junrao commented on code in PR #20334:
URL: https://github.com/apache/kafka/pull/20334#discussion_r2519403595
##########
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:
> 100%. It's never ok to fail to start a broker due to existing config state.
I think we want to be reasonable about that. Currently, if one has a ZK-only
based config, one can't start that broker in 4.0. But we provide warning and a
migration path in the release notes. For duplicate config entries, it is not to
hard to deal with it in a non-breaking way. But for things that are too hard to
maintain compatibility, it's probably better to allow a breaking change, but
provide enough warning and a migration path.
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