Just FYI.. for anyone that is curious and planing to upgrade their druid versions. Last week I upgraded our cluster from Druid 0.17.0 to Druid 28.0.1, running on HDFS as deep storage and MySQL for metadata. It was direct - stop old version, start new version on same box with pretty much same configs. Cluster is about 50 hosts. steps in order... • updated zookeeper from 3.4.2 to 3.9.2 - important to do first, otherwise, you will get bunch of "unsupported" errors in the service logs. • Upgraded broker hosts • Upgraded historicals one by one, because some needed to reindex • Upgraded middlemanagers one by one when they finished their tasks and no tasks where running • Upgraded Overlord • And last Coordinator No issues or conflicts and everything is running smoothly... 😀

Thank you for sharing. It is good to know that no config changes were needed. We are planning a similar upgrade from v24.0.2 to v27.0.0. Regards, AR.

The only thing is make sure your query handles null values and returns the same values. Because before that the fields that where empty where left empty and now they are getting filled by null value. And had to modify few json queries to get correct values again.