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In the current version, users might create a topic with the configuration internal = true. After the changes in this proposal got adopted, the semantic of this configuration will would change and might break the expected behavior. The upstream application must change the topic config key from `internal` to something else and change the application logic if necessary.

Rejected Alternatives

1. Specify a naming convention that all internal topic should start with the prefix `_`.
   1. It's hard to make all clients adjust their topic names.
2. Change several public APIs to make the clients pass a flag indicating if the topic is internal or not when it creates a topic. Add a new ZK path such as `topics/internal`.
   1. May require a new flag in TopicCommand