SAC: message delivery halts when a second consumer with a higher priority is introduced

[elhectormento]

Describe the bug

When using a quorum queue with the following arguments:

• x-single-active-consumer = true
• x-delivery-limit = -1

A first consumer is created with priority = 0 and QoS = 1. This consumer starts and requeues the delivered messages.
A second consumer is created with priority = 100.
Message delivery becomes stuck. No messages are delivered to either consumer after the second one is introduced.

Reproduction steps

1. Create a quorum queue with:

• x-single-active-consumer = true
• x-delivery-limit = -1.

2. Enqueue several messages.
3. Start Consumer A with:

• priority = 0
• QoS = 1
• Behavior: requeues all delivered messages.

4. Wait until at least one message is requeued.
5. Start Consumer B with:

• priority = 100

6. Observe that message delivery stops entirely.

Expected behavior

After Consumer B (with higher priority) is started, message delivery should switch to it, as per the x-single-active-consumer and priority settings.

Additional context

RabbitMQ versions tested:

• 4.0.9
• 4.1.0

rabbitmq.log

A test to check this issue:

import static org.assertj.core.api.Assertions.assertThat;
import static org.awaitility.Awaitility.await;

import com.rabbitmq.client.Channel;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.ConnectionFactory;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;
import org.testcontainers.containers.RabbitMQContainer;
import org.testcontainers.junit.jupiter.Container;
import org.testcontainers.junit.jupiter.Testcontainers;

@Testcontainers
class SingleActiveConsumerWithPrioTest {

  @Container
  private static final RabbitMQContainer rabbitMQContainer =
      new RabbitMQContainer("rabbitmq:4.1-alpine");

  private static final String QUEUE_NAME = "test-queue";

  private static ConnectionFactory connectionFactory;
  private Connection connection;

  @BeforeAll
  static void setUpAll() throws Exception {
    createConnectionFactory();
    prepareQueue();
  }

  static void createConnectionFactory() {
    connectionFactory = new ConnectionFactory();
    connectionFactory.setHost(rabbitMQContainer.getHost());
    connectionFactory.setPort(rabbitMQContainer.getAmqpPort());
    connectionFactory.setUsername(rabbitMQContainer.getAdminUsername());
    connectionFactory.setPassword(rabbitMQContainer.getAdminPassword());
  }

  static void prepareQueue() throws Exception {
    try (Connection connection = connectionFactory.newConnection()) {
      try (var channel = connection.createChannel()) {
        Map<String, Object> arguments = new HashMap<>();
        arguments.put("x-queue-type", "quorum");
        arguments.put("x-single-active-consumer", true);
        arguments.put("x-delivery-limit", -1);
        channel.queueDeclare(QUEUE_NAME, true, false, false, arguments);
        for (int i = 0; i < 100; i++) {
          // Publish some test messages
          channel.basicPublish("", QUEUE_NAME, null, ("Test message " + i).getBytes());
        }
      }
    }
  }

  @BeforeEach
  void setUp() throws Exception {
    connection = connectionFactory.newConnection();
  }

  @AfterEach
  void tearDown() throws Exception {
    if (connection != null && connection.isOpen()) {
      connection.close();
    }
  }

  @Test
  @DisplayName(
      "Given a consumer that requeues messages, when another consumer is created with more priority, then consuming gets stuck")
  void givenAConsumerThatRequeue_whenOtherConsumerIsCreatedWithMorePriority_thenConsumingGetsStuck()
      throws Exception {
    // given
    Channel channel1 = connection.createChannel();
    channel1.basicQos(1); // Set QoS to 1 to ensure single message delivery
    Map<String, Object> args1 = new HashMap<>();
    args1.put("x-priority", 0); // Lower priority for the first consumer
    AtomicInteger counterConsumer1 = new AtomicInteger(0);
    channel1.basicConsume(
        QUEUE_NAME,
        false,
        args1,
        (consumerTag, message) -> {
          System.out
              .format(
                  "Message received for consumer 1 %s: %s",
                  consumerTag, new String(message.getBody()))
              .println();
          counterConsumer1.incrementAndGet();
          // Simulate processing time
          try {
            Thread.sleep(1000);
          } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
          }
          channel1.basicNack(
              message.getEnvelope().getDeliveryTag(), false, true); // Requeue the message
        },
        consumerTag -> {});

