m1a2st commented on code in PR #20334:
URL: https://github.com/apache/kafka/pull/20334#discussion_r2508017832
##########
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:
For HeaderFrom fields, headers
Both configurations should allow duplicate values. we can see the [test
case](https://github.com/apache/kafka/blob/18045c6ac30921503deffbef1744bb365dc599fb/connect/transforms/src/test/java/org/apache/kafka/connect/transforms/HeaderFromTest.java#L240
) for reference.
> perhaps we should print warnings instead of throwing exception in 4.x.
If we want to avoid breaking compatibility due to the stricter
duplicate-value validation, printing a warning message is a good option.
We could also update the following configurations, which should include
duplicate-value warnings as well, since their validators are customized.
```
TopicCreationConfig#INCLUDE_REGEX_CONFIG
TopicCreationConfig#EXCLUDE_REGEX_CONFIG
DistributedConfig#INTER_WORKER_VERIFICATION_ALGORITHMS_CONFIG
RestServerConfig#LISTENERS_DEFAULT
Cast#SPEC_CONFIG
ReplaceField#RENAMES
QuorumConfig#QUORUM_VOTERS_CONFIG
QuorumConfig#QUORUM_BOOTSTRAP_SERVERS_CONFIG
QuotaConfig#LEADER_REPLICATION_THROTTLED_REPLICAS_CONFIG
QuotaConfig#FOLLOWER_REPLICATION_THROTTLED_REPLICAS_CONFIG
```
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