ETag / Optimistic Concurrency
What it is
A version token RAP attaches to an entity so a save is rejected if someone else changed the record since you read it. Declared in the BDEF (`etag master`), it implements optimistic locking over OData.
Why it matters for Clean Core
It's the Clean Core answer to 'two users edited the same order' without pessimistic SAP enqueue locks held across a stateless OData round-trip. RAP handles the conflict detection for you once you declare the ETag field.
Common pitfall
Omitting the ETag and being surprised by silent last-writer-wins overwrites in a Fiori app. Pick a reliable change-stamp field (a changed-at timestamp) and declare it as the ETag master.