jolshan commented on a change in pull request #10952:
URL: https://github.com/apache/kafka/pull/10952#discussion_r669899334



##########
File path: clients/src/main/java/org/apache/kafka/clients/MetadataCache.java
##########
@@ -130,13 +150,30 @@ MetadataCache mergeWith(String newClusterId,
                             Set<String> addInvalidTopics,
                             Set<String> addInternalTopics,
                             Node newController,
+                            Map<String, Uuid> topicIds,
                             BiPredicate<String, Boolean> retainTopic) {
 
         Predicate<String> shouldRetainTopic = topic -> retainTopic.test(topic, 
internalTopics.contains(topic));
 
         Map<TopicPartition, PartitionMetadata> newMetadataByPartition = new 
HashMap<>(addPartitions.size());
+        Map<String, Uuid> newTopicIds = new HashMap<>(topicIds.size());
+
+        // We want the most recent topic ID. We add the old one here for 
retained topics and then update with newest information in the MetadataResponse
+        // we add if a new topic ID is added or remove if the request did not 
support topic IDs for this topic.
+        for (Map.Entry<String, Uuid> entry : this.topicIds.entrySet()) {
+            if (shouldRetainTopic.test(entry.getKey())) {
+                newTopicIds.put(entry.getKey(), entry.getValue());
+            }
+        }
+
         for (PartitionMetadata partition : addPartitions) {
             newMetadataByPartition.put(partition.topicPartition, partition);
+            Uuid id = topicIds.get(partition.topic());
+            if (id != null)
+                newTopicIds.put(partition.topic(), id);
+            else
+                // Remove if the latest metadata does not have a topic ID

Review comment:
       Yeah. That was my reasoning. I thought the upgrade/downgrade case would 
be rare and the guarantees harder to reason about there.




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