ijuma commented on code in PR #20334:
URL: https://github.com/apache/kafka/pull/20334#discussion_r2530094198


##########
docs/upgrade.html:
##########
@@ -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:
   @junrao I agree that for major changes like zk to kraft, it's fine to 
require a migration step.
   
   I was referring to more incremental changes like "configs that are no longer 
used" or "stricter validation". For these cases, I don't think it's reasonable 
to break start-up. If we care that much about duplicate configs, we can simply 
remove the duplication (there is no change in semantics) - why force the user 
to deal with that?



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