andymg3 commented on code in PR #12305:
URL: https://github.com/apache/kafka/pull/12305#discussion_r913275951
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clients/src/main/java/org/apache/kafka/common/internals/Topic.java:
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@@ -33,7 +33,7 @@ public class Topic {
public static final String LEGAL_CHARS = "[a-zA-Z0-9._-]";
private static final Set<String> INTERNAL_TOPICS =
Collections.unmodifiableSet(
- Utils.mkSet(GROUP_METADATA_TOPIC_NAME,
TRANSACTION_STATE_TOPIC_NAME));
+ Utils.mkSet(GROUP_METADATA_TOPIC_NAME,
TRANSACTION_STATE_TOPIC_NAME, METADATA_TOPIC_NAME));
Review Comment:
Apologies for the late reply @divijvaidya. I think it's a good point about
the compatibility issue. If a user is relying on `isInternal` and they have a
topic `__cluster_metadata` on an existing ZK cluster we do break them. We could
wrap the code in a conditional check I think. @hachikuji thoughts? At first I
thought this sounded a bit overkill but ultimately we dont know if folks are
relying on this behavior and how critical it is to their applications. For
upgrading from ZK to KRaft it seems like we'd have to handle the topic already
existing somehow. But that issue is a bit separate from what @divijvaidya is
concerned about (please correct me if I'm wrong) I think.
This makes me wonder how we would handle other topics that we want to mark
as internal in the future. It feels like maybe Kafka could/should reserve topic
names that start with two underscores? Obviously this could also break
applications, and would have to be thought through a lot and done carefully.
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