+1 (non-binding) Just to remark: the proposal has two parts - compatibility as an invariant of the code, and RU as a functionality. Let's focus on compatibility now, because it is the foundation for RU.
Other projects also treat compatibility as its own documented contract, separate from specific upgrade tooling, e.g. Cassandra's "Compatibility guarantees" page [1], or Kafka's ongoing KIP-1156 [2]. Might be worth following a similar pattern here. [1] https://cwiki.apache.org/confluence/display/CASSANDRA2/CompatibilityGuarantees [2] https://cwiki.apache.org/confluence/display/KAFKA/KIP-1156:+Define+an+official+backward+compatibility+contract Best regards, Aleksandr Chesnokov
