scala - How to concatenate two streams in Apache Flink

Question

E.g. i want to compose stream of 1, 2, 3 and 4, 5 in single one, so result should be: 1, 2, 3, 4, 5. In other words: if first source is exhausted - get elements from second one. My closest attempt, which unfortunately does not preserve items order, is:

val a = streamEnvironment.fromElements(1, 2, 3)

val b = streamEnvironment.fromElements(4, 5)

val c = a.union(b)

c.map(x => println(s"X=$x")) // X=4, 5, 1, 2, 3 or something like that

Also did similar attempt with datetime included, but with same result.

Answer 1

This is not possible right now, at least not with the high level DataStream API.

It might be possible to implement a low-level operator that first reads on input and then the other input. However, this will completely block one input which does not work well with the way that Flink handles watermarks and performs checkpoints.

In the future, this will be possible using so-called side inputs.