    // Waits to see that the first consumer is consuming
    System.out.println("First consumer should consuming...");
    await().pollDelay(10, TimeUnit.SECONDS).timeout(11, TimeUnit.SECONDS).until(() -> true);

    assertThat(counterConsumer1.get()).isGreaterThan(0); // Ensure the first consumer has processed
    // at least one message

    // when
    Channel channel2 = connection.createChannel();
    Map<String, Object> args2 = new HashMap<>();
    args2.put("x-priority", 100); // Higher priority for the second consumer
    AtomicInteger counterConsumer2 = new AtomicInteger(0);
    channel2.basicConsume(
        QUEUE_NAME,
        false,
        args2,
        (consumerTag, message) -> {
          System.out
              .format(
                  "Message received for consumer 2 %s: %s",
                  consumerTag, new String(message.getBody()))
              .println();
          // Simulate processing time
          try {
            Thread.sleep(1000);
          } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
          }
          counterConsumer2.incrementAndGet();
          channel2.basicAck(message.getEnvelope().getDeliveryTag(), false);
        },
        consumerTag -> {});

    int consumption1 = counterConsumer1.get();

    // Waits to see if second consumer is consuming
    System.out.println("Second consumer should consuming...");
    await().pollDelay(10, TimeUnit.SECONDS).timeout(11, TimeUnit.SECONDS).until(() -> true);

    // then
    // The second consumer does not consume any messages
    assertThat(counterConsumer2.get()).isZero();
    // The first consumer stopped after second consumer started
    assertThat(counterConsumer1.get()).isEqualTo(consumption1);

    // cleanup
    channel1.close();
    channel2.close();
  }
}

[michaelklishin]

@elhectormento from the Single Active Consumer documentation:

If the queue is a quorum queue, and a new consumer registers with a higher priority, then the queue stops delivering messages to the current active consumer. 

When all messages are acknowledged, the new consumer becomes the active consumer.

Perhaps you have missed this part specifically:

When all messages are acknowledged, the new consumer becomes the active consumer.

We cannot suggest anything else without knowing what RabbitMQ version is used and logs from the node those consumes connect to, and if there are multiple nodes, all of them.

[michaelklishin]

There is a certain logic to it: the SAC switches the active consumer only when there are no pending unacknowledged deliveries to the current active consumer. Under a realistic scenario where a new consumer comes online but the previous one does not get stuck, eventually the switch will happen.

Pausing new deliveries to the current consumer can be the necessary measure to prevent the stream of deliveries to the current, lower priority consumer never ending.

"The switch will happen immediately" is a guarantee that the documentation explicitly does not advertise, and 99% of systems wouldn't care about. The remaining 1% will have to find a workaround.

[elhectormento]

I've updated the issue with the version and logs.
The versions where I checked it are 4.0.9 and 4.1.0.
About the pending unacknowledged deliveries, I'm doing a NACK with requeue, so I understand that there aren't any pending.

[michaelklishin]

@acogoluegnes also points out that the redelivery loop protection that quorum queues have, can play a role in a test like this.

If the current SAC keeps requeueing its deliveries, this will create a condition where there are always pending deliveries to the same active consumer, so the redelivery loop protection will kicks in, and the flow of deliveries will stop.

[elhectormento]

I check that removing the x-delivery-limit param the result is the same.

[acogoluegnes]

@kjnilsson confirmed it is a bug, I'll create a follow-up issue.

[acogoluegnes]

PR with a fix (requires some more work): #14145.