Schedule example

1 Start consumer to wait for incoming subscribed messages

import org.apache.rocketmq.client.consumer.DefaultMQPushConsumer;
import org.apache.rocketmq.client.consumer.listener.ConsumeConcurrentlyContext;
import org.apache.rocketmq.client.consumer.listener.ConsumeConcurrentlyStatus;
import org.apache.rocketmq.client.consumer.listener.MessageListenerConcurrently;
import org.apache.rocketmq.common.message.MessageExt;
import java.util.List;

public class ScheduledMessageConsumer {
public static void main(String[] args) throws Exception {
    // Instantiate message consumer
    DefaultMQPushConsumer consumer = new DefaultMQPushConsumer("ExampleConsumer");
    // Subscribe topics
    consumer.subscribe("TestTopic", "*");
    // Register message listener
    consumer.registerMessageListener(new MessageListenerConcurrently() {
        @Override
        public ConsumeConcurrentlyStatus consumeMessage(List<MessageExt> messages, ConsumeConcurrentlyContext context) {
            for (MessageExt message : messages) {
                // Print approximate delay time period
                System.out.println("Receive message[msgId=" + message.getMsgId() + "] "
                                   + (System.currentTimeMillis() - message.getStoreTimestamp()) + "ms later");
            }
            return ConsumeConcurrentlyStatus.CONSUME_SUCCESS;
        }
    });
    // Launch consumer
    consumer.start();
}

}

2 Send scheduled messages

import org.apache.rocketmq.client.producer.DefaultMQProducer;
import org.apache.rocketmq.common.message.Message;

public class ScheduledMessageProducer {
public static void main(String[] args) throws Exception {
    // Instantiate a producer to send scheduled messages
    DefaultMQProducer producer = new DefaultMQProducer("ExampleProducerGroup");
    // Launch producer
    producer.start();
    int totalMessagesToSend = 100;
    for (int i = 0; i < totalMessagesToSend; i++) {
        Message message = new Message("TestTopic", ("Hello scheduled message " + i).getBytes());
        // This message will be delivered to consumer 10 seconds later.
        message.setDelayTimeLevel(3);
        // Send the message
        producer.send(message);
    }

    // Shutdown producer after use.
    producer.shutdown();
}

}

3 Verification

You should see messages are consumed about 10 seconds later than their storing time.

4 Use scenarios for scheduled messages

For example, in e-commerce, if an order is submitted, a delay message can be sent, and the status of the order can be checked after 1 hour. If the order is still unpaid, the order can be cancelled and the inventory released.

5 Restrictions on the use of scheduled messages

// org/apache/rocketmq/store/config/MessageStoreConfig.java

private String messageDelayLevel = “1s 5s 10s 30s 1m 2m 3m 4m 5m 6m 7m 8m 9m 10m 20m 30m 1h 2h”;

Nowadays RocketMq does not support any time delay. It needs to set several fixed delay levels, which correspond to level 1 to 18 from 1s to 2h. Message consumption failure will enter the delay message queue. Message sending time is related to the set delay level and the number of retries.

See SendMessageProcessor.java