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:

• ProcessFunction
• KeyedProcessFunction
• CoProcessFunction
• KeyedCoProcessFunction
• ProcessWindowFunction
• ProcessAllWindowFunction

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);