KAFKA-17412: add doc for unclean.leader.election.enable in KRaft (#…

…17051)

Reviewers: Colin P. McCabe <[email protected]>

clients/src/main/java/org/apache/kafka/common/config/TopicConfig.java

@@ -165,7 +165,9 @@ public class TopicConfig {
     public static final String UNCLEAN_LEADER_ELECTION_ENABLE_CONFIG = "unclean.leader.election.enable";
     public static final String UNCLEAN_LEADER_ELECTION_ENABLE_DOC = "Indicates whether to enable replicas " +
         "not in the ISR set to be elected as leader as a last resort, even though doing so may result in data " +
-        "loss.";
+        "loss.<p>Note: In KRaft mode, when enabling this config dynamically, it needs to wait for the unclean leader election" +
+        "thread to trigger election periodically (default is 5 minutes). Please run `kafka-leader-election.sh` with `unclean` option " +
+         "to trigger the unclean leader election immediately if needed.</p>";
 
     public static final String MIN_IN_SYNC_REPLICAS_CONFIG = "min.insync.replicas";
     public static final String MIN_IN_SYNC_REPLICAS_DOC = "When a producer sets acks to \"all\" (or \"-1\"), " +

docs/upgrade.html

@@ -44,6 +44,9 @@ <h5><a id="upgrade_390_notable" href="#upgrade_390_notable">Notable changes in 3
             See <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-950%3A++Tiered+Storage+Disablement">KIP-950</a> for more details.</li>
         <li>Tiered storage quota is implemented. Users can set an "upper bound" on the rate at which logs are copied/read to/from the remote storage.
             See <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-956+Tiered+Storage+Quotas">KIP-956</a> for more details.</li>
+        <li><code>unclean.leader.election.enable</code> config is supported in KRaft. Compared with ZK mode, there is one behavior change in KRaft mode
+            when dynamically enabling <code>unclean.leader.election.enable</code> config. Please check
+            <a href="https://kafka.apache.org/documentation/#brokerconfigs_unclean.leader.election.enable">here</a> for more details.</li>
     </ul>
 
 <h4><a id="upgrade_3_8_0" href="#upgrade_3_8_0">Upgrading to 3.8.0 from any version 0.8.x through 3.7.x</a></h4>

server/src/main/java/org/apache/kafka/server/config/ReplicationConfigs.java

@@ -122,7 +122,10 @@ public class ReplicationConfigs {
     public static final String UNCLEAN_LEADER_ELECTION_INTERVAL_MS_DOC = "The frequency with which the controller checks if it should perform an unclean leader election for leaderless partitions.";
 
     public static final String UNCLEAN_LEADER_ELECTION_ENABLE_CONFIG = ServerTopicConfigSynonyms.serverSynonym(TopicConfig.UNCLEAN_LEADER_ELECTION_ENABLE_CONFIG);
-    public static final String UNCLEAN_LEADER_ELECTION_ENABLE_DOC = "Indicates whether to enable replicas not in the ISR set to be elected as leader as a last resort, even though doing so may result in data loss";
+    public static final String UNCLEAN_LEADER_ELECTION_ENABLE_DOC = "Indicates whether to enable replicas not in the ISR set to be elected as leader as a last resort, even though doing so may result in data loss" +
+            "<p>Note: In KRaft mode, when enabling this config dynamically, it needs to wait for the unclean leader election " +
+            "thread to trigger election periodically (default is 5 minutes). Please run `kafka-leader-election.sh` with `unclean` option " +
+            "to trigger the unclean leader election immediately if needed.</p>";
 
     public static final String INTER_BROKER_PROTOCOL_VERSION_CONFIG = "inter.broker.protocol.version";
     public static final String INTER_BROKER_PROTOCOL_VERSION_DEFAULT = MetadataVersion.latestProduction().version();