We received following error in Creating Tests for Apache Beam Pipeline with DirectTestRunner.
Cloning and editing the original object will be required to fix the issue. IllegalMutationException from Beam PTransform

org.apache.beam.sdk.util.IllegalMutationException: PTransform
org.apache.beam.sdk.values.KV. Input values must not be mutated in any way.

This problem does not occur in our realtime Flink Pipeline, but only in the Test Pipeline Code.

What are some problems that can ocur, if we modify the original object? Apache mentions input objects are supposed to be immutable.

Resource:

https://cloud.google.com/dataflow/docs/guides/develop-and-test-pipelines

Example Code:

private class WeatherStatsPipeline extends
  PTransform<PCollection<Integer>, PCollection<WeatherSummary>> {
  @Override
  public PCollection<WeatherSummary> expand(PCollection<Integer> input) {
       // modify the original input * 2 , etc
  }
}

@Rule
public final transient TestPipeline p = TestPipeline.create();

@Test
@Category(NeedsRunner.class)
public void testWeatherPipeline() {
  // Create test input consisting of temperature readings
  PCollection<Integer> tempCelsius =
      p.apply(Create.of(24, 22, 20, 22, 21, 21, 20));

  // CalculateWeatherStats calculates the min, max, and average temperature
  PCollection<WeatherSummary> result =
      tempCelsius.apply("Calculate weather statistics", new WeatherStatsPipeline());

   // Assert correct output from CalculateWeatherStats
   PAssert.thatSingleton(result).isEqualTo(new WeatherSummary.Builder()
       .withAverageTemp(21)
       .withMaxTemp(24)
       .withMinTemp(20)
       .build());

   p.run();
}