【RocketMQ】原理篇-订阅关系的一致性

本文已参与「新人创作礼」活动，一起开启掘金创作之路。

订阅关系的一致性指的是，同一个消费者组（Group ID相同）下所有Consumer实例所订阅的Topic与Tag及对消息的处理逻辑必须完全一致。否则，消息消费的逻辑就会混乱，甚至导致消息丢失。

1、正确订阅关系

多个消费者组订阅了多个Topic，并且每个消费者组里的多个消费者实例的订阅关系保持了一致。

2、错误订阅关系

一个消费者组订阅了多个Topic，但是该消费者组里的多个Consumer实例的订阅关系并没有保持一致。

2.1、订阅了不同Topic

该例中的错误在于，同一个消费者组中的两个Consumer实例订阅了不同的Topic。

Consumer实例1-1：（订阅了topic为jodie_test_A，tag为所有的消息）

Properties properties = new Properties();
properties.put(PropertyKeyConst.GROUP_ID,"GID_jodie_test_1");
Consumer consumer = ONSFactory.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实例1-2：（订阅了topic为jodie_test_B，tag为所有的消息）

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

2.2、订阅了不同Tag

该例中的错误在于，同一个消费者组中的两个Consumer订阅了相同Topic的不同Tag。

Consumer实例2-1：（订阅了topic为jodie_test_A，tag为TagA的消息）

Properties properties = new Properties();
properties.put(PropertyKeyConst.GROUP_ID, "GID_jodie_test_2");
Consumer consumer = ONSFactory.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实例2-2：（订阅了topic为jodie_test_A，tag为所有的消息）

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

2.3、订阅了不同数量的Topic

该例中的错误在于，同一个消费者组中的两个Consumer订阅了不同数量的Topic。

Consumer实例3-1：（该Consumer订阅了两个Topic）

Properties properties = new Properties();
properties.put(PropertyKeyConst.GROUP_ID, "GID_jodie_test_3");
Consumer consumer = ONSFactory.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;  
    }
});

Consumer实例3-2：（该Consumer订阅了一个Topic）

Properties properties = new Properties();
properties.put(PropertyKeyConst.GROUP_ID, "GID_jodie_test_3");
Consumer consumer = ONSFactory.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;   
    }
});