基础概念
exchange的作用是：
An exchange is a very simple thing.
On one side it receives messages from producers and on the other side it pushes them to queues.
The exchange must know exactly what to do with a message it receives.
Should it be appended to a particular queue? Should it be appended to many queues? Or should it get discarded.
There are a few exchange types available: direct, topic, headers and fanout
binding的作用是：
That relationship between exchange and a queue is called a binding.
by binding,the rabbitmq knows , the exchange should send messages to where? queue1 ? queue2 ? or queue3?
routing key 和 binding key的区别：
routing key是message的属性，而binding key describe the relationship of exchange and queue.
if routing key of A message matches bingding key, the A message will appended to the queue
Subscriber 和 Queue的关系
2种消费模型
2种消费模型：
- 5条消息，C1 消费2条，C2消费3条
- 5条消息，C1 消费了5条，C2 也消费了5条
如果想实现第2种消费模型，那么C1 和 C2需要分别创建一个queue，这样，就每个consumer消费各自的queue中的消息。
如果想实现第1种消费模型，即共享订阅，即Shared Subscription Available
由于mqtt协议要求客户端唯一（即clientId唯一），导致每一个client都会创建该clientId对应的queue，因此在3.1.1版本下，是无法实现共享订阅的。参考：
https://blog.csdn.net/qq_41034780/article/details/129087592
幸运的是，在MQTT V5，支持了共享订阅，这样就能实现第1种消费模型。
参考：https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901080
https://blog.rabbitmq.com/posts/2023/07/mqtt5/#feature-15-optional-server-feature-availability