A message(also called a "row", a "tuple", or an "event") is very similar to a row of data in a database table. A row is a group of fields that all have data about the same thing.
For example, a row may describe a single stock trade, and may contain the price at which the stock was sold, the number of shares sold, and the stock name or stock symbol. If there are multiple stock trades, each will be described in a separate row.
Most messages match the description given above. However, there are also "control" messages (as opposed to "data" messages). Control messages are described later.
Messages "travel" in a stream. For more information, see Data Stream .