https://issues.apache.org/jira/browse/KAFKA-10847 improves left/outer stream-stream join, by not emitting left/outer results eagerly, but only after the grace period passed.
While this change is desired, there is an issue with regard to upgrades: if users don't specify a grace period, we fall back to a 24h default. Thus, left/outer join results would only be emitted 24h after the join window end. This change in behavior could break existing applications when upgrading to 3.0.0 release. – And even if users do set a grace period explicitly, it's still unclear if the new delayed output behavior would work for them.
Thus, we propose to disable the fix of KAFAK-10847 by default, and let user opt-into the fix explicitly instead.
To allow users to enable the fix, we want to piggy-back on KIP-633 (https://issues.apache.org/jira/browse/KAFKA-8613) that deprecated the existing `JoinWindows.of()` and `JoinWindows#grace()` methods in favor of `JoinWindows.ofSizeAndGrace()` – if users don't update their code, we would keep the fix disabled, and thus, if users upgrade their app nothing changes. Only if users switch to the new `ofSizeAndGrace()` API, we enable the fix and thus give users the opportunity to opt-in expliclity and pick an appropriate grace period for their application.