每一个主题订阅三个可能的qos值为0、1和2。
The three possible QoS values for each topic subscription are 0, 1 and 2.
这里还需要考虑用户、主题和订阅之间的一些关系。
Some relationships between users, subjects and subscriptions would also need to be considered.
采用基于主题的路由，订阅者将获得所注册主题上的所有消息。
On topic based routing, the subscriber would get all the messages on the registered topic.
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