Start here. This is the direct spoken answer to practice first.
Why this question matters
Explain API behavior when data and operations cross services with separate data ownership. It matters in real backend work because the team needs observability, ownership, and rollback room to recover without guessing. The practical angle is data ownership, eventual consistency, API states, retries, compensation, read models, and service boundaries, tied to a concrete production decision.
I start by assigning one service as the owner of each piece of data and business rule. Other services use that owner's API or events instead of writing its tables directly. A synchronous call is appropriate when the caller needs an immediate decision, while an event is useful for follow-on work that can complete later. The user-facing operation needs a clear answer when a dependency is unavailable or only partially complete.