消息内容的格式必须得到消息发布者和订阅者的同意和理解。
The format of the message content must be agreed and understood by both publishers and subscribers.
订阅者是消息使用者，其接收特定主题下由消息发布者发布的消息。
A subscriber is a message consumer that receives messages under a particular topic, published by a message publisher.
发布者：创建通知消息的实体。
Publisher: an entity that creates notification message instances.
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