ActiveMQ some consumers not picking up tasks if they arrive after producer

Question

Im just getting starting with ActiveMQ and i seem to have a weird problem. (Source below)

There are 2 scenarios

Consumers connect to broker, waits for tasks on the queue. Producer arrives later, drops the list of tasks and they are rightly taken up by the different consumers and performed. This works fine and i have simulated it as well.
Producer connects first, drops the list of tasks. No consumers are connected at this point. Now when lets say 3 consumers - C1, C2 and C3 connect to the broker (in that order) i see that only C1 picks up and does the tasks that are dropped by the producer. C2 and C3 stay idle. Why does this happen?

I've also noticed 1 more thing in regards to the 2nd scenario -- If the producer keeps on dropping tasks inside the queue, C2 and C3 pick up tasks but if the producer has dropped tasks before (as mentioned) then C2 and C3 dont do anything.

Producer code

package com.activemq.apps;

import javax.jms.Connection;
import javax.jms.ConnectionFactory;
import javax.jms.Destination;
import javax.jms.JMSException;
import javax.jms.Message;
import javax.jms.MessageConsumer;
import javax.jms.MessageListener;
import javax.jms.MessageProducer;
import javax.jms.Session;
import javax.jms.TextMessage;

import org.apache.activemq.ActiveMQConnectionFactory;

import com.commons.helpers.Maths;

public class Publisher implements MessageListener {

    private static String _URL;
    private static String _TOPIC_PUBLISH;
    private static String _TOPIC_CONSUME;

    public Publisher (String URL, String TOPIC) {

        _URL = URL;
        _TOPIC_PUBLISH = TOPIC + "_REQUESTS";
        _TOPIC_CONSUME = TOPIC + "_RESPONSES";

    }

    public void initialize() {

        try
        {

            ConnectionFactory connectionFactory = new ActiveMQConnectionFactory(_URL);
            Connection connection = connectionFactory.createConnection();
            connection.start();

            Session session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);

            Destination destinationProducer = session.createQueue(_TOPIC_PUBLISH);
            Destination destinationConsumers = session.createQueue(_TOPIC_CONSUME);

            MessageProducer producer = session.createProducer(destinationProducer);
            MessageConsumer consumer = session.createConsumer(destinationConsumers);

            consumer.setMessageListener(this);

            int count = 0;

            System.out.println("Sending requests");

            while (true)
            {
                int randomSleepTime = Maths.rand(1000, 5000);

                String messageToSend = count + "_" + randomSleepTime;

                TextMessage message = session.createTextMessage(messageToSend);

                producer.send(message);

                System.out.println("Job #" + count + " | " + (randomSleepTime/1000) + "s");

                if (count++%10 == 0)
                    Thread.sleep(10000);

            }
        }

        catch (JMSException ex)
        {
            ex.printStackTrace();
        }

        catch (InterruptedException e) {
            e.printStackTrace();
        }

    }

    @Override
    public void onMessage(Message message) {

        if (message instanceof TextMessage)
        {
            TextMessage msg = (TextMessage) message;

            try {

                String response = msg.getText();
                String[] responseSplit = response.split("_");

                String clientId = responseSplit[1];
                String count = responseSplit[0];

                System.out.println("Got response from " + clientId + " Job #" + count);
            } 

            catch (JMSException e) {
                e.printStackTrace();
            }
        }

    }

}

Consumer code

package com.activemq.apps;

import java.util.UUID;

import javax.jms.Connection;
import javax.jms.ConnectionFactory;
import javax.jms.Destination;
import javax.jms.JMSException;
import javax.jms.Message;
import javax.jms.MessageConsumer;
import javax.jms.MessageListener;
import javax.jms.MessageProducer;
import javax.jms.Session;
import javax.jms.TextMessage;

import org.apache.activemq.ActiveMQConnectionFactory;

public class Consumer implements MessageListener {

    private static String _URL;
    private static String _TOPIC_PUBLISH;
    private static String _TOPIC_CONSUME;
    private static String _CLIENTID;

    private MessageProducer producer;
    private Session session;

    public Consumer (String URL, String TOPIC) {

        _URL = URL;
        _TOPIC_PUBLISH = TOPIC + "_RESPONSES";
        _TOPIC_CONSUME = TOPIC + "_REQUESTS";

    }

    public void initialize() {

        try
        {

            _CLIENTID = UUID.randomUUID().toString();

            ConnectionFactory connectionFactory = new ActiveMQConnectionFactory(_URL);
            Connection connection = connectionFactory.createConnection();
            connection.start();

            session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);

