Creating an Example Queue
The following example code illustrates a distributed queue that connects a producer and consumer.
Putting Items on the Queue
Let's put one integer on the queue every second, 100 integers total.
import com.hazelcast.core.Hazelcast;
import com.hazelcast.core.HazelcastInstance;
import com.hazelcast.core.IQueue;
public class ProducerMember {
public static void main( String[] args ) throws Exception {
HazelcastInstance hz = Hazelcast.newHazelcastInstance();
IQueue<Integer> queue = hz.getQueue( "queue" );
for ( int k = 1; k < 100; k++ ) {
queue.put( k );
System.out.println( "Producing: " + k );
Thread.sleep(1000);
}
queue.put( -1 );
System.out.println( "Producer Finished!" );
}
}
Producer puts a -1 on the queue to show that the put's are finished.
Taking Items off the Queue
Now, let's create a Consumer class to take a message from this queue, as shown below.
import com.hazelcast.core.Hazelcast;
import com.hazelcast.core.HazelcastInstance;
import com.hazelcast.core.IQueue;
public class ConsumerMember {
public static void main( String[] args ) throws Exception {
HazelcastInstance hz = Hazelcast.newHazelcastInstance();
IQueue<Integer> queue = hz.getQueue( "queue" );
while ( true ) {
int item = queue.take();
System.out.println( "Consumed: " + item );
if ( item == -1 ) {
queue.put( -1 );
break;
}
Thread.sleep( 5000 );
}
System.out.println( "Consumer Finished!" );
}
}
As seen in the above example code, Consumer waits 5 seconds before it consumes the next message. It stops once it receives -1. Also note that Consumer puts -1 back on the queue before the loop is ended.
When you first start Producer and then start Consumer, items produced on the queue will be consumed from the same queue.
Balancing the Queue Operations
From the above example code, you can see that an item is produced every second, and consumed every 5 seconds. Therefore, the consumer keeps growing. To balance the produce/consume operation, let's start another consumer. By this way, consumption is distributed to these two consumers, as seen in the sample outputs below.
The second consumer is started. After a while, here is the first consumer output:
...
Consumed 13
Consumed 15
Consumer 17
...
Here is the second consumer output:
...
Consumed 14
Consumed 16
Consumer 18
...
In the case of a lot of producers and consumers for the queue, using a list of queues may solve the queue bottlenecks. In this case, be aware that the order of the messages sent to different queues is not guaranteed. Since in most cases strict ordering is not important, a list of queues is a good solution.
NOTE: The items are taken from the queue in the same order they were put on the queue. However, if there is more than one consumer, this order is not guaranteed.