Consistent subscription relationships

Background information

In SOFAStack Message Queue, a consumer Group ID represents a consumer group. In most distributed applications, multiple consumer instances are attached to a single consumer Group ID.

A subscription relationship in a message queue consists of a topic and a tag. To maintain a consistent subscription relationship, all consumer instances within the same consumer Group ID must be consistent in the following two aspects:

• The subscribed topics must be identical.

• The tags for each subscribed topic must be identical.

Example of a correct subscription relationship

Multiple Group IDs subscribe to multiple topics. Within each Group ID, the subscription relationships for all consumer instances are consistent.

Example of an incorrect subscription relationship

A single Group ID subscribes to multiple topics, but the subscription relationships for the consumer instances within that Group ID are inconsistent.

Code examples of incorrect subscription relationships

Example 1

Two consumer instances in the same group subscribe to different topics.

• Consumer 1-1:

    Properties properties = new Properties();
    properties.put(PropertyKeyConst.GROUP_ID, "GID_jodie_test_1");
    Consumer consumer = OMS.builder().driver("sofamq").createConsumer(properties);
    consumer.subscribe("jodie_test_A", "*", new MessageListener() {
        public Action consume(Message message, ConsumeContext context) {
            System.out.println(message.getMsgID());
            return Action.CommitMessage;
        }
    });

• Consumer 1-2:

    Properties properties = new Properties();
    properties.put(PropertyKeyConst.GROUP_ID, "GID_jodie_test_1");
    Consumer consumer = OMS.builder().driver("sofamq").createConsumer(properties);
    consumer.subscribe("jodie_test_B ", "*", new MessageListener() {
        public Action consume(Message message, ConsumeContext context) {
            System.out.println(message.getMsgID());
            return Action.CommitMessage;
        }
    });

Example 2

The tags for a subscribed topic are inconsistent within the same group. Consumer 2-1 subscribes to TagA, while Consumer 2-2 does not specify a tag.

• Consumer 2-1:

    Properties properties = new Properties();
    properties.put(PropertyKeyConst.GROUP_ID, "GID_jodie_test_2");
    Consumer consumer = OMS.builder().driver("sofamq").createConsumer(properties);
    consumer.subscribe("jodie_test_A", "TagA", new MessageListener() {
        public Action consume(Message message, ConsumeContext context) {
            System.out.println(message.getMsgID());
            return Action.CommitMessage;
        }
    });

• Consumer 2-2:

    Properties properties = new Properties();
    properties.put(PropertyKeyConst.GROUP_ID, "GID_jodie_test_2");
    Consumer consumer = OMS.builder().driver("sofamq").createConsumer(properties);
    consumer.subscribe("jodie_test_A", "*", new MessageListener() {
        public Action consume(Message message, ConsumeContext context) {
            System.out.println(message.getMsgID());
            return Action.CommitMessage;
        }
    });

Example 3

The number of subscribed topics is inconsistent within the same group.

• Consumer 3-1:

    Properties properties = new Properties();
    properties.put(PropertyKeyConst.GROUP_ID, "GID_jodie_test_3");
    Consumer consumer = OMS.builder().driver("sofamq").createConsumer(properties);
    consumer.subscribe("jodie_test_A", "TagA", new MessageListener() {
        public Action consume(Message message, ConsumeContext context) {
            System.out.println(message.getMsgID());
            return Action.CommitMessage;
        }
    });
    consumer.subscribe("jodie_test_B", "TagB", new MessageListener() {
        public Action consume(Message message, ConsumeContext context) {
            System.out.println(message.getMsgID());
            return Action.CommitMessage;
        }
    });

The tags for a subscribed topic are inconsistent within the same group.

• Consumer 3-2:

    Properties properties = new Properties();
    properties.put(PropertyKeyConst.GROUP_ID, "GID_jodie_test_3");
    Consumer consumer = OMS.builder().driver("sofamq").createConsumer(properties);
    consumer.subscribe("jodie_test_A", "TagB", new MessageListener() {
        public Action consume(Message message, ConsumeContext context) {
            System.out.println(message.getMsgID());
            return Action.CommitMessage;
        }
    });