In addition to the main stream that results from DataStream operations, you can also produce any
number of additional side output result streams. The type of data in the result streams does not
have to match the type of data in the main stream and the types of the different side outputs can
also differ. This operation can be useful when you want to split a stream of data where you would
normally have to replicate the stream and then filter out from each stream the data that you don’t
want to have.
When using side outputs, you first need to define an OutputTag that will be used to identify a
side output stream:
Notice how the OutputTag is typed according to the type of elements that the side output stream
contains.
Emitting data to a side output is possible from the following functions:
You can use the Context parameter, which is exposed to users in the above functions, to emit
data to a side output identified by an OutputTag. Here is an example of emitting side output
data from a ProcessFunction:
For retrieving the side output stream you use getSideOutput(OutputTag)
on the result of the DataStream operation. This will give you a DataStream that is typed
to the result of the side output stream: