How to merge data windows in Apache Beam?

In Apache Beam, the merging operation of data windows can be achieved by using the Combine operator. The Combine operator combines multiple data elements into a single result, and the merging function can be specified to determine how the data is merged.

For example, suppose we have a PCollection containing a series of integers and we want to combine these integers into one sum. We can achieve this functionality using the Combine operator.

PCollection<Integer> numbers = ...; // assume we have a PCollection of integers

PCollection<Integer> sum = numbers.apply(Combine.globally(new SumIntegersFn()));

public static class SumIntegersFn extends CombineFn<Integer, Integer, Integer> {
  @Override
  public Integer createAccumulator() {
    return 0;
  }

  @Override
  public Integer addInput(Integer accumulator, Integer input) {
    return accumulator + input;
  }

  @Override
  public Integer mergeAccumulators(Iterable<Integer> accumulators) {
    int sum = 0;
    for (int acc : accumulators) {
      sum += acc;
    }
    return sum;
  }

  @Override
  public Integer extractOutput(Integer accumulator) {
    return accumulator;
  }
}

In the example above, we first create a Combine operator that combines integers into a sum. We implement the CombineFn interface and override the createAccumulator(), addInput(), mergeAccumulators(), and extractOutput() methods to complete the merging operation. Finally, we apply the Combine operator to a dataset and store the result in a new PCollection.

It is important to note that merging operations in Apache Beam are global operations that combine data from all data windows. If specific data windows need to be merged, one can use the window operator to specify window types and handle window information in the merge function.