            Destination destinationProducer = session.createQueue(_TOPIC_PUBLISH);
            Destination destinationConsumers = session.createQueue(_TOPIC_CONSUME);

            producer = session.createProducer(destinationProducer);
            MessageConsumer consumer = session.createConsumer(destinationConsumers);

            consumer.setMessageListener(this);

            System.out.println("Client: " + _CLIENTID + "\nWaiting to pick up tasks");
        }

        catch (JMSException ex)
        {
            ex.printStackTrace();
        }

    }

    @Override
    public void onMessage(Message message) {

        if (message instanceof TextMessage)
        {
            TextMessage msg = (TextMessage) message;

            try 
            {

                String[] messageSplits = msg.getText().split("_");

                String count = messageSplits[0];
                String timeString = messageSplits[1];

                int sleepFor = Integer.parseInt(timeString);

                System.out.println("Job #" + count + " | Sleeping for " + (sleepFor/1000) + "s");

                Thread.sleep(sleepFor);

                TextMessage sendToProducer = session.createTextMessage(count + "_" + _CLIENTID);

                producer.send(sendToProducer);
            } 

            catch (JMSException e) {
                e.printStackTrace();
            } 

            catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

    }

}

score 1 · Accepted Answer · answered Jul 04 '13 at 02:37

1

You have mentioned

Now when lets say 3 consumers - C1, C2 and C3 connect to the broker (in that order)

Since C1 connects first it starts getting all the messages on the queue immediately after it connects. That is expected. So i dont see any issue here. C2, C3 are not idle but C1 has got hold of the messages before C2 and C3 could.

I am not sure how many messages were sent by the producer. I assume the number of messages are less. To see what you expect try sending many messages from the producer, like thousands or millions and then start the consumers. The high number of messages are subjective and depends on the memory, network and other resources. You might see what you are expecting.

answered Jul 04 '13 at 02:37

techuser soma

4,766
5
23
43

I understand. I am sending 10 messages from the producer to the queue. What do you mean "C1 has got hold of the messages before C2 and C3 could" ? I see in the admin screen that messages are dequeue'd one by one by C1. I will try sending the high number of messages right now and get back to you. – Shrayas Jul 04 '13 at 04:39
Went back and increased the number of messages to 2000. Now C2 and C3 are picking up tasks too. Like you said, it must be a cap on the number of messages. – Shrayas Jul 04 '13 at 05:56

score 0 · Answer 2 · answered Jul 03 '13 at 05:15

0

I dont think there is anything weird here. Queues represent P2P mode which is supposed to have only one consumer. In our case we have 3 consumers, it's not forbidden, but there is no guarantee of any specific order in which consumers will receive messages.

answered Jul 03 '13 at 05:15

Evgeniy Dorofeev

133,369
30
199
275

I see. So the basic implementation itself is wrong? Should i be using topics instead then ? – Shrayas Jul 03 '13 at 05:23
Also, would you recommend some place where I can understand these concepts in a more clear fashion? Thanks in advance – Shrayas Jul 03 '13 at 05:26
I dont say its wrong. Topic will work differently, all 3 consumers will be getting the same messages. If you want to organize parallel processing Queue seems good. There is an alternative to MesssageListener. You can run consumers in parallel threads, each reads in a cycle with MessageConsumer.read(timeout). – Evgeniy Dorofeev Jul 03 '13 at 05:39
Try http://docs.oracle.com/javaee/6/api/ package description, it's at the bottom of the page – Evgeniy Dorofeev Jul 03 '13 at 05:45
Oh allright, will try that out too. But i can't seem to understand how only the first guy (C1) picks up the tasks while the others just sit idle and the instant more tasks are added by the producer, C2 and C3 kick into action. If they can dequeue tasks, shouldn't they do it on the first set of tasks as well? – Shrayas Jul 03 '13 at 06:53
Possibly, if you have some msgs on the queue and then if C1 connects first it takes all messages off queue and C2 and C3 which connect later dont get anything. – Evgeniy Dorofeev Jul 03 '13 at 06:58
Actually, I see the messages being dequeue'd one by one only in the admin console which means that C1 didnt take *all* the messages of the queue – Shrayas Jul 03 '13 at 10:22

score 0 · Answer 3 · answered Aug 22 '19 at 06:58

I believe you should check prefetchPolicy param for your consumer. By default, prefetch policy has default value 1000. It means that the first consumer fetches up to 1000 messages and other consumers are not able to fetch anything else. If you are trying load balance your messages across consumers consider to modify this param to 0 or 1.

ActiveMQ some consumers not picking up tasks if they arrive after producer

3 Answers3