Each stream or window has a schema, which defines the columns in the events produced by the stream or window.
Each column has a name and datatype. All events that output from a single stream or window have an identical set of columns. For example:
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An input stream called RFIDRaw, coming out of an RFID reader, may have columns for a ReaderID and a TagID, both containing string data.
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An input stream called Trades, coming from a stock exchange, may have columns for the Symbol (string), Volume (integer), Price (float), and Time (datetime).