mjsax commented on code in PR #15740:
URL: https://github.com/apache/kafka/pull/15740#discussion_r1938337081


##########
streams/src/main/java/org/apache/kafka/streams/kstream/KStream.java:
##########
@@ -685,6 +685,41 @@ <VR> KStream<K, VR> flatMapValues(final ValueMapper<? 
super V, ? extends Iterabl
     <VR> KStream<K, VR> flatMapValues(final ValueMapperWithKey<? super K, ? 
super V, ? extends Iterable<? extends VR>> mapper,
                                       final Named named);
 
+    /**
+     * Marking the {@code KStream} as partitioned signals the stream is 
partitioned as intended,
+     * and does not require further repartitioning by downstream key changing 
operations.
+     * <p>
+     * Note that {@link KStream#markAsPartitioned()} SHOULD NOT be used with 
interactive query(IQ) or {@link KStream#join}.
+     * For reasons that when repartitions happen, records are physically 
shuffled by a composite key defined in the stateful operation.
+     * However, if the repartitions were cancelled, records stayed in their 
original partition by its original key. IQ or joins
+     * assumes and uses the composite key instead of the original key.

Review Comment:
   In general, it's better to include docs update in the same PR as the 
actually code lands. This way, we cannot "forget" to update the docs.



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