A layer calls only the layer directly below it. Ingestion never writes to persistence directly — it goes through merge and precedence, which is the only component permitted to author a version.
The entitlement filter sits in the core platform, below every service. Authorisation applied per service is authorisation eventually forgotten in one of them.
Extensibility
The model and schema registry is a runtime component, not a build-time artefact: a new aspect type is registered, versioned and validated without a platform release (view 17).
The connector framework loads plugins. A new source type is a new package, not a fork.
Risk
Seven layers is a lot of indirection for a small team. The mitigation is that the metadata services layer is intentionally thin — most services are a few hundred lines over the core platform.
