Hi, Nikita! Thank you for pointing me to KIP-1161 and reminding me of this policy. I fully agree with your suggestion.
As you mentioned, an application that sets an invalid value for `application.server` already fails with `ConfigException` and Kafka Streams does not start. Therefore, moving the validation logic into `StreamsConfig` should not affect any application that is currently working correctly. Your argument sounds completely reasonable to me. Based on this, I think we do not need a two phase compatibility strategy with a warning first and an error later. A single phase that throws a `ConfigException` from the beginning should be sufficient. Let me update the KIP accordingly. Once the update is complete, I will let you know via the mailing list as soon as possible. Best regards, Sanghyeok An
