Hi all,

What would be a natural way to implement a parallel version of the following Flink program?

Suppose I have a stream of items of type DataItem with two concrete implementations of DataItem: Value and Barrier. Let’s say that Value holds an integer value, and Barrier acts as explicit punctuation.

public interface DataItem {}

public class Value implements DataItem {

   private final int val;

   public Value(int val) { this.val = val; }

   public int getVal() { return val; }


public class Barrier implements DataItem {}

The program should maintain a sum of values seen since the beginning of the stream. On each Barrier, the program should output the sum seen so far.

An obvious way to implement this would be with a FlatMapFunction, maintaining the sum as state and emitting it on each Barrier.

StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();

DataStream<DataItem> stream = env.fromElements(DataItem.class,

       new Value(1), new Barrier(), new Value(3), new Value(-1), new Barrier());

stream.flatMap(new FlatMapFunction<DataItem, Integer>() {

   private int sum = 0;


   public void flatMap(DataItem dataItem, Collector<Integer> collector) throws Exception {

       if (dataItem instanceof Value) {

           sum += ((Value) dataItem).getVal();

       } else {






// We should see 1 followed by 3 as output

However, such an operator cannot be parallelized, since the order of Values and Barriers matters. That’s why I need to set parallelism to 1 above. Is there a way to rewrite this to exploit parallelism?

(Another reason to set parallelism to 1 above is that I’m assuming there is a single instance of the FlatMapFunction. A proper implementation would take more care in using state. Feel free to comment on that as well.)

Best regards,

Filip Niksic