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The first step in aggregating data is defining an event stream by creating the event stream definition. A stream definition provides the initial structure and identification required for event processing. It includes a set of data types as properties, name ,version and other attributes. When an external data publisher sends data events to WSO2 DAS (the receiver), it needs to specify the name and version of the stream intended. Also, data events should be sent according to the structure defined in the stream definition. For more information on event streams, see Event Streams.