事件可以没有事件类型元数据与之关联——比如，有可能出现这种情况：事件是由现有非sca事件源创建的。
An event can have no event type metadata associated with it - for example, this can be the case for events which are created by pre-existing non-SCA event sources.
数据类型和格式是元数据。
服务使用的数据类型必须以元数据来描述，以使其他服务相互之间可以协同操作。
The data types that a service USES must be described in metadata to enable others to interoperate with the service.
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