private static boolean isKeyPreserving(PTransform<?, ?> transform) {
   // This is a hacky check for what is considered key-preserving to the direct runner.
   // The most obvious alternative would be a package-private marker interface, but
   // better to make this obviously hacky so it is less likely to proliferate. Meanwhile
   // we intend to allow explicit expression of key-preserving DoFn in the model.
   if (transform instanceof ParDo.BoundMulti) {
     ParDo.BoundMulti<?, ?> parDo = (ParDo.BoundMulti<?, ?>) transform;
     return parDo.getFn() instanceof ParDoMultiOverrideFactory.ToKeyedWorkItem;
   } else {
     return false;
   }
 }
    @Override
    public void translateNode(
        ParDo.BoundMulti<InputT, OutputT> transform, FlinkBatchTranslationContext context) {
      DoFn<InputT, OutputT> doFn = transform.getFn();
      rejectStateAndTimers(doFn);
      DataSet<WindowedValue<InputT>> inputDataSet =
          context.getInputDataSet(context.getInput(transform));

      List<TaggedPValue> outputs = context.getOutputs(transform);

      Map<TupleTag<?>, Integer> outputMap = Maps.newHashMap();
      // put the main output at index 0, FlinkMultiOutputDoFnFunction  expects this
      outputMap.put(transform.getMainOutputTag(), 0);
      int count = 1;
      for (TaggedPValue taggedValue : outputs) {
        if (!outputMap.containsKey(taggedValue.getTag())) {
          outputMap.put(taggedValue.getTag(), count++);
        }
      }

      // assume that the windowing strategy is the same for all outputs
      WindowingStrategy<?, ?> windowingStrategy = null;

      // collect all output Coders and create a UnionCoder for our tagged outputs
      List<Coder<?>> outputCoders = Lists.newArrayList();
      for (TaggedPValue taggedValue : outputs) {
        checkState(
            taggedValue.getValue() instanceof PCollection,
            "Within ParDo, got a non-PCollection output %s of type %s",
            taggedValue.getValue(),
            taggedValue.getValue().getClass().getSimpleName());
        PCollection<?> coll = (PCollection<?>) taggedValue.getValue();
        outputCoders.add(coll.getCoder());
        windowingStrategy = coll.getWindowingStrategy();
      }

      if (windowingStrategy == null) {
        throw new IllegalStateException("No outputs defined.");
      }

      UnionCoder unionCoder = UnionCoder.of(outputCoders);

      TypeInformation<WindowedValue<RawUnionValue>> typeInformation =
          new CoderTypeInformation<>(
              WindowedValue.getFullCoder(
                  unionCoder, windowingStrategy.getWindowFn().windowCoder()));

      List<PCollectionView<?>> sideInputs = transform.getSideInputs();

      // construct a map from side input to WindowingStrategy so that
      // the OldDoFn runner can map main-input windows to side input windows
      Map<PCollectionView<?>, WindowingStrategy<?, ?>> sideInputStrategies = new HashMap<>();
      for (PCollectionView<?> sideInput : sideInputs) {
        sideInputStrategies.put(sideInput, sideInput.getWindowingStrategyInternal());
      }

      @SuppressWarnings("unchecked")
      FlinkMultiOutputDoFnFunction<InputT, OutputT> doFnWrapper =
          new FlinkMultiOutputDoFnFunction(
              doFn,
              windowingStrategy,
              sideInputStrategies,
              context.getPipelineOptions(),
              outputMap);

      MapPartitionOperator<WindowedValue<InputT>, WindowedValue<RawUnionValue>> taggedDataSet =
          new MapPartitionOperator<>(
              inputDataSet, typeInformation, doFnWrapper, transform.getName());

      transformSideInputs(sideInputs, taggedDataSet, context);

      for (TaggedPValue output : outputs) {
        pruneOutput(
            taggedDataSet,
            context,
            outputMap.get(output.getTag()),
            (PCollection) output.getValue());
      }
    }