How would you use ETag and If-Match to stop two clients from silently overwriting each other's changes?

In production I define what the validator covers. A composite resource may depend on child rows, permissions, or derived state, so a parent rowversion might not represent every meaningful change. If automatic merging is safe, the client can refetch, reconcile, and submit against the new validator; the server should not blindly retry a stale user decision. I test simultaneous updates, stale deletes, missing preconditions, authorization changes, and validators across replicas. Observability tracks precondition failures separately from server errors because they often reveal normal collaboration or a client that is not refreshing state correctly.