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Public Interfaces

The following new methods method will be introduced:

interface KStream<K, V> {
	KStream<K, V> recursively(UnaryOperator<KStream<K, V>> op);
	KStream<K, V> recursively(UnaryOperator<KStream<K, V>> op, Produced<K, V> produced);
}

Note: UnaryOperator is java.util.function.UnaryOperator

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• op cannot be UnaryOperator.identity, or an equivalent function that simply returns its argument unmodified - this would produce an infinite recursive loop, since there's no opportunity refine the output to break out of the loop.
• op MUST "terminate"; that is, it must have some condition which eventually prevents further recursion of a record. In our example here, the terminating condition is the join, since the root node of our graph will have no parent, so the join will produce no output for the root node.
  • We can attempt to detect "definitely non-terminating" arguments by failing to detect operations that can cause the stream to terminate (e.g. filter, join, flatMap, etc.) in the process graph produced by the function.
  • We cannot guarantee that a function that includes terminating operations (filter, join, flatMap, etc.) actually terminates.

Automatic Repartitioning

If the op argument applies a key-changing operation (as it does in our example above), a repartition  topic may be automatically created. The optional Produced argument can be provided to customize repartitioning behaviour. This argument is ignored if a repartition  topic is not necessary.

• We use Produced  instead of Repartitioned, because when operating recursively, it would be an error to modify the number of partitions, since the topic MUST have the same number of partitions as the current Task.

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• • .

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Implementation

In KStreamImpl, implementation is fairly simple:

1. We call op, passing our current KStream as its argument. This produces our output KStream.
2. We determine if repartitioning is required on the op stream, and if it is, we automatically include a repartition node, equivalent to adding .repartition()  to the end of the op stream.We wire up the graphNode  from the output KStream  as a parent of the current KStream. This takes care of the recursion.
3. Finally, we return the output  KStream. This enables users to operate on the records that are being recursively produced, as above.

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The following tests will be added:

• Counting descendants of graph nodes arriving in-order (as above)
• Counting descendants of graph nodes arriving in any orderStreaming recursion:
• No repartitioning required
• Repartitioning handled by main stream.
Repartitioning handled by op argument.

Rejected Alternatives

It's currently possible to implement streaming recursion via explicit topics, albeit with a number of disadvantages:

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