junrao commented on code in PR #20334:
URL: https://github.com/apache/kafka/pull/20334#discussion_r2504944928
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docs/upgrade.html:
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@@ -113,6 +113,35 @@ <h5><a id="upgrade_420_notable"
href="#upgrade_420_notable">Notable changes in 4
<li>
The <code>num.replica.fetchers</code> config has a new lower bound of
1.
</li>
+ <li>
+ Improvements have been made to the validation rules and default values
of LIST-type configurations
+ (<a href="https://cwiki.apache.org/confluence/x/HArXF">KIP-1161</a>).
+ <ul>
+ <li>
+ LIST-type configurations now enforce stricter validation:
+ <ul>
+ <li>Null values are no longer accepted for most LIST-type
configurations, except those that explicitly
+ allow a null default value or where a null value has a
well-defined semantic meaning.</li>
+ <li>Duplicate entries within the same list are no longer
permitted.</li>
Review Comment:
It seems that configs like HeaderFrom.HEADERS_FIELD still allow duplicates.
So, we want to say sth like "Duplicate entries are no longer accepted for most
LIST-type configurations, except for those that explicitly stated". We also
want to change the description for HeaderFrom.HEADERS_FIELD to explicitly say
that duplicates are allowed.
Thinking a bit more about this. Since we typically only introduce breaking
changes in major releases, it might be useful to take a more conservative
approach by only logging a warning for duplicated entries for now and rejecting
them starting in 5.0.
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