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  1. Kafka
  2. KAFKA-7055

Kafka Streams Processor API allows you to add sinks and processors without parent



    • Type: Bug
    • Status: Resolved
    • Priority: Minor
    • Resolution: Fixed
    • Affects Version/s: 1.0.0
    • Fix Version/s: 2.0.0
    • Component/s: streams
    • Labels:


      The Kafka Streams Processor API allows you to define a Topology and connect sources, processors, and sinks. From reading through the code, it seems that you cannot forward a message to a downstream node unless it is explicitly connected to the upstream node (from which you are forwarding the message) as a child. Here is an example where you forward using name of downstream node rather than child index (https://github.com/apache/kafka/blob/trunk/streams/src/main/java/org/apache/kafka/streams/processor/internals/ProcessorContextImpl.java#L117).

      However, I've been able to connect processors and sinks to the topology without including parent names, i.e with empty vararg, using this method: https://github.com/apache/kafka/blob/trunk/streams/src/main/java/org/apache/kafka/streams/Topology.java#L423.

      As any attempt to forward a message to those nodes will throw a StreamsException, I suggest throwing an exception if a processor or sink is added without at least one upstream node. There is a method in `InternalTopologyBuilder` that allows you to connect processors by name after you add them to the topology, but it is not part of the external Processor API.

      In addition (or alternatively), I suggest making the error message for when users try to forward messages to a node that is not connected more descriptive, like this one for when a user attempts to access a state store that is not connected to the processor: https://github.com/apache/kafka/blob/trunk/streams/src/main/java/org/apache/kafka/streams/processor/internals/ProcessorContextImpl.java#L75-L81


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              • Assignee:
                nthean Nikki Thean
                nthean Nikki Thean
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