.commit(); 

But keep in mind that for a single message transactions don’t really make sense to client ack with no transaction is a better option.