Compatibility Mode
The registry setting that defines which schema changes are permitted — backward, forward, full, or none — and therefore what upgrade order is safe.
The choice encodes an operational assumption, and getting it wrong means an outage during a rollout rather than a build failure.
Backward — new schema can read data written with the old. Safe to upgrade consumers first. The common default.
Forward — old schema can read data written with the new. Safe to upgrade producers first.
Full — both. Required when upgrade order cannot be controlled, which is the realistic case for a long-lived event log with many independent consumers.
Transitive variants check against all previous versions rather than only the last, which matters for a topic retaining years of history — non-transitive checking allows a sequence of individually-safe changes that collectively break a replay from the beginning.
For retained event streams, full transitive is the conservative and usually correct setting.