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https://issues.apache.org/jira/browse/KAFKA-8377?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16844290#comment-16844290
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John Roesler commented on KAFKA-8377:
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I happened to be looking at the TransformValues today for another reason, and
noticed that during the `process` method, we also transform the oldValue under
some situations:
`final V1 oldValue = sendOldValues ? valueTransformer.transform(key,
change.oldValue) : null;`
Just like the valueGetter, I think this is only correct if the transformation
is idempotent. So, it might be worth considering this as well, when taking this
ticket on.
> KTable#transformValue might lead to incorrect result in joins
> -------------------------------------------------------------
>
> Key: KAFKA-8377
> URL: https://issues.apache.org/jira/browse/KAFKA-8377
> Project: Kafka
> Issue Type: Bug
> Components: streams
> Affects Versions: 2.0.0
> Reporter: Matthias J. Sax
> Priority: Major
> Labels: newbie++
>
> Kafka Streams uses an optimization to not materialize every result KTable. If
> a non-materialized KTable is input to a join, the lookup into the table
> results in a lookup of the parents table plus a call to the operator. For
> example,
> {code:java}
> KTable nonMaterialized = materializedTable.filter(...);
> KTable table2 = ...
> table2.join(nonMaterialized,...){code}
> If there is a table2 input record, the lookup to the other side is performed
> as a lookup into materializedTable plus applying the filter().
> For stateless operation like filter, this is safe. However,
> #transformValues() might have an attached state store. Hence, when an input
> record r is processed by #transformValues() with current state S, it might
> produce an output record r' (that is not materialized). When the join later
> does a lookup to get r from the parent table, there is no guarantee that
> #transformValues() again produces r' because its state might not be the same
> any longer.
> Hence, it seems to be required, to always materialize the result of a
> KTable#transformValues() operation if there is state. Note, that if there
> would be a consecutive filter() after tranformValue(), it would also be ok to
> materialize the filter() result. Furthermore, if there is no downstream
> join(), materialization is also not required.
> Basically, it seems to be unsafe to apply `KTableValueGetter` on a stateful
> #transformValues()` operator.
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