Example_Delay.md

# Schedule example

### 1 Start consumer to wait for incoming subscribed messages 

```java
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 

```java
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

```java 
// 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`