When using side outputs, you first need to define an OutputTag
that will be used to identify a side output stream:
// this needs to be an anonymous inner class, so that we can analyze the type OutputTag<String> outputTag = new OutputTag<String>("side-output") {};
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
:
DataStream<Integer> input = ...; final OutputTag<String> outputTag = new OutputTag<String>("side-output"){}; SingleOutputStreamOperator<Integer> mainDataStream = input .process(new ProcessFunction<Integer, Integer>() { @Override public void processElement( Integer value, Context ctx, Collector<Integer> out) throws Exception { // emit data to regular output out.collect(value); // emit data to side output ctx.output(outputTag, "sideout-" + String.valueOf(value)); } });
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:
final OutputTag<String> outputTag = new OutputTag<String>("side-output"){}; SingleOutputStreamOperator<Integer> mainDataStream = ...; DataStream<String> sideOutputStream = mainDataStream.getSideOutput(outputTag);