name: beam-dofn-modernizer
description: Rewrite Apache Beam DoFn methods (@ProcessElement, @OnTimer, @OnWindowExpiration) to remove legacy ProcessContext or OnTimerContext usage. Use this skill when you encounter DoFn methods that use context.element(), context.output(), etc., and need to modernize them using parameter injection (@Element, @Timestamp, @Pane, OutputReceiver, MultiOutputReceiver).

Modernizing Apache Beam DoFns

Apache Beam has moved towards parameter injection in DoFn methods to improve readability and allow for more efficient execution. This skill helps you migrate legacy ProcessContext and OnTimerContext usage to modern annotated parameters.

Core Mappings

When rewriting a @ProcessElement or @OnTimer method, replace the context argument with the corresponding parameters based on the usage:

Legacy Context Usage (e.g. ProcessContext c)	Modern Parameter Replacement
c.element()	@Element T element
c.timestamp()	@Timestamp Instant timestamp
c.pane()	PaneInfo pane
c.window()	BoundedWindow window
c.sideInput(PCollectionView<T> view)	@SideInput("viewName") T value
c.getPipelineOptions()	PipelineOptions options
c.output(value)	OutputReceiver<T> receiver then receiver.output(value)
c.output(tag, value)	MultiOutputReceiver receiver then receiver.get(tag).output(value)
c.outputWithTimestamp(value, ts)	OutputReceiver<T> receiver then receiver.outputWithTimestamp(value, ts)

Method Signature Changes

@ProcessElement

Legacy:

@ProcessElement
public void processElement(ProcessContext c) {
  T element = c.element();
  c.output(transform(element));
}

Modern:

@ProcessElement
public void processElement(
    @Element T element,
    @Timestamp Instant timestamp,
    OutputReceiver<V> receiver) {
  receiver.output(transform(element));
}

@OnTimer

Legacy:

@OnTimer("timerId")
public void onTimer(OnTimerContext c) {
  c.output(someValue);
}

Modern:

@OnTimer("timerId")
public void onTimer(
    @Timestamp Instant timestamp,
    BoundedWindow window,
    OutputReceiver<V> receiver) {
  receiver.output(someValue);
}

Best Practices

1. Specific OutputReceiver: If the method only outputs to the main output, use OutputReceiver<T>. If it outputs to multiple tags, use MultiOutputReceiver.
2. Element Type: Ensure the @Element parameter type matches the input type of the DoFn.
3. Imports: Don't forget to add imports for:
   • org.apache.beam.sdk.transforms.DoFn.Element
   • org.apache.beam.sdk.transforms.DoFn.Timestamp
   • org.apache.beam.sdk.transforms.DoFn.OutputReceiver
   • org.apache.beam.sdk.transforms.DoFn.MultiOutputReceiver (if needed)
   • org.apache.beam.sdk.values.PCollectionView (if using @SideInput)
   • org.apache.beam.sdk.transforms.DoFn.SideInput
   • org.apache.beam.sdk.transforms.windowing.PaneInfo
4. Side Inputs: When using @SideInput, make sure to use the correct name that matches the one passed to ParDo.withSideInput("name", view).
5. Parameter Naming and Redundant Variables: Use descriptive names for the @Element parameter (e.g., record, line, row) instead of a generic element if it improves readability. Do not create a redundant local variable to copy the element (e.g., MyType elm = element;), use the parameter directly.

Example Conversion

Before:

@ProcessElement
public void processElement(ProcessContext c) {
  KV<String, Integer> element = c.element();
  Instant ts = c.timestamp();
  if (element.getValue() > threshold) {
    c.output(element.getKey());
    c.output(specialTag, element.getValue());
  }
}

After:

@ProcessElement
public void processElement(
    @Element KV<String, Integer> element,
    @Timestamp Instant timestamp,
    MultiOutputReceiver receiver) {
  if (element.getValue() > threshold) {
    receiver.get(mainTag).output(element.getKey());
    receiver.get(specialTag).output(element.getValue());
  }
}

[!NOTE]
If you only have one output, use OutputReceiver<String> receiver and receiver.output(element.getKey()).

Side Input Conversion

Modernizing side inputs involves removing the PCollectionView from the DoFn constructor and using @SideInput parameter injection instead.

Before (Legacy):

PTransform/Pipeline side:

PCollectionView<String> myView = ...;
input.apply(ParDo.of(new MyFn(myView)).withSideInputs(myView));

DoFn side:

class MyFn extends DoFn<T, V> {
  private final PCollectionView<String> view;
  MyFn(PCollectionView<String> view) { this.view = view; }

  @ProcessElement
  public void processElement(ProcessContext c) {
    String value = c.sideInput(view);
    // ...
  }
}

Nullable Side Inputs

If a side input is optional and a DoFn has conditional logic based on whether the side input is present, it is best to split the DoFn into two separate classes: one that requires the side input and one that does not. This avoids creating complex, conditional DoFns and ensures type safety.

PTransform/Pipeline side:

PCollectionView<String> myView = ...;
input.apply(ParDo.of(new MyFn(myView)).withSideInputs(myView));
//or
input.apply(ParDo.of(new MyFn(null))); // to introduce null

DoFn side:

class MyFn extends DoFn<T, V> {
  private final PCollectionView<String> view;
  MyFn(PCollectionView<String> view) { this.view = view; }

  @ProcessElement
  public void processElement(ProcessContext c) {
    String value = null;
    if (this.view != null) { // can do conditional side input
      value = c.sideInput(this.view);
    }

    // ...
  }
}

After (Modern):

PTransform/Pipeline side:

PCollectionView<String> myView = ...;
input.apply(ParDo.of(new MyFn()).withSideInput("sideInputName", myView));

DoFn side:

class MyFn extends DoFn<T, V> {
  @ProcessElement
  public void processElement(
      @Element T element,
      @SideInput("sideInputName") String value) {
    // value is injected directly
  }
}