@Override
  public WatermarkFunction getWatermarkFn() {
    return this.filterFn instanceof WatermarkFunction ? (WatermarkFunction) this.filterFn : null;
  }
The spec for an operator that filters input messages based on some conditions.

@param <M> type of input message
samza-core/src/main/java/org/apache/samza/operators/spec/FilterOperatorSpec.java